Pitch attitude control is controlling the movement of
the helicopter about its lateral axis. After interpreting
the helicopter’s pitch attitude by reference to the pitch
instruments (attitude indicator, altimeter, airspeed
indicator, and vertical speed indicator (VSI)), cyclic control
adjustments are made to affect the desired pitch attitude. In
this chapter, the pitch attitudes depicted are approximate
and vary with different helicopters.
Bank attitude control is controlling the angle made by the
lateral tilt of the rotor and the natural horizon, or the movement
of the helicopter about its longitudinal axis. After interpreting
the helicopter’s bank instruments (attitude indicator, heading
indicator, and turn indicator), cyclic control adjustments are
made to attain the desired bank attitude.
Power control is the application of collective pitch with
corresponding throttle control, where applicable. In straightand-
level fl ight, changes of collective pitch are made to
correct for altitude deviation if the error is more than 100
feet, or the airspeed is off by more than 10 knots. If the error
is less than that amount, a pilot should use a slight cyclic
climb or descent.
In order to fl y a helicopter by reference to the instruments, it
is important to know the approximate power settings required
for a particular helicopter in various load confi gurations and
fl ight conditions.
Trim, in helicopters, refers to the use of the cyclic centering
button, if the helicopter is so equipped, to relieve all
possible cyclic pressures. Trim also refers to the use of pedal
adjustment to center the ball of the turn indicator. Pedal trim
is required during all power changes.
The proper adjustment of collective pitch and cyclic friction
helps a pilot relax during instrument fl ight. Friction should
be adjusted to minimize overcontrolling and to prevent
creeping, but not applied to such a degree that control
movement is limited. In addition, many helicopters equipped
for instrument fl ight contain stability augmentation systems
or an autopilot to help relieve pilot workload.
Straight-and-Level Flight
Straight-and-level unaccelerated fl ight consists of maintaining
the desired altitude, heading, airspeed, and pedal trim.
Pitch Control
The pitch attitude of a helicopter is the angular relation of
its longitudinal axis to the natural horizon. If available, the
attitude indicator is used to establish the desired pitch attitude.
In level fl ight, pitch attitude varies with airspeed and center of
gravity (CG). At a constant altitude and a stabilized airspeed,
the pitch attitude is approximately level.
Attitude Indicator
The attitude indicator gives a direct indication of the pitch
attitude of the helicopter. In visual fl ight, attain the desired
pitch attitude by using the cyclic to raise and lower the nose
6-4
Figure 6-3. The initial pitch correction at normal cruise is one bar
width or less.
Figure 6-2. The fl ight instruments for pitch control are the airspeed indicator, attitude indicator, altimeter, and vertical speed
indicator.
of the helicopter in relation to the natural horizon. During
instrument fl ight, follow exactly the same procedure in
raising or lowering the miniature aircraft in relation to the
horizon bar.
There is some delay between control application and resultant
instrument change. This is the normal control lag in the
helicopter and should not be confused with instrument lag.
The attitude indicator may show small misrepresentations
of pitch attitude during maneuvers involving acceleration,
deceleration, or turns. This precession error can be detected
quickly by cross-checking the other pitch instruments.
If the miniature aircraft is properly adjusted on the ground, it
may not require readjustment in fl ight. If the miniature aircraft
is not on the horizon bar after level off at normal cruising
airspeed, adjust it as necessary while maintaining level fl ight
with the other pitch instruments. Once the miniature aircraft
has been adjusted in level fl ight at normal cruising airspeed,
leave it unchanged so it gives an accurate picture of pitch
attitude at all times.
When making initial pitch attitude corrections to maintain
altitude, the changes of attitude should be small and smoothly
applied. The initial movement of the horizon bar should not
exceed one bar width high or low. If a further
adjustment is required, an additional correction of one-half bar
normally corrects any deviation from the desired altitude. This
one-and-one-half bar correction is normally the maximum
pitch attitude correction from level fl ight attitude.
After making the correction, cross-check the other pitch
instruments to determine whether the pitch attitude change
is suffi cient. If additional correction is needed to return to
altitude, or if the airspeed varies more than 10 knots from
that desired, adjust the power.
Altimeter
The altimeter gives an indirect indication of the pitch
attitude of the helicopter in straight-and-level fl ight. Since
the altitude should remain constant in level fl ight, deviation
from the desired altitude indicates a need for a change in
pitch attitude and power as necessary. When losing altitude,
raise the pitch attitude and adjust power as necessary. When
gaining altitude, lower the pitch attitude and adjust power
as necessary. Indications for power changes are explained
in the next paragraph.
The rate at which the altimeter moves helps to determine pitch
attitude. A very slow movement of the altimeter indicates
a small deviation from the desired pitch attitude, while a
6-5
fast movement of the altimeter indicates a large deviation
from the desired pitch attitude. Make any corrective action
promptly, with small control changes. Also, remember that
movement of the altimeter should always be corrected by
two distinct changes. The fi rst is a change of attitude to stop
the altimeter movement; the second is a change of attitude to
return smoothly to the desired altitude. If altitude and airspeed
are more than 100 feet and 10 knots low, respectively, apply
power in addition to an increase of pitch attitude. If the
altitude and airspeed are high by more than 100 feet and 10
knots, reduce power and lower the pitch attitude.
There is a small lag in the movement of the altimeter;
however, for all practical purposes, consider that the altimeter
gives an immediate indication of a change, or a need for
change in pitch attitude. Since the altimeter provides the
most pertinent information regarding pitch in level fl ight, it
is considered primary for pitch.
Vertical Speed Indicator (VSI)
The VSI gives an indirect indication of the pitch attitude of
the helicopter and should be used in conjunction with the
other pitch instruments to attain a high degree of accuracy
and precision. The instrument indicates zero when in level
fl ight. Any movement of the needle from the zero position
shows a need for an immediate change in pitch attitude to
return it to zero. Always use the VSI in conjunction with
the altimeter in level fl ight. If a movement of the VSI is
detected, immediately use the proper corrective measures
to return it to zero. If the correction is made promptly, there
is usually little or no change in altitude. If the needle of the
VSI does not indicate zero, the altimeter indicates a gain or
loss of altitude.
The initial movement of the vertical speed needle is
instantaneous and indicates the trend of the vertical movement
of the helicopter. A period of time is necessary for the VSI to
reach its maximum point of defl ection after a correction has
been made. This time element is commonly referred to as
instrument lag. The lag is directly proportional to the speed
and magnitude of the pitch change. When employing smooth
control techniques and small adjustments in pitch attitude are
made, lag is minimized, and the VSI is easy to interpret.
Overcontrolling can be minimized by fi rst neutralizing the
controls and allowing the pitch attitude to stabilize, then
readjusting the pitch attitude by noting the indications of the
other pitch instruments.
Occasionally, the VSI may be slightly out of calibration.
This could result in the instrument indicating a slight climb
or descent even when the helicopter is in level fl ight. If the
instrument cannot be calibrated properly, this error must be
taken into consideration when using the VSI for pitch control.
For example, if a descent of 100 feet per minute (fpm) is the
vertical speed indication when the helicopter is in level fl ight,
use that indication as level fl ight. Any deviation from that
reading would indicate a change in attitude.
Airspeed Indicator
The airspeed indicator gives an indirect indication of
helicopter pitch attitude. With a given power setting and
pitch attitude, the airspeed remains constant. If the airspeed
increases, the nose is too low and should be raised. If the
airspeed decreases, the nose is too high and should be
lowered. A rapid change in airspeed indicates a large change
in pitch attitude, and a slow change in airspeed indicates a
small change in pitch attitude. There is very little lag in the
indications of the airspeed indicator. If, while making attitude
changes, there is some lag between control application and
change of airspeed, it is most likely due to cyclic control lag.
Generally, a departure from the desired airspeed, due to an
inadvertent pitch attitude change, also results in a change in
altitude. For example, an increase in airspeed due to a low
pitch attitude results in a decrease in altitude. A correction in
the pitch attitude regains both airspeed and altitude.
Bank Control
The bank attitude of a helicopter is the angular relation of
its lateral axis to the natural horizon. To maintain a straight
course in visual fl ight, keep the lateral axis of the helicopter
level with the natural horizon. Assuming the helicopter is in
coordinated fl ight, any deviation from a laterally level attitude
produces a turn.
Attitude Indicator
The attitude indicator gives a direct indication of the bank
attitude of the helicopter. For instrument fl ight, the miniature
aircraft and the horizon bar of the attitude indicator are
substituted for the actual helicopter and the natural horizon.
Any change in bank attitude of the helicopter is indicated
instantly by the miniature aircraft. For proper interpretation
of this instrument, imagine being in the miniature aircraft. If
the helicopter is properly trimmed and the rotor tilts, a turn
begins. The turn can be stopped by leveling the miniature
aircraft with the horizon bar. The ball in the turn-and-slip
indicator should always be kept centered through proper
pedal trim.
The angle of bank is indicated by the pointer on the banking
scale at the top of the instrument. Small bank
angles, which may not be seen by observing the miniature
aircraft, can easily be determined by referring to the banking
scale pointer.
6-6
Figure 6-5. The banking scale at the top of the attitude indicator indicates varying degrees of bank. In this example, the helicopter is
banked approximately 15° to the right.
Figure 6-4. The fl ight instruments used for bank control are the attitude, heading, and turn indicators.
Pitch-and-bank attitudes can be determined simultaneously
on the attitude indicator. Even though the miniature aircraft
is not level with the horizon bar, pitch attitude can be
established by observing the relative position of the miniature
aircraft and the horizon bar.
The attitude indicator may show small misrepresentations
of bank attitude during maneuvers that involve turns. This
precession error can be detected immediately by closely
cross-checking the other bank instruments during these
maneuvers. Precession is normally noticed when rolling
out of a turn. If, upon completion of a turn, the miniature
aircraft is level and the helicopter is still turning, make a
small change of bank attitude to center the turn needle and
stop the movement of the heading indicator.
Heading Indicator
In coordinated fl ight, the heading indicator gives an indirect
indication of a helicopter’s bank attitude. When a helicopter is
banked, it turns. When the lateral axis of a helicopter is level,
it fl ies straight. Therefore, in coordinated fl ight when the
heading indicator shows a constant heading, the helicopter is
level laterally. A deviation from the desired heading indicates
a bank in the direction the helicopter is turning. A small angle
of bank is indicated by a slow change of heading; a large angle
of bank is indicated by a rapid change of heading. If a turn
is noticed, apply opposite cyclic until the heading indicator
6-7
Figure 6-6. Coordinated fl ight is indicated by centering of the ball.
indicates the desired heading, simultaneously ensuring the
ball is centered. When making the correction to the desired
heading, do not use a bank angle greater than that required
to achieve a standard rate turn. In addition, if the number
of degrees of change is small, limit the bank angle to the
number of degrees to be turned. Bank angles greater than
these require more skill and precision in attaining the desired
results. During straight-and-level fl ight, the heading indicator
is the primary reference for bank control.
Turn Indicator
During coordinated fl ight, the needle of the turn-and-slip
indicator gives an indirect indication of the bank attitude
of the helicopter. When the needle is displaced from the
vertical position, the helicopter is turning in the direction of
the displacement. Thus, if the needle is displaced to the left,
the helicopter is turning left. Bringing the needle back to
the vertical position with the cyclic produces straight fl ight.
A close observation of the needle is necessary to accurately
interpret small deviations from the desired position.
Cross-check the ball of the turn-and-slip indicator to determine
if the helicopter is in coordinated fl ight. If
the rotor is laterally level and pedal pressure properly
compensates for torque, the ball remains in the center. To
center the ball, level the helicopter laterally by reference to
the other bank instruments, then center the ball with pedal
trim. Torque correction pressures vary as power changes are
made. Always check the ball after such changes.
Common Errors During Straight-and-Level Flight
1. Failure to maintain altitude
2. Failure to maintain heading
3. Overcontrolling pitch and bank during corrections
4. Failure to maintain proper pedal trim
5. Failure to cross-check all available instruments
Power Control During Straight-and-Level Flight
Establishing specifi c power settings is accomplished through
collective pitch adjustments and throttle control, where
necessary. For reciprocating-powered helicopters, power
indication is observed on the manifold pressure gauge.
For turbine-powered helicopters, power is observed on the
torque gauge. (Although most Instrument Flight Rules (IFR)-
certifi ed helicopters are turbine powered, depictions within
this chapter use a reciprocating-powered helicopter as this
is where training is most likely conducted.)
At any given airspeed, a specifi c power setting determines
whether the helicopter is in level fl ight, in a climb, or in a
descent. For example, cruising airspeed maintained with
cruising power results in level fl ight. If a pilot increases the
power setting and holds the airspeed constant, the helicopter
climbs. Conversely, if the pilot decreases power and holds
the airspeed constant, the helicopter descends.
6-8
Figure 6-7. Flight instrument indications in straight-and-level fl ight with power increasing.
If the altitude is held constant, power determines the airspeed.
For example, at a constant altitude, cruising power results
in cruising airspeed. Any deviation from the cruising power
setting results in a change of airspeed. When power is added
to increase airspeed, the nose of the helicopter pitches up and
yaws to the right in a helicopter with a counterclockwise main
rotor blade rotation. When power is reduced to
decrease airspeed, the nose pitches down and yaws to the
left. The yawing effect is most pronounced in
single-rotor helicopters, and is absent in helicopters with
counter-rotating rotors. To counteract the yawing tendency
of the helicopter, apply pedal trim during power changes.
To maintain a constant altitude and airspeed in level fl ight,
coordinate pitch attitude and power control. The relationship
between altitude and airspeed determines the need for a
change in power and/or pitch attitude. If the altitude is
constant and the airspeed is high or low, change the power to
obtain the desired airspeed. During the change in power, make
an accurate interpretation of the altimeter, then counteract
any deviation from the desired altitude by an appropriate
change of pitch attitude. If the altitude is low and the airspeed
is high, or vice versa, a change in pitch attitude alone may
return the helicopter to the proper altitude and airspeed. If
both airspeed and altitude are low, or if both are high, changes
in both power and pitch attitude are necessary.
To make power control easy when changing airspeed, it is
necessary to know the approximate power settings for the
various airspeeds at which the helicopter is fl own. When the
airspeed is to be changed by any appreciable amount, adjust
the power so that it is over or under that setting necessary
to maintain the new airspeed. As the power approaches the
desired setting, include the manifold pressure in the crosscheck
to determine when the proper adjustment has been
accomplished. As the airspeed is changing, adjust the pitch
attitude to maintain a constant altitude. A constant heading
should be maintained throughout the change. As the desired
airspeed is approached, adjust power to the new cruising
power setting and further adjust pitch attitude to maintain
altitude. The instrument indications for straight-and-level
fl ight at normal cruise and during the transition from normal
cruise to slow cruise are illustrated in Figures 6-9 and 6-10.
After the airspeed stabilizes at slow cruise, the attitude
indicator shows an approximate level pitch attitude.
The altimeter is the primary pitch instrument during level
fl ight, whether fl ying at a constant airspeed or during a
change in airspeed. Altitude should not change during
airspeed transitions, and the heading indicator remains the
primary bank instrument. Whenever the airspeed is changed
by an appreciable amount, the manifold pressure gauge is
momentarily the primary instrument for power control.
When the airspeed approaches the desired reading, the
airspeed indicator again becomes the primary instrument
for power control.
6-9
Figure 6-8. Flight instrument indications in straight-and-level fl ight with power decreasing.
Figure 6-9. Flight instrument indications in straight-and-level fl ight at normal cruise speed.
6-10
Figure 6-10. Flight instrument indications in straight-and-level fl ight with airspeed decreasing.
Entry
To enter a constant airspeed climb from cruise airspeed when
the climb speed is lower than cruise speed, simultaneously
increase power to the climb power setting and adjust pitch
attitude to the approximate climb attitude. The increase in
power causes the helicopter to start climbing and only very
slight back cyclic pressure is needed to complete the change
from level to climb attitude. The attitude indicator should
be used to accomplish the pitch change. If the transition
from level fl ight to a climb is smooth, the VSI shows an
immediate upward trend and then stops at a rate appropriate
to the stabilized airspeed and attitude. Primary and supporting
instruments for climb entry are illustrated in Figure 6-11.
When the helicopter stabilizes at a constant airspeed and
attitude, the airspeed indicator becomes primary for pitch.
The manifold pressure continues to be primary for power and
should be monitored closely to determine if the proper climb
power setting is being maintained. Primary and supporting
instruments for a stabilized constant airspeed climb are shown
in Figure 6-12.
The technique and procedures for entering a constant rate
climb are very similar to those previously described for a
constant airspeed climb. For training purposes, a constant
To produce straight-and-level fl ight, the cross-check of the
pitch-and-bank instruments should be combined with the
power control instruments. With a constant power setting, a
normal cross-check should be satisfactory. When changing
power, the speed of the cross-check must be increased to
cover the pitch-and-bank instruments adequately. This is
necessary to counteract any deviations immediately.
Common Errors During Airspeed Changes
1. Improper use of power
2. Overcontrolling pitch attitude
3. Failure to maintain heading
4. Failure to maintain altitude
5. Improper pedal trim
Straight Climbs (Constant Airspeed and
Constant Rate)
For any power setting and load condition, there is only
one airspeed that gives the most effi cient rate of climb.
To determine this, consult the climb data for the type of
helicopter being fl own. The technique varies according to
the airspeed on entry and whether a constant airspeed or
constant rate climb is made.
6-11
Figure 6-11. Flight instrument indications during climb entry for a constant-airspeed climb.
Figure 6-12. Flight instrument indications in a stabilized constant-airspeed climb.
6-12
Figure 6-13. Flight Instrument Indications in a Stabilized Constant-Rate Climb.
rate climb is entered from climb airspeed. Use the rate
appropriate for the particular helicopter being flown.
Normally, in helicopters with low climb rates, 500 fpm is
appropriate. In helicopters capable of high climb rates, use
a rate of 1,000 fpm.
To enter a constant rate climb, increase power to the
approximate setting for the desired rate. As power is
applied, the airspeed indicator is primary for pitch until the
vertical speed approaches the desired rate. At this time, the
VSI becomes primary for pitch. Change pitch attitude by
reference to the attitude indicator to maintain the desired
vertical speed. When the VSI becomes primary for pitch, the
airspeed indicator becomes primary for power. Primary and
supporting instruments for a stabilized constant rate climb are
illustrated in Figure 6-13. Adjust power to maintain desired
airspeed. Pitch attitude and power corrections should be
closely coordinated. To illustrate this, if the vertical speed
is correct but the airspeed is low, add power. As power is
increased, it may be necessary to lower the pitch attitude
slightly to avoid increasing the vertical rate. Adjust the pitch
attitude smoothly to avoid overcontrolling. Small power
corrections are usually suffi cient to bring the airspeed back
to the desired indication.
Level Off
The level off from a constant airspeed climb must be started
before reaching the desired altitude. Although the amount
of lead varies with the type of helicopter being fl own and
pilot technique, the most important factor is vertical speed.
As a rule of thumb, use 10 percent of the vertical velocity
as the lead point. For example, if the rate of climb is 500
fpm, initiate the level off approximately 50 feet before the
desired altitude. When the proper lead altitude is reached, the
altimeter becomes primary for pitch. Adjust the pitch attitude
to the level fl ight attitude for that airspeed. Cross-check the
altimeter and VSI to determine when level fl ight has been
attained at the desired altitude. If cruise airspeed is higher
than climb airspeed, leave the power at the climb power
setting until the airspeed approaches cruise airspeed, and
then reduce it to the cruise power setting. The level off from
a constant rate climb is accomplished in the same manner as
the level off from a constant airspeed climb.
Straight Descents (Constant Airspeed
and Constant Rate)
A descent may be performed at any normal airspeed the
helicopter can attain, but the airspeed must be determined
prior to entry. The technique is determined by the type of
descent, a constant airspeed or a constant rate.
Entry
If airspeed is higher than descending airspeed, and a constant
airspeed descent is desired, reduce power to a descent
power setting and maintain a constant altitude using cyclic
pitch control. This slows the helicopter. As the helicopter
6-13
approaches the descending airspeed, the airspeed indicator
becomes primary for pitch and the manifold pressure is
primary for power. Holding the airspeed constant causes the
helicopter to descend. For a constant rate descent, reduce the
power to the approximate setting for the desired rate. If the
descent is started at the descending airspeed, the airspeed
indicator is primary for pitch until the VSI approaches the
desired rate. At this time, the VSI becomes primary for
pitch, and the airspeed indicator becomes primary for power.
Coordinate power and pitch attitude control as previously
described on page 6-10 for constant rate climbs.
Level Off
The level off from a constant airspeed descent may be
made at descending airspeed or at cruise airspeed, if this is
higher than descending airspeed. As in a climb level off, the
amount of lead depends on the rate of descent and control
technique. For a level off at descending airspeed, the lead
should be approximately 10 percent of the vertical speed. At
the lead altitude, simultaneously increase power to the setting
necessary to maintain descending airspeed in level fl ight. At
this point, the altimeter becomes primary for pitch, and the
airspeed indicator becomes primary for power.
To level off at an airspeed higher than descending airspeed,
increase the power approximately 100 to 150 feet prior to
reaching the desired altitude. The power setting should be that
which is necessary to maintain the desired airspeed in level
fl ight. Hold the vertical speed constant until approximately
50 feet above the desired altitude. At this point, the altimeter
becomes primary for pitch and the airspeed indicator becomes
primary for power. The level off from a constant rate descent
should be accomplished in the same manner as the level off
from a constant airspeed descent.
Common Errors During Straight Climbs and
Descents
1. Failure to maintain heading
2. Improper use of power
3. Poor control of pitch attitude
4. Failure to maintain proper pedal trim
5. Failure to level off on desired altitude
Turns
Turns made by reference to the fl ight instruments should
be made at a precise rate. Turns described in this chapter
are those not exceeding a standard rate of 3° per second
as indicated on the turn-and-slip indicator. True airspeed
determines the angle of bank necessary to maintain a standard
rate turn. A rule of thumb to determine the approximate angle
of bank required for a standard rate turn is to use 15 percent
of the airspeed. A simple way to determine this amount is
to divide the airspeed by 10 and add one-half the result. For
example, at 60 knots approximately 9° of bank is required
(60 ÷ 10 = 6, 6 + 3 = 9); at 80 knots approximately 12° of
bank is needed for a standard rate turn.
To enter a turn, apply lateral cyclic in the direction of the
desired turn. The entry should be accomplished smoothly,
using the attitude indicator to establish the approximate bank
angle. When the turn indicator indicates a standard rate turn,
it becomes primary for bank. The attitude indicator now
becomes a supporting instrument. During level turns, the
altimeter is primary for pitch, and the airspeed indicator is
primary for power. Primary and supporting instruments for a
stabilized standard rate turn are illustrated in Figure 6-14. If
an increase in power is required to maintain airspeed, slight
forward cyclic pressure may be required since the helicopter
tends to pitch up as collective pitch is increased. Apply pedal
trim, as required, to keep the ball centered.
To recover to straight-and-level fl ight, apply cyclic in the
direction opposite the turn. The rate of roll-out should be the
same as the rate used when rolling into the turn. As the turn
recovery is initiated, the attitude indicator becomes primary
for bank. When the helicopter is approximately level, the
heading indicator becomes primary for bank as in straightand-
level fl ight. Cross-check the airspeed indicator and ball
closely to maintain the desired airspeed and pedal trim.
Turn to a Predetermined Heading
A helicopter turns as long as its lateral axis is tilted;
therefore, the recovery must start before the desired heading
is reached. The amount of lead varies with the rate of turn
and piloting technique.
As a guide, when making a 3° per second rate of turn, use a
lead of one-half the bank angle. For example, if using a 12°
bank angle, use half of that, or 6°, as the lead point prior to the
desired heading. Use this lead until the exact amount required
by a particular technique can be determined. The bank angle
should never exceed the number of degrees to be turned.
As in any standard rate turn, the rate of recovery should be
the same as the rate of entry. During turns to predetermined
headings, cross-check the primary and supporting pitch, bank,
and power instruments closely.
Timed Turns
A timed turn is a turn in which the clock and turn-and-slip
indicator are used to change heading a defi nite number of
degrees in a given time. For example, using a standard rate
turn, a helicopter turns 45° in 15 seconds. Using a half6-
14
Figure 6-14. Flight Instrument Indications in a Standard-Rate Turn to the Left.
standard rate turn, the helicopter turns 45° in 30 seconds.
Timed turns can be used if the heading indicator becomes
inoperative.
Prior to performing timed turns, the turn coordinator should
be calibrated to determine the accuracy of its indications.
To do this, establish a standard rate turn by referring to the
turn-and-slip indicator. Then, as the sweep second hand of
the clock passes a cardinal point (12, 3, 6, or 9), check the
heading on the heading indicator. While holding the indicated
rate of turn constant, note the heading changes at 10-second
intervals. If the helicopter turns more or less than 30° in
that interval, a smaller or larger defl ection of the needle is
necessary to produce a standard rate turn. After the turnand-
slip indicator has been calibrated during turns in each
direction, note the corrected defl ections, if any, and apply
them during all timed turns.
Use the same cross-check and control technique in making
timed turns that is used to make turns to a predetermined
heading, but substitute the clock for the heading indicator.
The needle of the turn-and-slip indicator is primary for bank
control, the altimeter is primary for pitch control, and the
airspeed indicator is primary for power control. Begin the
roll-in when the clock’s second hand passes a cardinal point;
hold the turn at the calibrated standard rate indication, or
half-standard rate for small changes in heading; then begin
the roll-out when the computed number of seconds has
elapsed. If the roll-in and roll-out rates are the same, the time
taken during entry and recovery need not be considered in
the time computation.
If practicing timed turns with a full instrument panel, check
the heading indicator for the accuracy of the turns. If executing
turns without the heading indicator, use the magnetic compass
at the completion of the turn to check turn accuracy, taking
compass deviation errors into consideration.
Change of Airspeed in Turns
Changing airspeed in turns is an effective maneuver for
increasing profi ciency in all three basic instrument skills.
Since the maneuver involves simultaneous changes in all
components of control, proper execution requires a rapid
cross-check and interpretation, as well as smooth control.
Profi ciency in the maneuver also contributes to confi dence in
the instruments during attitude and power changes involved
in more complex maneuvers.
Pitch and power control techniques are the same as those
used during airspeed changes in straight-and-level fl ight.
As discussed previously, the angle of bank necessary for a
given rate of turn is proportional to the true airspeed. Since
the turns are executed at standard rate, the angle of bank
must be varied in direct proportion to the airspeed change in
order to maintain a constant rate of turn. During a reduction
of airspeed, decrease the angle of bank and increase the pitch
attitude to maintain altitude and a standard rate turn.
6-15
Altimeter and turn indicator readings should remain constant
throughout the turn. The altimeter is primary for pitch control,
and the turn needle is primary for bank control. Manifold
pressure is primary for power control while the airspeed is
changing. As the airspeed approaches the new indication, the
airspeed indicator becomes primary for power control.
Two methods of changing airspeed in turns may be used.
In the fi rst method, airspeed is changed after the turn is
established. In the second method, the airspeed change
is initiated simultaneously with the turn entry. The fi rst
method is easier, but regardless of the method used, the rate
of cross-check must be increased as power is reduced. As
the helicopter decelerates, check the altimeter and VSI for
needed pitch changes, and the bank instruments for needed
bank changes. If the needle of the turn-and-slip indicator
shows a deviation from the desired defl ection, change the
bank. Adjust pitch attitude to maintain altitude. When the
airspeed approaches that desired, the airspeed indicator
becomes primary for power control. Adjust the power to
maintain the desired airspeed. Use pedal trim to ensure the
maneuver is coordinated.
Until control technique is very smooth, frequently crosscheck
the attitude indicator to keep from overcontrolling
and to provide approximate bank angles appropriate for the
changing airspeeds.
Compass Turns
The use of gyroscopic heading indicators makes heading
control very easy. However, if the heading indicator fails
or the helicopter is not equipped with one, use the magnetic
compass for heading reference. When making compass-only
turns, a pilot needs to adjust for the lead or lag created by
acceleration and deceleration errors so that the helicopter
rolls out on the desired heading. When turning to a heading
of north, the lead for the roll-out must include the number of
degrees of latitude plus the lead normally used in recovery
from turns. During a turn to a south heading, maintain the
turn until the compass passes south the number of degrees
of latitude, minus the normal roll-out lead. For example,
when turning from an easterly direction to north, where the
latitude is 30°, start the roll-out when the compass reads
37° (30° plus one-half the 15° angle of bank, or whatever
amount is appropriate for the rate of roll-out). When turning
from an easterly direction to south, start the roll-out when
the magnetic compass reads 203° (180° plus 30° minus onehalf
the angle of bank). When making similar turns from a
westerly direction, the appropriate points at which to begin
the roll-out would be 323° for a turn to north, and 157° for
a turn to south.
30° Bank Turn
A turn using 30° of bank is seldom necessary or advisable
in instrument meteorological conditions (IMC), and is
considered an unusual attitude in a helicopter. However, it
is an excellent maneuver to practice to increase the ability to
react quickly and smoothly to rapid changes of attitude. Even
though the entry and recovery techniques are the same as for
any other turn, it is more diffi cult to control pitch because
of the decrease in vertical lift as the bank increases. Also,
because of the decrease in vertical lift, there is a tendency
to lose altitude and/or airspeed. Therefore, to maintain a
constant altitude and airspeed, additional power is required.
Do not initiate a correction, however, until the instruments
indicate the need for one. During the maneuver, note the
need for a correction on the altimeter and VSI, check the
attitude indicator, and then make the necessary adjustments.
After making a change, check the altimeter and VSI again to
determine whether or not the correction was adequate.
Climbing and Descending Turns
For climbing and descending turns, the techniques described
previously for straight climbs, descents, and standard rate
turns are combined. For practice, simultaneously turn and
start the climb or descent. The primary and supporting
instruments for a stabilized constant airspeed left climbing
turn are illustrated in Figure 6-15. The level off from a
climbing or descending turn is the same as the level off from
a straight climb or descent. To return to straight-and-level
fl ight, stop the turn and then level off, or level off and then
stop the turn, or simultaneously level off and stop the turn.
During climbing and descending turns, keep the ball of the
turn indicator centered with pedal trim.
Common Errors During Turns
1. Failure to maintain desired turn rate
2. Failure to maintain altitude in level turns
3. Failure to maintain desired airspeed
4. Variation in the rate of entry and recovery
5. Failure to use proper lead in turns to a heading
6. Failure to properly compute time during timed turns
7. Failure to use proper leads and lags during the compass
turns
8. Improper use of power
9. Failure to use proper pedal trim
6-16
Figure 6-15. Flight Instrument Indications for a Stabilized Left Climbing Turn at a Constant Airspeed.
Unusual Attitudes
Any maneuver not required for normal helicopter instrument
fl ight is an unusual attitude and may be caused by any one
or combination of factors such as turbulence, disorientation,
instrument failure, confusion, preoccupation with fl ight deck
duties, carelessness in cross-checking, errors in instrument
interpretation, or lack of profi ciency in aircraft control. Due
to the instability characteristics of the helicopter, unusual
attitudes can be extremely critical. As soon as an unusual
attitude is detected, make a recovery to straight-and-level
fl ight as soon as possible with a minimum loss of altitude.
To recover from an unusual attitude, a pilot should correct
bank-and-pitch attitude and adjust power as necessary. All
components are changed almost simultaneously, with little
lead of one over the other. A pilot must be able to perform
this task with and without the attitude indicator. If the
helicopter is in a climbing or descending turn, adjust bank,
pitch, and power. The bank attitude should be corrected
by referring to the turn-and-slip indicator and attitude
indicator. Pitch attitude should be corrected by reference to
the altimeter, airspeed indicator, VSI, and attitude indicator.
Adjust power by referring to the airspeed indicator and
manifold pressure.
Since the displacement of the controls used in recovery from
unusual attitudes may be greater than those used for normal
fl ight, make careful adjustments as straight-and-level fl ight
is approached. Cross-check the other instruments closely to
avoid overcontrolling.
Common Errors During Unusual Attitude
Recoveries
1. Failure to make proper pitch correction
2. Failure to make proper bank correction
3. Failure to make proper power correction
4. Overcontrolling pitch and/or bank attitude
5. Overcontrolling power
6. Excessive loss of altitude
Emergencies
Emergencies during instrument fl ight are handled similarly
to those occurring during VFR fl ight. A thorough knowledge
of the helicopter and its systems, as well as good aeronautical
knowledge and judgment, is the best preparation for
emergency situations. Safe operations begin with prefl ight
planning and a thorough prefl ight inspection. Plan a route
of fl ight to include adequate landing sites in the event of an
emergency landing. Make sure all resources, such as maps,
publications, fl ashlights, and fi re extinguishers are readily
available for use in an emergency.
During any emergency, fi rst fl y the aircraft. This means ensure
the helicopter is under control, and determine emergency
6-17
landing sites. Then perform the emergency checklist memory
items, followed by items written in the rotorcraft fl ight
manual (RFM). When all these items are under control, notify
air traffi c control (ATC). Declare any emergency on the last
assigned ATC frequency. If one was not issued, transmit on
the emergency frequency 121.5. Set the transponder to the
emergency squawk code 7700. This code triggers an alarm
or special indicator in radar facilities.
When experiencing most in-fl ight emergencies, such as low
fuel or complete electrical failure, land as soon as possible.
In the event of an electrical fi re, turn off all nonessential
equipment and land immediately. Some essential electrical
instruments, such as the attitude indicator, may be required
for a safe landing. A navigation radio failure may not require
an immediate landing if the fl ight can continue safely. In
this case, land as soon as practical. ATC may be able to
provide vectors to a safe landing area. For specifi c details
on what to do during an emergency, refer to the RFM for
the helicopter.
Autorotations
Both straight-ahead and turning autorotations should be
practiced by reference to instruments. This training ensures
prompt corrective action to maintain positive aircraft control
in the event of an engine failure.
To enter autorotation, reduce collective pitch smoothly to
maintain a safe rotor RPM and apply pedal trim to keep the
ball of the turn-and-slip indicator centered. The pitch attitude
of the helicopter should be approximately level as shown by
the attitude indicator. The airspeed indicator is the primary
pitch instrument and should be adjusted to the recommended
autorotation speed. The heading indicator is primary for bank
in a straight-ahead autorotation. In a turning autorotation, a
standard rate turn should be maintained by reference to the
needle of the turn-and-slip indicator.
Common Errors During Autorotations
1. Uncoordinated entry due to improper pedal trim
2. Poor airspeed control due to improper pitch attitude
3. Poor heading control in straight-ahead autorotations
4. Failure to maintain proper rotor RPM
5. Failure to maintain a standard rate turn during turning
autorotations
Servo Failure
Most helicopters certifi ed for single-pilot IFR fl ight are required
to have autopilots, which greatly reduces pilot workload. If an
autopilot servo fails, however, resume manual control of the
helicopter. The amount of workload increase depends on which
servo fails. If a cyclic servo fails, a pilot may want to land
immediately because the workload increases tremendously. If
an antitorque or collective servo fails, continuing to the next
suitable landing site might be possible.
Instrument Takeoff
The procedures and techniques described here should be
modifi ed as necessary to conform to those set forth in the
operating instructions for the particular helicopter being
flown. During training, instrument takeoffs should not
be attempted except when receiving instruction from an
appropriately certifi cated, profi cient fl ight instructor pilot.
Adjust the miniature aircraft in the attitude indicator, as
appropriate, for the aircraft being fl own. After the helicopter
is aligned with the runway or takeoff pad, to prevent forward
movement of a helicopter equipped with a wheel-type landing
gear, set the parking brakes or apply the toe brakes. If the
parking brake is used, it must be unlocked after the takeoff
has been completed. Apply suffi cient friction to the collective
pitch control to minimize overcontrolling and to prevent
creeping. Excessive friction should be avoided since it limits
collective pitch movement.
After checking all instruments for proper indications, start
the takeoff by applying collective pitch and a predetermined
power setting. Add power smoothly and steadily to gain
airspeed and altitude simultaneously and to prevent settling to
the ground. As power is applied and the helicopter becomes
airborne, use the antitorque pedals initially to maintain the
desired heading. At the same time, apply forward cyclic to
begin accelerating to climbing airspeed. During the initial
acceleration, the pitch attitude of the helicopter, as read on the
attitude indicator, should be one- to two-bar widths low. The
primary and supporting instruments after becoming airborne
are illustrated in Figure 6-16. As the airspeed increases to the
appropriate climb airspeed, adjust pitch gradually to climb
attitude. As climb airspeed is reached, reduce power to the
climb power setting and transition to a fully coordinated
straight climb.
During the initial climb out, minor heading corrections
should be made with pedals only until suffi cient airspeed is
attained to transition to fully coordinated fl ight. Throughout
the instrument takeoff, instrument cross-check and
interpretations must be rapid and accurate, and aircraft control
positive and smooth.
6-18
Figure 6-16. Flight Instrument Indications During an Instrument Takeoff.
Common Errors During Instrument Takeoffs
1. Failure to maintain heading
2. Overcontrolling pedals
3. Failure to use required power
4. Failure to adjust pitch attitude as climbing airspeed is
reached
Changing Technology
Advances in technology have brought about changes in
the instrumentation found in all types of aircraft, including
helicopters. Electronic displays commonly referred to as
“glass cockpits” are becoming more common. Primary fl ight
displays (PFDs) and multi-function displays (MFDs) are
changing not only what information is available to a pilot
but also how that information is displayed.
Illustrations of technological advancements in instrumentation
are described as follows. In Figure 6-17, a typical PFD
depicts an aircraft flying straight-and-level at 3,000
feet and 100 knots. Figure 6-18 illustrates a nose-low
pitch attitude in a right turn. MFDs can be confi gured to
provide navigation information such as the moving map in
Figure 6-19 or information pertaining to aircraft systems as
in Figure 6-20.
6-19
Figure 6-17. PFD Indications During Straight-and-Level Flight.
Figure 6-18. PFD Indications During a Nose-Low Pitch Attitude in a Right Turn.
6-20
Figure 6-19. MFD Display of a Moving Map.
Figure 6-20. MFD Display of Aircraft Systems.
7-1
Introduction
This chapter provides the basic radio principles applicable to
navigation equipment, as well as an operational knowledge
of how to use these systems in instrument flight. This
information provides the framework for all instrument
procedures, including standard instrument departure
procedures (SIDS), departure procedures (DPs), holding
patterns, and approaches, because each of these maneuvers
consists mainly of accurate attitude instrument fl ying and
accurate tracking using navigation systems.
Navigation
Systems
Chapter 7
7-2
Figure 7-1. Ground, Space, and Sky Wave Propogation.
Basic Radio Principles
A radio wave is an electromagnetic (EM) wave with
frequency characteristics that make it useful. The wave
will travel long distances through space (in or out of the
atmosphere) without losing too much strength. An antenna
is used to convert electric current into a radio wave so it can
travel through space to the receiving antenna, which converts
it back into an electric current for use by a receiver.
How Radio Waves Propagate
All matter has a varying degree of conductivity or resistance
to radio waves. The Earth itself acts as the greatest resistor
to radio waves. Radiated energy that travels near the ground
induces a voltage in the ground that subtracts energy from the
wave, decreasing the strength of the wave as the distance from
the antenna becomes greater. Trees, buildings, and mineral
deposits affect the strength to varying degrees. Radiated
energy in the upper atmosphere is likewise affected as the
energy of radiation is absorbed by molecules of air, water,
and dust. The characteristics of radio wave propagation vary
according to the signal frequency and the design, use, and
limitations of the equipment.
Ground Wave
A ground wave travels across the surface of the Earth. You
can best imagine a ground wave’s path as being in a tunnel
or alley bounded by the surface of the Earth and by the
ionosphere, which keeps the ground wave from going out
into space. Generally, the lower the frequency, the farther
the signal will travel.
Ground waves are usable for navigation purposes because
they travel reliably and predictably along the same route
day after day, and are not infl uenced by too many outside
factors. The ground wave frequency range is generally from
the lowest frequencies in the radio range (perhaps as low as
100 Hz) up to approximately 1,000 kHz (1 MHz). Although
there is a ground wave component to frequencies above this,
up to 30 MHz, the ground wave at these higher frequencies
loses strength over very short distances.
Sky Wave
The sky wave, at frequencies of 1 to 30 MHz, is good for
long distances because these frequencies are refracted or
“bent” by the ionosphere, causing the signal to be sent back
to Earth from high in the sky and received great distances
away. Used by high frequency (HF) radios in
aircraft, messages can be sent across oceans using only 50
to 100 watts of power. Frequencies that produce a sky wave
are not used for navigation because the pathway of the signal
from transmitter to receiver is highly variable. The wave is
“bounced” off of the ionosphere, which is always changing
due to the varying amount of the sun’s radiation reaching it
(night/day and seasonal variations, sunspot activity, etc.). The
sky wave is, therefore, unreliable for navigation purposes.
For aeronautical communication purposes, the sky wave
(HF) is about 80 to 90 percent reliable. HF is being gradually
replaced by more reliable satellite communication.
Space Wave
When able to pass through the ionosphere, radio waves
of 15 MHz and above (all the way up to many GHz), are
considered space waves. Most navigation systems operate
with signals propagating as space waves. Frequencies above
100 MHz have nearly no ground or sky wave components.
They are space waves, but (except for global positioning
system (GPS)) the navigation signal is used before it reaches
the ionosphere so the effect of the ionosphere, which can
cause some propagation errors, is minimal. GPS errors
caused by passage through the ionosphere are signifi cant
and are corrected for by the GPS receiver system.
Space waves have another characteristic of concern to users.
Space waves refl ect off hard objects and may be blocked if
the object is between the transmitter and the receiver. Site
and terrain error, as well as propeller/rotor modulation error
in very high omnidirectional range (VOR) systems is caused
by this bounce. Instrument landing system (ILS) course
distortion is also the result of this phenomenon, which led
to the need for establishment of ILS critical areas.
7-3
Figure 7-2. ADF Indicator Instrument and Receiver.
Generally, space waves are “line of sight” receivable, but
those of lower frequencies will “bend” somewhat over the
horizon. The VOR signal at 108 to 118 MHz is a lower
frequency than distance measuring equipment (DME) at 962
to 1213 MHz. Therefore, when an aircraft is fl own “over the
horizon” from a VOR/DME station, the DME will normally
be the fi rst to stop functioning.
Disturbances to Radio Wave Reception
Static distorts the radio wave and interferes with normal
reception of communications and navigation signals. Lowfrequency
airborne equipment such as automatic direction
fi nder (ADF) and LORAN are particularly subject to static
disturbance. Using very high frequency (VHF) and ultrahigh
frequency (UHF) frequencies avoids many of the
discharge noise effects. Static noise heard on navigation
or communication radio frequencies may be a warning of
interference with navigation instrument displays. Some of
the problems caused by precipitation static (P-static) are:
• Complete loss of VHF communications.
• Erroneous magnetic compass readings.
• Aircraft fl ying with one wing low while using the
autopilot.
• High-pitched squeal on audio.
• Motorboat sound on audio.
• Loss of all avionics.
• Inoperative very-low frequency (VLF) navigation
system.
• Erratic instrument readouts.
• Weak transmissions and poor radio reception.
• St. Elmo’s Fire.
Traditional Navigation Systems
Nondirectional Radio Beacon (NDB)
The nondirectional radio beacon (NDB) is a ground-based
radio transmitter that transmits radio energy in all directions.
The ADF, when used with an NDB, determines the bearing
from the aircraft to the transmitting station. The indicator
may be mounted in a separate instrument in the aircraft
panel. The ADF needle points to the NDB
ground station to determine the relative bearing (RB) to the
transmitting station. It is the number of degrees measured
clockwise between the aircraft’s heading and the direction
from which the bearing is taken. The aircraft’s magnetic
heading (MH) is the direction the aircraft is pointed with
respect to magnetic north. The magnetic bearing (MB) is the
direction to or from a radio transmitting station measured
relative to magnetic north.
NDB Components
The ground equipment, the NDB, transmits in the frequency
range of 190 to 535 kHz. Most ADFs will also tune the AM
broadcast band frequencies above the NDB band (550 to
1650 kHz). However, these frequencies are not approved
for navigation because stations do not continuously identify
themselves, and they are much more susceptible to sky
wave propagation especially from dusk to dawn. NDB
stations are capable of voice transmission and are often used
for transmitting the automated weather observing system
(AWOS). The aircraft must be in operational range of the
NDB. Coverage depends on the strength of the transmitting
station. Before relying on ADF indications, identify the
station by listening to the Morse code identifi er. NDB stations
are usually two letters or an alpha-numeric combination.
ADF Components
The airborne equipment includes two antennas, a receiver,
and the indicator instrument. The “sense” antenna (nondirectional)
receives signals with nearly equal effi ciency
from all directions. The “loop” antenna receives signals
better from two directions (bidirectional). When the loop
and sense antenna inputs are processed together in the ADF
radio, the result is the ability to receive a radio signal well in
all directions but one, thus resolving all directional ambiguity.
The indicator instrument can be one of four kinds: fi xedcard
ADF, rotatable compass-card ADF, or radio magnetic
7-4
Figure 7-3. Relative bearing (RB) on a fi xed-card indicator. Note
that the card always indicates 360°, or north. In this case, the
relative bearing to the station is 135° to the right. If the aircraft
were on a magnetic heading of 360°, then the magnetic bearing
(MB) would also be 135°.
Figure 7-4. Relative bearing (RB) on a movable-card indicator. By
placing the aircraft’s magnetic heading (MH) of 045° under the
top index, the relative bearing (RB) of 135° to the right will also
be the magnetic bearing (no wind conditions) which will take you
to the transmitting station.
indicator (RMI) with either one needle or dual needle. Fixedcard
ADF (also known as the relative bearing indicator (RBI))
always indicates zero at the top of the instrument, with the
needle indicating the RB to the station. Figure 7-3 indicates
an RB of 135°; if the MH is 045°, the MB to the station is
180°. (MH + RB = MB to the station.)
The movable-card ADF allows the pilot to rotate the
aircraft’s present heading to the top of the instrument so
that the head of the needle indicates MB to the station and
the tail indicates MB from the station. Figure 7-4 indicates
a heading of 045°, MB to the station of 180°, and MB from
the station of 360°.
The RMI differs from the movable-card ADF in that it
automatically rotates the azimuth card (remotely controlled
by a gyrocompass) to represent aircraft heading. The RMI
has two needles, which can be used to indicate navigation
information from either the ADF or the VOR receiver. When
a needle is being driven by the ADF, the head of the needle
indicates the MB TO the station tuned on the ADF receiver.
The tail of the needle is the bearing FROM the station. When
a needle of the RMI is driven by a VOR receiver, the needle
indicates where the aircraft is radially with respect to the
VOR station. The needle points to the bearing TO the station,
as read on the azimuth card. The tail of the needle points to
the radial of the VOR the aircraft is currently on or crossing.
Figure 7-5 indicates a heading of 005°, the MB to the station
is 015°, and the MB from the station is 195°.
Function of ADF
The ADF can be used to plot your position, track inbound
and outbound, and intercept a bearing. These procedures
are used to execute holding patterns and nonprecision
instrument approaches.
Orientation
The ADF needle points TO the station, regardless of aircraft
heading or position. The RB indicated is thus the angular
relationship between the aircraft heading and the station,
measured clockwise from the nose of the aircraft. Think of
the nose/tail and left/right needle indications, visualizing the
ADF dial in terms of the longitudinal axis of the aircraft.
When the needle points to 0°, the nose of the aircraft points
directly to the station; with the pointer on 210°, the station
is 30° to the left of the tail; with the pointer on 090°, the
station is off the right wingtip. The RB alone does not indicate
aircraft position. The RB must be related to aircraft heading
in order to determine direction to or from the station.
Station Passage
When you are near the station, slight deviations from
the desired track result in large defl ections of the needle.
Therefore, it is important to establish the correct drift
correction angle as soon as possible. Make small heading
corrections (not over 5°) as soon as the needle shows a
deviation from course, until it begins to rotate steadily toward
a wingtip position or shows erratic left/right oscillations. You
7-5
Figure 7-5. Radio magnetic indicator (RMI). Because the aircraft’s
magnetic heading is automatically changed, the relative bearing
(RB), in this case 095°, will indicate the magnetic bearing (095°)
to the station (no wind conditions) and the magnetic heading that
will take you there.
are abeam a station when the needle points 90° off your track.
Hold your last corrected heading constant and time station
passage when the needle shows either wingtip position or
settles at or near the 180° position. The time interval from
the fi rst indications of station proximity to positive station
passage varies with altitude—a few seconds at low levels to
3 minutes at high altitude.
Homing
The ADF may be used to “home” in on a station. Homing
is fl ying the aircraft on any heading required to keep the
needle pointing directly to the 0° RB position. To home in
on a station, tune the station, identify the Morse code signal,
and then turn the aircraft to bring the ADF azimuth needle to
the 0° RB position. Turns should be made using the heading
indicator. When the turn is complete, check the ADF needle
and make small corrections as necessary.
Figure 7-6 illustrates homing starting from an initial MH of
050° and an RB of 300°, indicating a 60° left turn is needed
to produce an RB of zero. Turn left, rolling out at 50° minus
60° equals 350°. Small heading corrections are then made
to zero the ADF needle.
If there is no wind, the aircraft will home to the station on a
direct track over the ground. With a crosswind, the aircraft
will follow a circuitous path to the station on the downwind
side of the direct track to the station.
Tracking
Tracking uses a heading that will maintain the desired track
to or from the station regardless of crosswind conditions.
Interpretation of the heading indicator and needle is done to
maintain a constant MB to or from the station.
To track inbound, turn to the heading that will produce a zero
RB. Maintain this heading until off-course drift is indicated
by displacement of the needle, which will occur if there is a
crosswind (needle moving left = wind from the left; needle
moving right = wind from the right). A rapid rate of bearing
change with a constant heading indicates either a strong
crosswind or close proximity to the station or both. When
there is a defi nite (2° to 5°) change in needle reading, turn
in the direction of needle defl ection to intercept the initial
MB. The angle of interception must be greater than the
number of degrees of drift, otherwise the aircraft will slowly
drift due to the wind pushing the aircraft. If repeated often
enough, the track to the station will appear circular and the
distance greatly increased as compared to a straight track.
The intercept angle depends on the rate of drift, the aircraft
speed, and station proximity. Initially, it is standard to double
the RB when turning toward your course.
For example, if your heading equals your course and the
needle points 10° left, turn 20° left, twice the initial RB.
This will be your intercept angle to capture the
RB. Hold this heading until the needle is defl ected 20° in
the opposite direction. That is, the defl ection of the needle
equals the interception angle (in this case 20°). The track has
been intercepted, and the aircraft will remain on track as long
as the RB remains the same number of degrees as the wind
correction angle (WCA), the angle between the desired track
and the heading of the aircraft necessary to keep the aircraft
tracking over the desired track. Lead the interception to avoid
overshooting the track. Turn 10° toward the inbound course.
You are now inbound with a 10° left correction angle.
NOTE: In Figure 7-7, for the aircraft closest to the station, the
WCA is 10° left and the RB is 10° right. If those values do
not change, the aircraft will track directly to the station. If you
observe off-course defl ection in the original direction, turn
again to the original interception heading. When the desired
course has been re-intercepted, turn 5° toward the inbound
course, proceeding inbound with a 15° drift correction. If the
initial 10° drift correction is excessive, as shown by needle
defl ection away from the wind, turn to parallel the desired
course and let the wind drift you back on course. When the
needle is again zeroed, turn into the wind with a reduced
drift correction angle.
7-6
Figure 7-6. ADF Homing With a Crosswind.
7-7
Figure 7-7. ADF Tracking Inbound.
7-8
Operational Errors of ADF
Some of the common pilot-induced errors associated with
ADF navigation are listed below to help you avoid making
the same mistakes. The errors are:
1. Improper tuning and station identifi cation. Many pilots
have made the mistake of homing or tracking to the
wrong station.
2. Positively identifying any malfunctions of the RMI
slaving system or ignoring the warning fl ag.
3. Dependence on homing rather than proper tracking.
This commonly results from sole reliance on the ADF
indications, rather than correlating them with heading
indications.
4. Poor orientation, due to failure to follow proper steps
in orientation and tracking.
5. Careless interception angles, very likely to happen if
you rush the initial orientation procedure.
6. Overshooting and undershooting predetermined MBs,
often due to forgetting the course interception angles
used.
7. Failure to maintain selected headings. Any heading
change is accompanied by an ADF needle change.
The instruments must be read in combination before
any interpretation is made.
8. Failure to understand the limitations of the ADF and
the factors that affect its use.
9. Overcontrolling track corrections close to the station
(chasing the ADF needle), due to failure to understand
or recognize station approach.
10. Failure to keep the heading indicator set so it agrees
with the magnetic compass.
Very High Frequency Omnidirectional Range
(VOR)
VOR is the primary navigational aid (NAVAID) used by civil
aviation in the National Airspace System (NAS). The VOR
ground station is oriented to magnetic north and transmits
azimuth information to the aircraft, providing 360 courses
TO or FROM the VOR station. When DME is installed with
the VOR, it is referred to as a VOR/DME and provides both
azimuth and distance information. When military tactical air
navigation (TACAN) equipment is installed with the VOR,
it is known as a VORTAC and provides both azimuth and
distance information.
To track outbound, the same principles apply: needle moving
left = wind from the left, needle moving right = wind from the
right. Wind correction is made toward the needle defl ection.
The only exception is while the turn to establish the WCA is
being made, the direction of the azimuth needle defl ections is
reversed. When tracking inbound, needle defl ection decreases
while turning to establish the WCA, and needle defl ection
increases when tracking outbound. Note the example of
course interception and outbound tracking in Figure 7-8.
Intercepting Bearings
ADF orientation and tracking procedures may be applied to
intercept a specifi ed inbound or outbound MB. To intercept
an inbound bearing of 355°, the following steps may be used.
1. Determine your position in relation to the station by
paralleling the desired inbound bearing. In this case,
turn to a heading of 355°. Note that the station is to
the right front of the aircraft.
2. Determine the number of degrees of needle defl ection
from the nose of the aircraft. In this case, the needle’s
RB from the aircraft’s nose is 40° to the right. A rule
of thumb for interception is to double this RB amount
as an interception angle (80°).
3. Turn the aircraft toward the desired MB the number of
degrees determined for the interception angle which
as indicated (in two above) is twice the initial RB
(40°), or in this case 80°. Therefore, the right turn will
be 80° from the initial MB of 355°, or a turn to 075°
magnetic (355° + 80° + 075°).
4. Maintain this interception heading of 075° until the
needle is defl ected the same number of degrees “left”
from the zero position as the angle of interception
080°, (minus any lead appropriate for the rate at which
the bearing is changing).
5. Turn left 80° and the RB (in a no wind condition and
with proper compensation for the rate of the ADF
needle movement) should be 0°, or directly off the
nose. Additionally, the MB should be 355° indicating
proper interception of the desired course.
NOTE: The rate of an ADF needle movement or any bearing
pointer for that matter will be faster as aircraft position
becomes closer to the station or waypoint (WP).
Interception of an outbound MB can be accomplished by the
same procedures as for the inbound intercept, except that
it is necessary to substitute the 180° position for the zero
position on the needle.
7-9
Figure 7-8. ADF Interception and Tracking Outbound.
7-10
Figure 7-10. VOR Radials.
Figure 7-9. Interception of Bearing.
the aircraft altitude, class of facility, location of the facility,
terrain conditions within the usable area of the facility, and
other factors. Above and beyond certain altitude and distance
limits, signal interference from other VOR facilities and a
weak signal make it unreliable. Coverage is typically at least
40 miles at normal minimum instrument fl ight rules (IFR)
altitudes. VORs with accuracy problems in parts of their
service volume are listed in Notices to Airmen (NOTAMs)
and in the Airport/Facility Directory (A/FD) under the name
of the NAVAID.
VOR Components
The ground equipment consists of a VOR ground station,
which is a small, low building topped with a fl at white disc,
upon which are located the VOR antennas and a fi berglass
cone-shaped tower. The station includes an
automatic monitoring system. The monitor automatically
turns off defective equipment and turns on the standby
transmitter. Generally, the accuracy of the signal from the
ground station is within 1°.
VOR facilities are aurally identifi ed by Morse code, or
voice, or both. The VOR can be used for ground-to-air
communication without interference with the navigation
signal. VOR facilities operate within the 108.0 to 117.95 MHz
frequency band and assignment between 108.0 and 112.0
The courses oriented FROM the station are called radials. The
VOR information received by an aircraft is not infl uenced
by aircraft attitude or heading. Radials can
be envisioned to be like the spokes of a wheel on which the
aircraft is on one specifi c radial at any time. For example,
aircraft A (heading 180°) is inbound on the 360° radial; after
crossing the station, the aircraft is outbound on the 180°
radial at A1. Aircraft B is shown crossing the 225° radial.
Similarly, at any point around the station, an aircraft can be
located somewhere on a specifi c VOR radial. Additionally,
a VOR needle on an RMI will always point to the course
that will take you to the VOR station where conversely the
ADF needle points to the station as a RB from the aircraft. In
the example above, the ADF needle at position A would be
pointed straight ahead, at A1 to the aircraft’s 180° position
(tail) and at B, to the aircraft’s right.
The VOR receiver measures and presents information to
indicate bearing TO or FROM the station. In addition to the
navigation signals transmitted by the VOR, a Morse code
signal is transmitted concurrently to identify the facility, as
well as voice transmissions for communication and relay of
weather and other information.
VORs are classifi ed according to their operational uses. The
standard VOR facility has a power output of approximately
200 watts, with a maximum usable range depending upon
7-11
Figure 7-12. The VOR Indicator Instrument.
Figure 7-11. VOR Transmitter (Ground Station).
Figure 7-13. A Typical Horizontal Situation Indicator (HSI).
MHz is in even-tenth increments to preclude any confl ict
with ILS localizer frequency assignment, which uses the
odd tenths in this range.
The airborne equipment includes an antenna, a receiver, and
the indicator instrument. The receiver has a frequency knob to
select any of the frequencies between 108.0 to 117.95 MHz.
The On/Off/volume control turns on the navigation receiver
and controls the audio volume. The volume has no effect on
the operation of the receiver. You should listen to the station
identifi er before relying on the instrument for navigation.
VOR indicator instruments have at least the essential
components shown in the instrument illustrated in
Figure 7-12.
Omnibearing Selector (OBS)
The desired course is selected by turning the OBS knob until
the course is aligned with the course index mark or displayed
in the course window.
Course Deviation Indicator (CDI)
The deviation indicator is composed of an instrument face
and a needle hinged to move laterally across the instrument
face. The needle centers when the aircraft is on the selected
radial or its reciprocal. Full needle defl ection from the center
position to either side of the dial indicates the aircraft is 12°
or more off course, assuming normal needle sensitivity. The
outer edge of the center circle is 2° off course; with each dot
representing an additional 2°.
TO/FROM Indicator
The TO/FROM indicator shows whether the selected course
will take the aircraft TO or FROM the station. It does not
indicate whether the aircraft is heading to or from the
station.
Flags or Other Signal Strength Indicators
The device that indicates a usable or an unreliable signal may
be an “OFF” fl ag. It retracts from view when signal strength
is suffi cient for reliable instrument indications. Alternately,
insuffi cient signal strength may be indicated by a blank or
OFF in the TO/FROM window.
The indicator instrument may also be a horizontal situation
indicator (HSI) which combines the heading indicator
and CDI. The combination of navigation
information from VOR/Localizer (LOC) or from LORAN
or GPS, with aircraft heading information provides a visual
picture of the aircraft’s location and direction. This decreases
pilot workload especially with tasks such as course intercepts,
fl ying a back-course approach, or holding pattern entry. (See
7-12
Figure 7-14. An HSI display as seen on the pilot’s primary fl ight display (PFD) on an electronic fl ight instrument. Note that only attributes
related to the HSI are labeled.
Chapter 3, Flight Instruments, for operational characteristics.)
Function of VOR
Orientation
The VOR does not account for the aircraft heading. It only
relays the aircraft direction from the station and will have the
same indications regardless of which way the nose is pointing.
Tune the VOR receiver to the appropriate frequency of the
selected VOR ground station, turn up the audio volume, and
identify the station’s signal audibly. Then, rotate the OBS
to center the CDI needle and read the course under or over
the index.
In Figure 7-12, 360° TO is the course indicated, while in
Figure 7-15, 180° TO is the course. The latter indicates that
the aircraft (which may be heading in any direction) is, at this
moment, located at any point on the 360° radial (line from the
station) except directly over the station or very close to it, as
between points I and S in Figure 7-15. The CDI will deviate
from side to side as the aircraft passes over or nearly over
the station because of the volume of space above the station
where the zone of confusion exists. This zone of confusion is
caused by lack of adequate signal directly above the station
due to the radiation pattern of the station’s antenna, and
because the resultant of the opposing reference and variable
signals is small and constantly changing.
The CDI in Figure 7-15 indicates 180°, meaning that the
aircraft is on the 180° or the 360° radial of the station. The TO/
FROM indicator resolves the ambiguity. If the TO indicator is
showing, then it is 180° TO the station. The FROM indication
indicates the radial of the station the aircraft is presently on.
Movement of the CDI from center, if it occurs at a relatively
constant rate, indicates the aircraft is moving or drifting off the
180°/360° line. If the movement is rapid or fl uctuating, this
is an indication of impending station passage (the aircraft is
near the station). To determine the aircraft’s position relative
to the station, rotate the OBS until FROM appears in the
window, and then center the CDI needle. The index indicates
the VOR radial where the aircraft is located. The inbound (to
the station) course is the reciprocal of the radial.
If the VOR is set to the reciprocal of the intended course,
the CDI will refl ect reverse sensing. To correct for needle
defl ection, turn away from the needle. To avoid this reverse
sensing situation, set the VOR to agree with the intended
course.
7-13
Figure 7-15. CDI Interpretation. The CDI as typically found on analog systems (right) and as found on electronic fl ight instruments
(left).
7-14
A single NAVAID will allow a pilot to determine the aircraft’s
position relative to a radial. Indications from a second
NAVAID are needed in order to narrow the aircraft’s position
down to an exact location on this radial.
Tracking TO and FROM the Station
To track to the station, rotate the OBS until TO appears, then
center the CDI. Fly the course indicated by the index. If the CDI
moves off center to the left, follow the needle by correcting
course to the left, beginning with a 20° correction.
When flying the course indicated on the index, a left
defl ection of the needle indicates a crosswind component
from the left. If the amount of correction brings the needle
back to center, decrease the left course correction by half. If
the CDI moves left or right now, it should do so much more
slowly, and smaller heading corrections can be made for the
next iteration.
Keeping the CDI centered will take the aircraft to the station.
To track to the station, the OBS value at the index is not
changed. To home to the station, the CDI needle is periodically
centered, and the new course under the index is used for the
aircraft heading. Homing will follow a circuitous route to the
station, just as with ADF homing.
To track FROM the station on a VOR radial, you should
fi rst orient the aircraft’s location with respect to the station
and the desired outbound track by centering the CDI needle
with a FROM indication. The track is intercepted by either
fl ying over the station or establishing an intercept heading.
The magnetic course of the desired radial is entered under the
index using the OBS and the intercept heading held until the
CDI centers. Then the procedure for tracking to the station is
used to fl y outbound on the specifi ed radial.
Course Interception
If the desired course is not the one being fl own, fi rst orient
the aircraft’s position with respect to the VOR station and the
course to be fl own, and then establish an intercept heading.
The following steps may be used to intercept a predetermined
course, either inbound or outbound. Steps 1–3 may be omitted
when turning directly to intercept the course without initially
turning to parallel the desired course.
1. Turn to a heading to parallel the desired course, in the
same direction as the course to be fl own.
2. Determine the difference between the radial to be
intercepted and the radial on which the aircraft is
located (205° – 160° = 045°).
3. Double the difference to determine the interception
angle, which will not be less than 20° nor greater
than 90° (45° x 2 = 090°). 205° + 090° = 295° for the
intercept)
4. Rotate the OBS to the desired radial or inbound
course.
5. Turn to the interception heading.
6. Hold this heading constant until the CDI center, which
indicates the aircraft is on course. (With practice in
judging the varying rates of closure with the course
centerline, pilots learn to lead the turn to prevent
overshooting the course.)
7. Turn to the MH corresponding to the selected
course, and follow tracking procedures inbound or
outbound.
Course interception is illustrated in Figure 7-16.
VOR Operational Errors
Typical pilot-induced errors include:
1. Careless tuning and identifi cation of station.
2. Failure to check receiver for accuracy/sensitivity.
3. Turning in the wrong direction during an orientation.
This error is common until visualizing position rather
than heading.
4. Failure to check the ambiguity (TO/FROM) indicator,
particularly during course reversals, resulting
in reverse sensing and corrections in the wrong
direction.
5. Failure to parallel the desired radial on a track
interception problem. Without this step, orientation
to the desired radial can be confusing. Since pilots
think in terms of left and right of course, aligning the
aircraft position to the radial/course is essential.
6. Overshooting and undershooting radials on interception
problems.
7. Overcontrolling corrections during tracking, especially
close to the station.
8. Misinterpretation of station passage. On VOR
receivers not equipped with an ON/OFF flag, a
voice transmission on the combined communication
and navigation radio (NAV/COM) in use for VOR
may cause the same TO/FROM fl uctuations on the
ambiguity meter as shown during station passage.
Read the whole receiver—TO/FROM, CDI, and
OBS—before you make a decision. Do not utilize a
VOR reading observed while transmitting.
9. Chasing the CDI, resulting in homing instead of
tracking. Careless heading control and failure to
bracket wind corrections make this error common.
7-15
Figure 7-16. Course Interception (VOR).
7-16
VOR Accuracy
The effectiveness of the VOR depends upon proper use and
adjustment of both ground and airborne equipment.
The accuracy of course alignment of the VOR is generally
plus or minus 1°. On some VORs, minor course roughness
may be observed, evidenced by course needle or brief fl ag
alarm. At a few stations, usually in mountainous terrain,
the pilot may occasionally observe a brief course needle
oscillation, similar to the indication of “approaching station.”
Pilots fl ying over unfamiliar routes are cautioned to be on
the alert for these vagaries, and in particular, to use the TO/
FROM indicator to determine positive station passage.
Certain propeller revolutions per minute (RPM) settings
or helicopter rotor speeds can cause the VOR CDI to
fl uctuate as much as plus or minus 6°. Slight changes to
the RPM setting will normally smooth out this roughness.
Pilots are urged to check for this modulation phenomenon
prior to reporting a VOR station or aircraft equipment for
unsatisfactory operation.
VOR Receiver Accuracy Check
VOR system course sensitivity may be checked by noting
the number of degrees of change as the OBS is rotated to
move the CDI from center to the last dot on either side. The
course selected should not exceed 10° or 12° either side. In
addition, Title 14 of the Code of Federal Regulations (14
CFR) part 91 provides for certain VOR equipment accuracy
checks, and an appropriate endorsement, within 30 days prior
to fl ight under IFR. To comply with this requirement and to
ensure satisfactory operation of the airborne system, use the
following means for checking VOR receiver accuracy:
1. VOR test facility (VOT) or a radiated test signal from
an appropriately rated radio repair station.
2. Certifi ed checkpoints on the airport surface.
3. Certifi ed airborne checkpoints.
VOR Test Facility (VOT)
The Federal Aviation Administration (FAA) VOT transmits
a test signal which provides users a convenient means to
determine the operational status and accuracy of a VOR
receiver while on the ground where a VOT is located.
Locations of VOTs are published in the A/FD. Two means of
identifi cation are used. One is a series of dots and the other is
a continuous tone. Information concerning an individual test
signal can be obtained from the local fl ight service station
(FSS.) The airborne use of VOT is permitted; however, its
use is strictly limited to those areas/altitudes specifi cally
authorized in the A/FD or appropriate supplement.
To use the VOT service, tune in the VOT frequency 108.0
MHz on the VOR receiver. With the CDI centered, the
OBS should read 0° with the TO/FROM indication showing
FROM or the OBS should read 180° with the TO/FROM
indication showing TO. Should the VOR receiver operate an
RMI, it would indicate 180° on any OBS setting.
A radiated VOT from an appropriately rated radio repair
station serves the same purpose as an FAA VOT signal, and
the check is made in much the same manner as a VOT with
some differences.
The frequency normally approved by the Federal
Communications Commission (FCC) is 108.0 MHz;
however, repair stations are not permitted to radiate the
VOR test signal continuously. The owner or operator of the
aircraft must make arrangements with the repair station to
have the test signal transmitted. A representative of the repair
station must make an entry into the aircraft logbook or other
permanent record certifying to the radial accuracy and the
date of transmission.
Certifi ed Checkpoints
Airborne and ground checkpoints consist of certifi ed radials
that should be received at specifi c points on the airport surface
or over specifi c landmarks while airborne in the immediate
vicinity of the airport. Locations of these checkpoints are
published in the A/FD.
Should an error in excess of ±4° be indicated through use of
a ground check, or ±6° using the airborne check, IFR fl ight
shall not be attempted without fi rst correcting the source of
the error. No correction other than the correction card fi gures
supplied by the manufacturer should be applied in making
these VOR receiver checks.
If a dual system VOR (units independent of each other except
for the antenna) is installed in the aircraft, one system may
be checked against the other. Turn both systems to the same
VOR ground facility and note the indicated bearing to that
station. The maximum permissible variation between the two
indicated bearings is 4°.
Distance Measuring Equipment (DME)
When used in conjunction with the VOR system, DME makes
it possible for pilots to determine an accurate geographic
position of the aircraft, including the bearing and distance
TO or FROM the station. The aircraft DME transmits
interrogating radio frequency (RF) pulses, which are received
by the DME antenna at the ground facility. The signal triggers
ground receiver equipment to respond to the interrogating
7-17
Figure 7-17. DME Arc Interception.
aircraft. The airborne DME equipment measures the elapsed
time between the interrogation signal sent by the aircraft and
reception of the reply pulses from the ground station. This
time measurement is converted into distance in nautical miles
(NM) from the station.
Some DME receivers provide a groundspeed in knots by
monitoring the rate of change of the aircraft’s position relative
to the ground station. Groundspeed values are accurate only
when tracking directly to or from the station.
DME Components
VOR/DME, VORTAC, ILS/DME, and LOC/DME
navigation facilities established by the FAA provide course
and distance information from collocated components under
a frequency pairing plan. DME operates on frequencies
in the UHF spectrum between 962 MHz and 1213 MHz.
Aircraft receiving equipment which provides for automatic
DME selection assures reception of azimuth and distance
information from a common source when designated VOR/
DME, VORTAC, ILS/DME, and LOC/DME are selected.
Some aircraft have separate VOR and DME receivers, each
of which must be tuned to the appropriate navigation facility.
The airborne equipment includes an antenna and a receiver.
The pilot-controllable features of the DME receiver
include:
Channel (Frequency) Selector
Many DMEs are channeled by an associated VHF radio, or
there may be a selector switch so a pilot can select which
VHF radio is channeling the DME. For a DME with its
own frequency selector, use the frequency of the associated
VOR/DME or VORTAC station.
On/Off/Volume Switch
The DME identifi er will be heard as a Morse code identifi er
with a tone somewhat higher than that of the associated VOR
or LOC. It will be heard once for every three or four times
the VOR or LOC identifi er is heard. If only one identifi er is
heard about every 30 seconds, the DME is functional, but
the associated VOR or LOC is not.
Mode Switch
The mode switch selects between distance (DIST) or distance
in NMs, groundspeed, and time to station. There may also
be one or more HOLD functions which permit the DME to
stay channeled to the station that was selected before the
switch was placed in the hold position. This is useful when
you make an ILS approach at a facility that has no collocated
DME, but there is a VOR/DME nearby.
Altitude
Some DMEs correct for slant-range error.
Function of DME
A DME is used for determining the distance from a ground
DME transmitter. Compared to other VHF/UHF NAVAIDs,
a DME is very accurate. The distance information can be
used to determine the aircraft position or fl ying a track that
is a constant distance from the station. This is referred to as
a DME arc.
DME Arc
There are many instrument approach procedures (IAPs) that
incorporate DME arcs. The procedures and techniques given
here for intercepting and maintaining such arcs are applicable
to any facility that provides DME information. Such a facility
may or may not be collocated with the facility that provides
fi nal approach guidance.
As an example of fl ying a DME arc, refer to Figure 7-17 and
follow these steps:
1. Track inbound on the OKT 325° radial, frequently
checking the DME mileage readout.
2. A 0.5 NM lead is satisfactory for groundspeeds of 150
knots or less; start the turn to the arc at 10.5 miles. At
higher groundspeeds, use a proportionately greater
lead.
7-18
Figure 7-18. Using DME and RMI To Maintain an Arc.
3. Continue the turn for approximately 90°. The roll-out
heading will be 055° in a no wind condition.
4. During the last part of the intercepting turn, monitor
the DME closely. If the arc is being overshot (more
than 1.0 NM), continue through the originally planned
roll-out heading. If the arc is being undershot, roll-out
of the turn early.
The procedure for intercepting the 10 DME when outbound
is basically the same, the lead point being 10 NM minus 0.5
NM, or 9.5 NM.
When fl ying a DME arc with wind, it is important to keep a
continuous mental picture of the aircraft’s position relative to
the facility. Since the wind-drift correction angle is constantly
changing throughout the arc, wind orientation is important.
In some cases, wind can be used in returning to the desired
track. High airspeeds require more pilot attention because of
the higher rate of deviation and correction.
Maintaining the arc is simplifi ed by keeping slightly inside
the curve; thus, the arc is turning toward the aircraft and
interception may be accomplished by holding a straight
course. When outside the curve, the arc is “turning away”
and a greater correction is required.
To fl y the arc using the VOR CDI, center the CDI needle
upon completion of the 90° turn to intercept the arc. The
aircraft’s heading will be found very near the left or right
side (270° or 90° reference points) of the instrument. The
readings at that side location on the instrument will give
primary heading information while on the arc. Adjust the
aircraft heading to compensate for wind and to correct for
7-19
distance to maintain the correct arc distance. Recenter the
CDI and note the new primary heading indicated whenever
the CDI gets 2°–4° from center.
With an RMI, in a no wind condition, pilots should
theoretically be able to fl y an exact circle around the facility
by maintaining an RB of 90° or 270°. In actual practice,
a series of short legs are fl own. To maintain the arc in
Figure 7-18, proceed as follows:
1. With the RMI bearing pointer on the wingtip reference
(90° or 270° position) and the aircraft at the desired
DME range, maintain a constant heading and allow the
bearing pointer to move 5°–10° behind the wingtip.
This will cause the range to increase slightly.
2. Turn toward the facility to place the bearing pointer
5–10° ahead of the wingtip reference, and then
maintain heading until the bearing pointer is again
behind the wingtip. Continue this procedure to
maintain the approximate arc.
3. If a crosswind causes the aircraft to drift away from
the facility, turn the aircraft until the bearing pointer is
ahead of the wingtip reference. If a crosswind causes
the aircraft to drift toward the facility, turn until the
bearing is behind the wingtip.
4. As a guide in making range corrections, change the RB
10°–20° for each half-mile deviation from the desired
arc. For example, in no-wind conditions, if the aircraft
is 1/2 to 1 mile outside the arc and the bearing pointer
is on the wingtip reference, turn the aircraft 20° toward
the facility to return to the arc.
Without an RMI, orientation is more diffi cult since there is
no direct azimuth reference. However, the procedure can be
fl own using the OBS and CDI for azimuth information and
the DME for arc distance.
Intercepting Lead Radials
A lead radial is the radial at which the turn from the arc to the
inbound course is started. When intercepting a radial from
a DME arc, the lead will vary with arc radius and ground
speed. For the average general aviation aircraft, fl ying arcs
such as those depicted on most approach charts at speeds
of 150 knots or less, the lead will be under 5°. There is no
difference between intercepting a radial from an arc and
intercepting it from a straight course.
With an RMI, the rate of bearing movement should be
monitored closely while fl ying the arc. Set the course of the
radial to be intercepted as soon as possible and determine
the approximate lead. Upon reaching this point, start the
intercepting turn. Without an RMI, the technique for radial
interception is the same except for azimuth information,
which is available only from the OBS and CDI.
The technique for intercepting a localizer from a DME arc
is similar to intercepting a radial. At the depicted lead radial
(LR 070° or LR 084° in Figures 7-19, 7-20, and 7-21), a
pilot having a single VOR/LOC receiver should set it to the
localizer frequency. If the pilot has dual VOR/LOC receivers,
one unit may be used to provide azimuth information and the
other set to the localizer frequency. Since these lead radials
provide 7° of lead, a half-standard rate turn should be used
until the LOC needle starts to move toward center.
DME Errors
A DME/DME fi x (a location based on two DME lines of
position from two DME stations) provides a more accurate
aircraft location than using a VOR and a DME fi x.
DME signals are line-of-sight; the mileage readout is the
straight line distance from the aircraft to the DME ground
facility and is commonly referred to as slant range distance.
Slant range refers to the distance from the aircraft’s antenna
to the ground station (A line at an angle to the ground
transmitter. GPS systems provide distance as the horizontal
measurement from the WP to the aircraft. Therefore, at 3,000
feet and 0.5 miles the DME (slant range) would read 0.6 NM
while the GPS distance would show the actual horizontal
distance of .5 DME. This error is smallest at low altitudes
and/or at long ranges. It is greatest when the aircraft is closer
to the facility, at which time the DME receiver will display
altitude (in NM) above the facility. Slant range error is
negligible if the aircraft is one mile or more from the ground
facility for each 1,000 feet of altitude above the elevation of
the facility.
Area Navigation (RNAV)
Area navigation (RNAV) equipment includes VOR/DME,
LORAN, GPS, and inertial navigation systems (INS). RNAV
equipment is capable of computing the aircraft position,
actual track, groundspeed, and then presenting meaningful
information to the pilot. This information may be in the form
of distance, cross-track error, and time estimates relative to
the selected track or WP. In addition, the RNAV equipment
installations must be approved for use under IFR. The Pilot’s
Operating Handbook/Airplane Flight Manual (POH/AFM)
should always be consulted to determine what equipment is
installed, the operations that are approved, and the details of
equipment use. Some aircraft may have equipment that allows
input from more than one RNAV source, thereby providing
a very accurate and reliable navigation source.
7-20
Figure 7-19. An aircraft is displayed heading southwest to intercept the localizer approach, using the 16 NM DME Arc off of ORM.
7-21
Figure 7-20. The same aircraft illustrated in Figure 7-19 shown on the ORM radial near TIGAE intersection turning inbound for the
localizer.
7-22
Figure 7-21. Aircraft is illustrated inbound on the localizer
course.
Figure 7-22. RNAV Computation.
7-23
Figure 7-23. Onboard RNAV receivers have changed signifi cantly.
Originally, RNAV receivers typically computed combined data
from VOR, VORTAC, and/or DME. That is generally not the case
now. Today, GPS such as the GNC 300 and the Bendix King KLS
88 LORAN receivers compute waypoints based upon embedded
databases and aircraft positional information.
VOR/DME RNAV
VOR RNAV is based on information generated by the present
VORTAC or VOR/DME system to create a WP using an
airborne computer. As shown in Figure 7-22, the value of
side A is the measured DME distance to the VOR/DME. Side
B, the distance from the VOR/DME to the WP, and angle 1
(VOR radial or the bearing from the VORTAC to the WP)
are values set in the fl ight deck control. The bearing from
the VOR/DME to the aircraft, angle 2, is measured by the
VOR receiver. The airborne computer continuously compares
angles 1 and 2 and determines angle 3 and side C, which is
the distance in NMs and magnetic course from the aircraft
to the WP. This is presented as guidance information on the
fl ight deck display.
VOR/DME RNAV Components
Although RNAV fl ight deck instrument displays vary among
manufacturers, most are connected to the aircraft CDI with a
switch or knob to select VOR or RNAV guidance. There is
usually a light or indicator to inform the pilot whether VOR
or RNAV is selected. The display includes the
WP, frequency, mode in use, WP radial and distance, DME
distance, groundspeed, and time to station.
Most VOR/DME RNAV systems have the following airborne
controls:
1. Off/On/Volume control to select the frequency of the
VOR/DME station to be used.
2. MODE select switch used to select VOR/DME mode,
with:
a. Angular course width deviation (standard VOR
operation); or
b. Linear cross-track deviation as standard (±5 NM
full scale CDI).
3. RNAV mode, with direct to WP with linear cross-track
deviation of ±5 NM.
4. RNAV/APPR (approach mode) with linear deviation
of ±1.25 NM as full scale CDI defl ection.
5. WP select control. Some units allow the storage of more
than one WP; this control allows selection of any WP
in storage.
6. Data input controls. These controls allow user input
of WP number or ident, VOR or LOC frequency, WP
radial and distance.
While DME groundspeed readout is accurate only when
tracking directly to or from the station in VOR/DME mode,
in RNAV mode the DME groundspeed readout is accurate on
any track.
Function of VOR/DME RNAV
The advantages of the VOR/DME RNAV system stem from
the ability of the airborne computer to locate a WP wherever it
is convenient, as long as the aircraft is within reception range
of both nearby VOR and DME facilities. A series of these
WPs make up an RNAV route. In addition to the published
routes, a random RNAV route may be fl own under IFR if it is
approved by air traffi c control (ATC). RNAV DPs and standard
terminal arrival routes (STARs) are contained in the DP and
STAR booklets.
VOR/DME RNAV approach procedure charts are also
available. Note in the VOR/DME RNAV chart excerpt shown
in Figure 7-24 that the WP identifi cation boxes contain the
following information: WP name, coordinates, frequency,
identifi er, radial distance (facility to WP), and reference facility
elevation. The initial approach fi x (IAF), fi nal approach fi x
(FAF), and missed approach point (MAP) are labeled.
To fl y a route or to execute an approach under IFR, the RNAV
equipment installed in the aircraft must be approved for the
appropriate IFR operations.
In vertical navigation (VNAV) mode, vertical guidance is
provided, as well as horizontal guidance in some installations. A
WP is selected at a point where the descent begins, and another
WP is selected where the descent ends. The RNAV equipment
computes the rate of descent relative to the groundspeed; on
some installations, it displays vertical guidance information
on the GS indicator. When using this type of equipment
during an instrument approach, the pilot must keep in mind
that the vertical guidance information provided is not part of
the nonprecision approach. Published nonprecision approach
altitudes must be observed and complied with, unless otherwise
directed by ATC.
7-24
Figure 7-25. Aircraft/DME/Waypoint Relationship.
Figure 7-24. VOR/DME RNAV Rwy 25 Approach (Excerpt).
To fl y to a WP using RNAV, observe the following procedure
:
1. Select the VOR/DME frequency.
2. Select the RNAV mode.
3. Select the radial of the VOR that passes through the WP
(225°).
4. Select the distance from the DME to the WP (12
NM).
5. Check and confi rm all inputs, and center the CDI needle
with the TO indicator showing.
6. Maneuver the aircraft to fl y the indicated heading
plus or minus wind correction to keep the CDI needle
centered.
7. The CDI needle will indicate distance off course of 1
NM per dot; the DME readout will indicate distance in
NM from the WP; the groundspeed will read closing
speed (knots) to the WP; and the time to station (TTS)
will read time to the WP.
VOR/DME RNAV Errors
The limitation of this system is the reception volume.
Published approaches have been tested to ensure this is not
a problem. Descents/approaches to airports distant from the
VOR/DME facility may not be possible because, during
the approach, the aircraft may descend below the reception
altitude of the facility at that distance.
Long Range Navigation (LORAN)
LORAN uses a network of land-based transmitters to provide
an accurate long-range navigation system. The FAA and the
United States Coast Guard (USCG) arranged the stations
into chains. The signal from station is a carefully structured
sequence of brief RF pulses centered at 100 kHz. At that
frequency, signals travel considerable distances as ground
waves, from which accurate navigation information is
available. The airborne receiver monitors all of the stations
within the selected chain, then measures the arrival time
difference (TD) between the signals. All of the points having
the same TD from a station pair create a line of position
7-25
Figure 7-26. A control panel from a military aircraft after LORAN
was fi rst put into use. The receiver is remotely mounted and weighs
over 25 pounds. Its size is about six times that of the LORAN fully
integrated receiver.
(LOP). The aircraft position is determined at the intersection
of two or more LOPs. Then the computer converts the
known location to latitude and longitude coordinates.
While continually computing latitude/longitude fi xes, the
computer is able to determine and display:
1. Track over the ground since last computation;
2. Groundspeed by dividing distance covered since last
computation by the time since last computation (and
averaging several of these);
3. Distance to destination;
4. Destination time of arrival; and
5. Cross-track error.
The Aeronautical Information Manual (AIM) provides a
detailed explanation of how LORAN works. LORAN is
a very accurate navigation system if adequate signals are
received. There are two types of accuracy that must be
addressed in any discussion of LORAN accuracy.
Repeatable accuracy is the accuracy measured when a user
notes the LORAN position, moves away from that location,
then uses the LORAN to return to that initial LORAN
position. Distance from that initial position is the error.
Propagation and terrain errors will be essentially the same as
when the fi rst position was taken, so those errors are factored
out by using the initial position. Typical repeatable accuracy
for LORAN can be as good as 0.01 NM, or 60 feet, if the
second position is determined during the day and within a
short period of time (a few days).
Absolute accuracy refers to the ability to determine present
position in space independently, and is most often used by
pilots. When the LORAN receiver is turned on and position
is determined, absolute accuracy applies. Typical LORAN
absolute accuracy will vary from about 0.1 NM to as much
as 2.5 NM depending on distance from the station, geometry
of the TD LOP crossing angles, terrain and environmental
conditions, signal-to-noise ratio (signal strength), and some
design choices made by the receiver manufacturer.
Although LORAN use diminished with the introduction
of Global Navigation Satellite Systems such as the United
States’ GPS, its use has since increased. Three items aided
in this resurgence:
• In 1996, a commission called the Gore Commission
evaluated GPS’ long-term use as a sole navigation
aid. Although GPS was hailed originally as the
eventual sole NAVAID, which would replace the need
for most currently existing NAVAIDs by the year
2020, the Commission questioned single-link failure
potential and its effect on the NAS. For this reason,
the forecasted decommissioning of the VOR has been
amended and their expectant lifecycle extended into
the future. Additionally, the use of LORAN continues
to be evaluated for facilitating carrying GPS corrective
timing signals.
• The GPS is controlled by the DOD presenting certain
unforecasted uncertainties for commercial use on an
uninterrupted basis.
As a result of these and other key factors, it was determined that
LORAN would remain. In recognition of GPS vulnerabilities
as a GNSS, there are plans to maintain other systems that
could provide en route and terminal accuracy such as LORAN.
Therefore as LORAN is further modernized it’s a possibility
that it may be used to augment GPS and provide backup to
GPS during unlikely but potential outages. Or if combined
with GPS and other systems such as newer miniaturized lowcost
inertial navigation systems (INS), superior accuracy and
seamless backup will always be available.
LORAN Components
The LORAN receiver incorporates a radio receiver, signal
processor, navigation computer, control/display, and antenna.
When turned on, the receivers go through an initialization
or warm-up period, then inform the user they are ready to
be programmed. LORAN receivers vary widely in their
appearance, method of user programming, and navigation
information display. Therefore, it is necessary to become
familiar with the unit, including programming and output
interpretation. The LORAN operating manual should be in the
aircraft at all times and available to the pilot. IFR-approved
LORAN units require that the manual be aboard and that the
pilot be familiar with the unit’s functions, before fl ight.
7-26
Figure 7-27. A typical example (GNS 480) of a stand-alone GPS
receiver and display.
Function of LORAN
After initialization, select for the present location WP
(the airport), and select GO TO in order to determine if
the LORAN is functioning properly. Proper operation is
indicated by a low distance reading (0 to 0.5 NM). The
simplest mode of navigation is referred to as GO TO: you
select a WP from one of the databases and choose the
GO TO mode. Before use in fl ight, verify that the latitude
and longitude of the chosen WP is correct by reference to
another approved information source. An updatable LORAN
database that supports the appropriate operations (e.g., en
route, terminal, and instrument approaches) is required when
operating under IFR.
In addition to displaying bearing, distance, time to the WP,
and track and speed over the ground, the LORAN receiver
may have other features such as flight planning (WP
sequential storage), emergency location of several nearest
airports, vertical navigation capabilities, and more.
LORAN Errors
System Errors
LORAN is subject to interference from many external
sources, which can cause distortion of or interference with
LORAN signals. LORAN receiver manufacturers install
“notch fi lters” to reduce or eliminate interference. Proximity
to 60 Hz alternating current power lines, static discharge,
P-static, electrical noise from generators, alternators, strobes,
and other onboard electronics may decrease the signalto-
noise ratio to the point where the LORAN receiver’s
performance is degraded.
Proper installation of the antenna, good electrical bonding,
and an effective static discharge system are the minimum
requirements for LORAN receiver operation. Most receivers
have internal tests that verify the timing alignment of the
receiver clock with the LORAN pulse, and measure and
display signal-to-noise ratio. A signal will be activated to alert
the pilot if any of the parameters for reliable navigation are
exceeded on LORAN sets certifi ed for IFR operations.
LORAN is most accurate when the signal travels over sea
water during the day and least accurate when the signal
comes over land and large bodies of fresh water or ice at
night; furthermore, the accuracy degrades as distance from
the station increases. However, LORAN accuracy is generally
better than VOR accuracy.
Operational Errors
Some of the typical pilot-induced errors of LORAN operation
are:
1. Use of a nonapproved LORAN receiver for IFR
operations. The pilot should check the aircraft’s POH/
AFM LORAN supplement to be certain the unit’s
functions are well understood (this supplement must
be present in the aircraft for approved IFR operations).
There should be a copy of FAA Form 337, Major
Repair and Alteration, present in the aircraft’s records,
showing approval of use of this model LORAN for
IFR operations in this aircraft.
2. Failure to double-check the latitude/longitude values
for a WP to be used. Whether the WP was accessed
from the airport, NDB, VOR, or intersection database,
the values of latitude and longitude should still be
checked against the values in the A/FD or other
approved source. If the WP data is entered in the user
database, its accuracy must be checked before use.
3. Attempting to use LORAN information with degraded
signals.
Advanced Technologies
Global Navigation Satellite System (GNSS)
The Global Navigation Satellite System (GNSS) is a
constellation of satellites providing a high-frequency signal
which contains time and distance that is picked up by a
receiver thereby. The receiver which picks up
multiple signals from different satellites is able to triangulate
its position from these satellites.
7-27
Figure 7-28. Typical GPS Satellite Array.
Three GNSSs exist today: the GPS, a United States system;
the Russian GNSS (GLONASS); and Galileo, a European
system.
1. GLONASS is a network of 24 satellites, which can be
picked up by any GLONASS receiver, allowing the
user to pinpoint their position.
2. Galileo is a network of 30 satellites that continuously
transmit high-frequency radio signals containing time
and distance data that can be picked up by a Galileo
receiver with operational expectancy by 2008.
3. The GPS came on line in 1992 with 24 satellites, and
today utilizes 30 satellites.
Global Positioning System (GPS)
The GPS is a satellite-based radio navigation system, which
broadcasts a signal that is used by receivers to determine
precise position anywhere in the world. The receiver tracks
multiple satellites and determines a measurement that is then
used to determine the user location.
The Department of Defense (DOD) developed and deployed
GPS as a space-based positioning, velocity, and time system.
The DOD is responsible for operation of the GPS satellite
constellation, and constantly monitors the satellites to ensure
proper operation. The GPS system permits Earth-centered
coordinates to be determined and provides aircraft position
referenced to the DOD World Geodetic System of 1984
(WGS-84). Satellite navigation systems are unaffected
by weather and provide global navigation coverage that
fully meets the civil requirements for use as the primary
means of navigation in oceanic airspace and certain remote
areas. Properly certifi ed GPS equipment may be used as a
supplemental means of IFR navigation for domestic en route,
terminal operations, and certain IAPs. Navigational values,
such as distance and bearing to a WP and groundspeed, are
computed from the aircraft’s current position (latitude and
longitude) and the location of the next WP. Course guidance
is provided as a linear deviation from the desired track of a
Great Circle route between defi ned WPs.
GPS may not be approved for IFR use in other countries.
Prior to its use, pilots should ensure that GPS is authorized
by the appropriate countries.
GPS Components
GPS consists of three distinct functional elements: space,
control, and user.
The space element consists of over 30 Navstar satellites. This
group of satellites is called a constellation. The satellites
are in six orbital planes (with four in each plane) at about
11,000 miles above the Earth. At least fi ve satellites are
in view at all times. The GPS constellation broadcasts a
pseudo-random code timing signal and data message that the
aircraft equipment processes to obtain satellite position and
status data. By knowing the precise location of each satellite
and precisely matching timing with the atomic clocks on
the satellites, the aircraft receiver/processor can accurately
measure the time each signal takes to arrive at the receiver
and, therefore, determine aircraft position.
The control element consists of a network of ground-based
GPS monitoring and control stations that ensure the accuracy
of satellite positions and their clocks. In its present form, it
has fi ve monitoring stations, three ground antennas, and a
master control station.
The user element consists of antennas and receiver/processors
on board the aircraft that provide positioning, velocity,
and precise timing to the user. GPS equipment used while
operating under IFR must meet the standards set forth in
Technical Standard Order (TSO) C-129 (or equivalent); meet
the airworthiness installation requirements; be “approved” for
that type of IFR operation; and be operated in accordance with
the applicable POH/AFM or fl ight manual supplement.
An updatable GPS database that supports the appropriate
operations (e.g., en route, terminal, and instrument
approaches) is required when operating under IFR. The
aircraft GPS navigation database contains WPs from the
7-28
geographic areas where GPS navigation has been approved
for IFR operations. The pilot selects the desired WPs from
the database and may add user-defi ned WPs for the fl ight.
Equipment approved in accordance with TSO C-115a, visual
fl ight rules (VFR), and hand-held GPS systems do not meet
the requirements of TSO C-129 and are not authorized for
IFR navigation, instrument approaches, or as a principal
instrument fl ight reference. During IFR operations, these
units (TSO C-115a) may be considered only an aid to
situational awareness.
Prior to GPS/WAAS IFR operation, the pilot must review
appropriate NOTAMs and aeronautical information. This
information is available on request from an Automated
Flight Service Station. The FAA will provide NOTAMs to
advise pilots of the status of the WAAS and level of service
available.
Function of GPS
GPS operation is based on the concept of ranging and
triangulation from a group of satellites in space which act
as precise reference points. The receiver uses data from a
minimum of four satellites above the mask angle (the lowest
angle above the horizon at which it can use a satellite).
The aircraft GPS receiver measures distance from a satellite
using the travel time of a radio signal. Each satellite transmits
a specifi c code, called a course/acquisition (CA) code, which
contains information about satellite position, the GPS system
time, and the health and accuracy of the transmitted data.
Knowing the speed at which the signal traveled (approximately
186,000 miles per second) and the exact broadcast time,
the distance traveled by the signal can be computed from
the arrival time. The distance derived from this method of
computing distance is called a pseudo-range because it is not
a direct measurement of distance, but a measurement based
on time. In addition to knowing the distance to a satellite, a
receiver needs to know the satellite’s exact position in space,
its ephemeris. Each satellite transmits information about its
exact orbital location. The GPS receiver uses this information
to establish the precise position of the satellite.
Using the calculated pseudo-range and position information
supplied by the satellite, the GPS receiver/processor
mathematically determines its position by triangulation
from several satellites. The GPS receiver needs at least four
satellites to yield a three-dimensional position (latitude,
longitude, and altitude) and time solution. The GPS receiver
computes navigational values (distance and bearing to
a WP, groundspeed, etc.) by using the aircraft’s known
latitude/longitude and referencing these to a database built
into the receiver.
The GPS receiver verifi es the integrity (usability) of the
signals received from the GPS constellation through receiver
autonomous integrity monitoring (RAIM) to determine if a
satellite is providing corrupted information. RAIM needs
a minimum of fi ve satellites in view, or four satellites and
a barometric altimeter baro-aiding to detect an integrity
anomaly. For receivers capable of doing so, RAIM needs
six satellites in view (or fi ve satellites with baro-aiding)
to isolate a corrupt satellite signal and remove it from the
navigation solution.
Generally, there are two types of RAIM messages. One
type indicates that there are not enough satellites available
to provide RAIM and another type indicates that the RAIM
has detected a potential error that exceeds the limit for the
current phase of fl ight. Without RAIM capability, the pilot
has no assurance of the accuracy of the GPS position.
Aircraft using GPS navigation equipment under IFR for
domestic en route, terminal operations, and certain IAPs,
must be equipped with an approved and operational alternate
means of navigation appropriate to the fl ight. The avionics
necessary to receive all of the ground-based facilities
appropriate for the route to the destination airport and any
required alternate airport must be installed and operational.
Ground-based facilities necessary for these routes must also
be operational. Active monitoring of alternative navigation
equipment is not required if the GPS receiver uses RAIM for
integrity monitoring. Active monitoring of an alternate means
of navigation is required when the RAIM capability of the
GPS equipment is lost. In situations where the loss of RAIM
capability is predicted to occur, the fl ight must rely on other
approved equipment, delay departure, or cancel the fl ight.
GPS Substitution
IFR En Route and Terminal Operations
GPS systems, certified for IFR en route and terminal
operations, may be used as a substitute for ADF and DME
receivers when conducting the following operations within
the United States NAS.
1. Determining the aircraft position over a DME fi x. This
includes en route operations at and above 24,000 feet
mean sea level (MSL) (FL 240) when using GPS for
navigation.
2. Flying a DME arc.
3. Navigating TO/FROM an NDB/compass locator.
4. Determining the aircraft position over an NDB/compass
locator.
5. Determining the aircraft position over a fi x defi ned by
an NDB/compass locator bearing crossing a VOR/LOC
course.
7-29
6. Holding over an NDB/compass locator.
GPS Substitution for ADF or DME
Using GPS as a substitute for ADF or DME is subject to the
following restrictions:
1. This equipment must be installed in accordance with
appropriate airworthiness installation requirements
and operated within the provisions of the applicable
POH/AFM, or supplement.
2. The required integrity for these operations must be
provided by at least en route RAIM, or equivalent.
3. WPs, fi xes, intersections, and facility locations to be
used for these operations must be retrieved from the
GPS airborne database. The database must be current.
If the required positions cannot be retrieved from the
airborne database, the substitution of GPS for ADF
and/or DME is not authorized
4. Procedures must be established for use when RAIM
outages are predicted or occur. This may require the
fl ight to rely on other approved equipment or require
the aircraft to be equipped with operational NDB and/or
DME receivers. Otherwise, the fl ight must be rerouted,
delayed, canceled, or conducted under VFR.
5. The CDI must be set to terminal sensitivity (1 NM)
when tracking GPS course guidance in the terminal
area.
6. A non-GPS approach procedure must exist at the
alternate airport when one is required. If the non-GPS
approaches on which the pilot must rely require DME
or ADF, the aircraft must be equipped with DME or
ADF avionics as appropriate.
7. Charted requirements for ADF and/or DME can be met
using the GPS system, except for use as the principal
instrument approach navigation source.
NOTE: The following provides guidance, which is not
specifi c to any particular aircraft GPS system. For specifi c
system guidance, refer to the POH/AFM, or supplement, or
contact the system manufacturer.
To Determine Aircraft Position Over a DME Fix:
1. Verify aircraft GPS system integrity monitoring
is functioning properly and indicates satisfactory
integrity.
2. If the fi x is identifi ed by a fi ve-letter name which is
contained in the GPS airborne database, select either
the named fi x as the active GPS WP or the facility
establishing the DME fi x as the active GPS WP. When
using a facility as the active WP, the only acceptable
facility is the DME facility which is charted as the one
used to establish the DME fi x. If this facility is not in
the airborne database, it is not authorized for use.
3. If the fi x is identifi ed by a fi ve-letter name which is not
contained in the GPS airborne database, or if the fi x is
not named, select the facility establishing the DME fi x
or another named DME fi x as the active GPS WP.
4. When selecting the named fi x as the active GPS WP,
a pilot is over the fi x when the GPS system indicates
the active WP.
5. If selecting the DME providing facility as the active
GPS WP, a pilot is over the fi x when the GPS distance
from the active WP equals the charted DME value, and
the aircraft is established on the appropriate bearing
or course.
To Fly a DME Arc:
1. Verify aircraft GPS system integrity monitoring
is functioning properly and indicates satisfactory
integrity.
2. Select from the airborne database the facility providing
the DME arc as the active GPS WP. The only
acceptable facility is the DME facility on which the arc
is based. If this facility is not in your airborne database,
you are not authorized to perform this operation.
3. Maintain position on the arc by reference to the GPS
distance instead of a DME readout.
To Navigate TO or FROM an NDB/Compass
Locator:
1. Verify aircraft GPS system integrity monitoring
is functioning properly and indicates satisfactory
integrity.
2. Select the NDB/compass locator facility from the
airborne database as the active WP. If the chart depicts
the compass locator collocated with a fi x of the same
name, use of that fi x as the active WP in place of the
compass locator facility is authorized.
3. Select and navigate on the appropriate course to or
from the active WP.
To Determine Aircraft Position Over an NDB/
Compass Locator:
1. Verify aircraft GPS system integrity monitoring
is functioning properly and indicates satisfactory
integrity.
7-30
2. Select the NDB/compass locator facility from the
airborne database. When using an NDB/compass
locator, the facility must be charted and be in the
airborne database. If the facility is not in the airborne
database, pilots are not authorized to use a facility WP
for this operation.
3. A pilot is over the NDB/compass locator when the
GPS system indicates arrival at the active WP.
To Determine Aircraft Position Over a Fix Made up
of an NDB/Compass Locator Bearing Crossing a
VOR/LOC Course:
1. Verify aircraft GPS system integrity monitoring
is functioning properly and indicates satisfactory
integrity.
2. A fi x made up by a crossing NDB/compass locator
bearing is identifi ed by a fi ve-letter fi x name. Pilots
may select either the named fi x or the NDB/compass
locator facility providing the crossing bearing to
establish the fi x as the active GPS WP. When using
an NDB/compass locator, that facility must be charted
and be in the airborne database. If the facility is not
in the airborne database, pilots are not authorized to
use a facility WP for this operation.
3. When selecting the named fi x as the active GPS WP,
pilot is over the fi x when the GPS system indicates
the pilot is at the WP.
4. When selecting the NDB/compass locator facility
as the active GPS WP, pilots are over the fi x when
the GPS bearing to the active WP is the same as
the charted NDB/compass locator bearing for the
fi x fl ying the prescribed track from the non-GPS
navigation source.
To Hold Over an NDB/Compass Locator:
1. Verify aircraft GPS system integrity monitoring
is functioning properly and indicates satisfactory
integrity.
2. Select the NDB/compass locator facility from the
airborne database as the active WP. When using a
facility as the active WP, the only acceptable facility
is the NDB/compass locator facility which is charted.
If this facility is not in the airborne database, its use
is not authorized.
3. Select nonsequencing (e.g., “HOLD” or “OBS”) mode
and the appropriate course in accordance with the
POH/AFM, or supplement.
4. Hold using the GPS system in accordance with the
POH/AFM, or supplement.
IFR Flight Using GPS
Prefl ight preparations should ensure that the GPS is properly
installed and certifi ed with a current database for the type
of operation. The GPS operation must be conducted in
accordance with the FAA-approved POH/AFM or fl ight
manual supplement. Flightcrew members must be thoroughly
familiar with the particular GPS equipment installed in the
aircraft, the receiver operation manual, and the POH/AFM
or fl ight manual supplement. Unlike ILS and VOR, the
basic operation, receiver presentation to the pilot and some
capabilities of the equipment can vary greatly. Due to these
differences, operation of different brands, or even models
of the same brand of GPS receiver under IFR should not be
attempted without thorough study of the operation of that
particular receiver and installation. Using the equipment in
fl ight under VFR conditions prior to attempting IFR operation
will allow further familiarization.
Required prefl ight preparations should include checking
NOTAMs relating to the IFR fl ight when using GPS as a
supplemental method of navigation. GPS satellite outages
are issued as GPS NOTAMs both domestically and
internationally. Pilots may obtain GPS RAIM availability
information for an airport by specifi cally requesting GPS
aeronautical information from an automated fl ight service
station (AFSS) during preflight briefings. GPS RAIM
aeronautical information can be obtained for a 3-hour
period: the estimated time of arrival (ETA), and 1 hour
before to 1 hour after the ETA hour, or a 24-hour time frame
for a specifi c airport. FAA briefers will provide RAIM
information for a period of 1 hour before to 1 hour after
the ETA, unless a specifi c timeframe is requested by the
pilot. If fl ying a published GPS departure, the pilot should
also request a RAIM prediction for the departure airport.
Some GPS receivers have the capability to predict RAIM
availability. The pilot should also ensure that the required
underlying ground-based navigation facilities and related
aircraft equipment appropriate to the route of fl ight, terminal
operations, instrument approaches for the destination, and
alternate airports/heliports will be operational for the ETA.
If the required ground-based facilities and equipment will
not be available, the fl ight should be rerouted, rescheduled,
canceled, or conducted under VFR.
Except for programming and retrieving information from
the GPS receiver, planning the fl ight is accomplished in a
similar manner to conventional NAVAIDs. Departure WP,
DP, route, STAR, desired approach, IAF, and destination
airport are entered into the GPS receiver according to the
manufacturer’s instructions. During prefl ight, additional
information may be entered for functions such as ETA, fuel
planning, winds aloft, etc.
7-31
Figure 7-29. A GPS Stand-Alone Approach.
When the GPS receiver is turned on, it begins an internal
process of test and initialization. When the receiver is
initialized, the user develops the route by selecting a WP
or series of WPs, verifi es the data, and selects the active
fl ight plan. This procedure varies widely among receivers
made by different manufacturers. GPS is a complex system,
offering little standardization between receiver models. It is
the pilot’s responsibility to be familiar with the operation of
the equipment in the aircraft.
The GPS receiver provides navigational values such as track,
bearing, groundspeed, and distance. These are computed from
the aircraft’s present latitude and longitude to the location of
the next WP. Course guidance is provided between WPs. The
pilot has the advantage of knowing the aircraft’s actual track
over the ground. As long as track and bearing to the WP are
matched up (by selecting the correct aircraft heading), the
aircraft is going directly to the WP.
GPS Instrument Approaches
There is a mixture of GPS overlay approaches (approaches
with “or GPS” in the title) and GPS stand-alone approaches
in the United States.
NOTE: GPS instrument approach operations outside the
United States must be authorized by the appropriate country
authority.
While conducting these IAPs, ground-based NAVAIDs are
not required to be operational and associated aircraft avionics
need not be installed, operational, turned on, or monitored;
however, monitoring backup navigation systems is always
recommended when available.
Pilots should have a basic understanding of GPS approach
procedures and practice GPS IAPs under visual meteorological
conditions (VMC) until thoroughly proficient with all
aspects of their equipment (receiver and installation) prior
to attempting fl ight in instrument meteorological conditions
(IMC).
All IAPs must be retrievable from the current GPS database
supplied by the manufacturer or other FAA-approved
source. Flying point to point on the approach does not
assure compliance with the published approach procedure.
The proper RAIM sensitivity will not be available and the
CDI sensitivity will not automatically change to 0.3 NM.
Manually setting CDI sensitivity does not automatically
change the RAIM sensitivity on some receivers. Some
existing nonprecision approach procedures cannot be coded
for use with GPS and will not be available as overlays.
7-32
GPS approaches are requested and approved by ATC using
the GPS title, such as “GPS RWY 24” or “RNAV RWY
35.” Using the manufacturer’s recommended procedures, the
desired approach and the appropriate IAF are selected from
the GPS receiver database. Pilots should fl y the full approach
from an initial approach waypoint (IAWP) or feeder fi x unless
specifi cally cleared otherwise. Randomly joining an approach
at an intermediate fi x does not ensure terrain clearance.
When an approach has been loaded in the fl ight plan, GPS
receivers will give an “arm” annunciation 30 NM straight
line distance from the airport/heliport reference point. The
approach mode should be “armed” when within 30 NM
distance so the receiver will change from en route CDI
(±5 NM) and RAIM (±2 NM) sensitivity to ±1 NM terminal
sensitivity. Where the IAWP is within 30 NM, a CDI
sensitivity change will occur once the approach mode is
armed and the aircraft is within 30 NM. Where the IAWP
is beyond the 30 NM point, CDI sensitivity will not change
until the aircraft is within 30 NM even if the approach is
armed earlier. Feeder route obstacle clearance is predicated
on the receiver CDI and RAIM being in terminal CDI
sensitivity within 30 NM of the airport/heliport reference
point; therefore, the receiver should always be armed no later
than the 30 NM annunciation.
Pilots should pay particular attention to the exact operation
of their GPS receivers for performing holding patterns and in
the case of overlay approaches, operations such as procedure
turns. These procedures may require manual intervention
by the pilot to stop the sequencing of WPs by the receiver
and to resume automatic GPS navigation sequencing once
the maneuver is complete. The same WP may appear in the
route of fl ight more than once and consecutively (e.g., IAWP,
fi nal approach waypoint (FAWP), missed approach waypoint
(MAWP) on a procedure turn). Care must be exercised to
ensure the receiver is sequenced to the appropriate WP for
the segment of the procedure being fl own, especially if one
or more fl y-over WPs are skipped (e.g., FAWP rather than
IAWP if the procedure turn is not fl own). The pilot may need
to sequence past one or more fl y-overs of the same WP in
order to start GPS automatic sequencing at the proper place
in the sequence of WPs.
When receiving vectors to fi nal, most receiver operating
manuals suggest placing the receiver in the nonsequencing
mode on the FAWP and manually setting the course. This
provides an extended fi nal approach course in cases where
the aircraft is vectored onto the fi nal approach course outside
of any existing segment which is aligned with the runway.
Assigned altitudes must be maintained until established on a
published segment of the approach. Required altitudes at WPs
outside the FAWP or step-down fi xes must be considered.
Calculating the distance to the FAWP may be required in
order to descend at the proper location.
When within 2 NM of the FAWP with the approach mode
armed, the approach mode will switch to active, which results
in RAIM and CDI sensitivity changing to the approach
mode. Beginning 2 NM prior to the FAWP, the full scale
CDI sensitivity will change smoothly from ±1 NM to ±0.3
NM at the FAWP. As sensitivity changes from ±1 NM to
±0.3 NM approaching the FAWP, and the CDI not centered,
the corresponding increase in CDI displacement may give
the impression the aircraft is moving further away from the
intended course even though it is on an acceptable intercept
heading. If digital track displacement information (cross-track
error) is available in the approach mode, it may help the pilot
remain position oriented in this situation. Being established
on the fi nal approach course prior to the beginning of the
sensitivity change at 2 NM will help prevent problems in
interpreting the CDI display during ramp-down. Requesting
or accepting vectors, which will cause the aircraft to intercept
the fi nal approach course within 2 NM of the FAWP, is not
recommended.
Incorrect inputs into the GPS receiver are especially critical
during approaches. In some cases, an incorrect entry can
cause the receiver to leave the approach mode. Overriding
an automatically selected sensitivity during an approach
will cancel the approach mode annunciation. If the approach
mode is not armed by 2 NM prior to the FAWP, the approach
mode will not become active at 2 NM prior to the FAWP and
the equipment will fl ag. In these conditions, the RAIM and
CDI sensitivity will not ramp down, and the pilot should not
descend to minimum descent altitude (MDA), but fl y to the
MAWP and execute a missed approach. The approach active
annunciator and/or the receiver should be checked to ensure
the approach mode is active prior to the FAWP.
A GPS missed approach requires pilot action to sequence the
receiver past the MAWP to the missed approach portion of
the procedure. The pilot must be thoroughly familiar with
the activation procedure for the particular GPS receiver
installed in the aircraft and must initiate appropriate action
after the MAWP. Activating the missed approach prior to the
MAWP will cause CDI sensitivity to change immediately to
terminal (±1 NM) sensitivity, and the receiver will continue
to navigate to the MAWP. The receiver will not sequence past
the MAWP. Turns should not begin prior to the MAWP. If
the missed approach is not activated, the GPS receiver will
display an extension of the inbound fi nal approach course and
the along track distance (ATD) will increase from the MAWP
until it is manually sequenced after crossing the MAWP.
7-33
Missed approach routings in which the fi rst track is via a
course rather than direct to the next WP require additional
action by the pilot to set the course. Being familiar with all
of the required inputs is especially critical during this phase
of fl ight.
Departures and Instrument Departure Procedures
(DPs)
The GPS receiver must be set to terminal (±1 NM) CDI
sensitivity and the navigation routes contained in the database
in order to fl y published IFR charted departures and DPs.
Terminal RAIM should be provided automatically by the
receiver. (Terminal RAIM for departure may not be available
unless the WPs are part of the active fl ight plan rather than
proceeding direct to the fi rst destination.) Certain segments
of a DP may require some manual intervention by the pilot,
especially when radar vectored to a course or required to
intercept a specifi c course to a WP. The database may not
contain all of the transitions or departures from all runways
and some GPS receivers do not contain DPs in the database.
It is necessary that helicopter procedures be fl own at 70 knots
or less since helicopter departure procedures and missed
approaches use a 20:1 obstacle clearance surface (OCS),
which is double the fi xed-wing OCS. Turning areas are based
on this speed also. Missed approach routings in which the
fi rst track is via a course rather than direct to the next WP
require additional action by the pilot to set the course. Being
familiar with all of the required inputs is especially critical
during this phase of fl ight.
GPS Errors
Normally, with 30 satellites in operation, the GPS
constellation is expected to be available continuously
worldwide. Whenever there are fewer than 24 operational
satellites, GPS navigational capability may not be available
at certain geographic locations. Loss of signals may also
occur in valleys surrounded by high terrain, and any time
the aircraft’s GPS antenna is “shadowed” by the aircraft’s
structure (e.g., when the aircraft is banked).
Certain receivers, transceivers, mobile radios, and portable
receivers can cause signal interference. Some VHF
transmissions may cause “harmonic interference.” Pilots
can isolate the interference by relocating nearby portable
receivers, changing frequencies, or turning off suspected
causes of the interference while monitoring the receiver’s
signal quality data page.
GPS position data can be affected by equipment characteristics
and various geometric factors, which typically cause errors
of less than 100 feet. Satellite atomic clock inaccuracies,
receiver/processors, signals reflected from hard objects
(multi-path), ionospheric and tropospheric delays, and
satellite data transmission errors may cause small position
errors or momentary loss of the GPS signal.
System Status
The status of GPS satellites is broadcast as part of the data
message transmitted by the GPS satellites. GPS status
information is also available by means of the United States
Coast Guard navigation information service: (703) 313-
5907, or on the internet at http://www.navcen.uscg.gov/.
Additionally, satellite status is available through the Notice
to Airmen (NOTAM) system.
The GPS receiver verifi es the integrity (usability) of the
signals received from the GPS constellation through receiver
autonomous integrity monitoring (RAIM) to determine if
a satellite is providing corrupted information. At least one
satellite, in addition to those required for navigation, must
be in view for the receiver to perform the RAIM function;
thus, RAIM needs a minimum of fi ve satellites in view, or
four satellites and a barometric altimeter (baro-aiding) to
detect an integrity anomaly. For receivers capable of doing
so, RAIM needs six satellites in view (or fi ve satellites with
baro-aiding) to isolate the corrupt satellite signal and remove
it from the navigation solution.
RAIM messages vary somewhat between receivers; however,
there are two most commonly used types. One type indicates
that there are not enough satellites available to provide
RAIM integrity monitoring and another type indicates that
the RAIM integrity monitor has detected a potential error
that exceeds the limit for the current phase of fl ight. Without
RAIM capability, the pilot has no assurance of the accuracy
of the GPS position.
Selective Availability. Selective Availability (SA) is a method
by which the accuracy of GPS is intentionally degraded.
This feature is designed to deny hostile use of precise GPS
positioning data. SA was discontinued on May 1, 2000,
but many GPS receivers are designed to assume that SA
is still active. New receivers may take advantage of the
discontinuance of SA based on the performance values in
ICAO Annex 10, and do not need to be designed to operate
outside of that performance.
7-34
GPS Familiarization
Pilots should practice GPS approaches under visual
meteorological conditions (VMC) until thoroughly profi cient
with all aspects of their equipment (receiver and installation)
prior to attempting fl ight by IFR in instrument meteorological
conditions (IMC). Some of the tasks which the pilot should
practice are:
1. Utilizing the receiver autonomous integrity monitoring
(RAIM) prediction function;
2. Inserting a DP into the fl ight plan, including setting
terminal CDI sensitivity, if required, and the conditions
under which terminal RAIM is available for departure
(some receivers are not DP or STAR capable);
3. Programming the destination airport;
4. Programming and flying the overlay approaches
(especially procedure turns and arcs);
5. Changing to another approach after selecting an
approach;
6. Programming and flying “direct” missed
approaches;
7. Programming and flying “routed” missed
approaches;
8. Entering, flying, and exiting holding patterns,
particularly on overlay approaches with a second WP
in the holding pattern;
9. Programming and fl ying a “route” from a holding
pattern;
10. Programming and flying an approach with radar
vectors to the intermediate segment;
11. Indication of the actions required for RAIM failure
both before and after the FAWP; and
12. Programming a radial and distance from a VOR (often
used in departure instructions).
Differential Global Positioning Systems (DGPS)
Differential global positioning systems (DGPS) are designed
to improve the accuracy of global navigation satellite systems
(GNSS) by measuring changes in variables to provide satellite
positioning corrections.
Because multiple receivers receiving the same set of satellites
produce similar errors, a reference receiver placed at a known
location can compute its theoretical position accurately and
can compare that value to the measurements provided by the
navigation satellite signals. The difference in measurement
between the two signals is an error that can be corrected by
providing a reference signal correction.
As a result of this differential input accuracy of the
satellite system can be increased to meters. The Wide Area
Augmentation System (WAAS) and Local Area Augmentation
System (LAAS) are examples of differential global positioning
systems.
Wide Area Augmentation System (WAAS)
The WAAS is designed to improve the accuracy, integrity,
and availability of GPS signals. WAAS allows GPS to be
used, as the aviation navigation system, from takeoff through
Category I precision approaches. The International Civil
Aviation Organization (ICAO) has defi ned Standards for
satellite-based augmentation systems (SBAS), and Japan
and Europe are building similar systems that are planned
to be interoperable with WAAS: EGNOS, the European
Geostationary Navigation Overlay System, and MSAS,
the Japanese Multifunctional Transport Satellite (MTSAT)
Satellite-based Augmentation System. The result will be a
worldwide seamless navigation capability similar to GPS but
with greater accuracy, availability, and integrity.
Unlike traditional ground-based navigation aids, WAAS
will cover a more extensive service area in which surveyed
wide-area ground reference stations are linked to the WAAS
network. Signals from the GPS satellites are monitored by
these stations to determine satellite clock and ephemeris
corrections. Each station in the network relays the data to a
wide-area master station where the correction information is
computed. A correction message is prepared and uplinked to
a geostationary satellite (GEO) via a ground uplink and then
broadcast on the same frequency as GPS to WAAS receivers
within the broadcast coverage area.
In addition to providing the correction signal, WAAS
provides an additional measurement to the aircraft receiver,
improving the availability of GPS by providing, in effect,
an additional GPS satellite in view. The integrity of GPS is
improved through real-time monitoring, and the accuracy
is improved by providing differential corrections to reduce
errors. As a result, performance improvement
is suffi cient to enable approach procedures with GPS/WAAS
glide paths. At this time the FAA has completed installation of
25 wide area ground reference systems, two master stations,
and four ground uplink stations.
General Requirements
WAAS avionics must be certified in accordance with
TSO-C145A, Airborne Navigation Sensors Using the GPS
Augmented by the WAAS; or TSO-146A for stand-alone
systems. GPS/WAAS operation must be conducted in
accordance with the FAA-approved aircraft fl ight manual
(AFM) and flight manual supplements. Flight manual
supplements must state the level of approach procedure that
the receiver supports.
7-35
Figure 7-30. WAAS Satellite Representation.
7-36
Figure 7-32. LAAS Representation.
Figure 7-31. WAAS Satellite Representation.
Instrument Approach Capabilities
WAAS receivers support all basic GPS approach functions
and will provide additional capabilities with the key benefi t
to generate an electronic glide path, independent of ground
equipment or barometric aiding. This eliminates several
problems such as cold temperature effects, incorrect altimeter
setting or lack of a local altimeter source, and allows approach
procedures to be built without the cost of installing ground
stations at each airport. A new class of approach procedures
which provide vertical guidance requirements for precision
approaches has been developed to support satellite navigation
use for aviation applications. These new procedures called
Approach with Vertical Guidance (APV) include approaches
such as the LNAV/VNAV procedures presently being fl own
with barometric vertical navigation.
Local Area Augmentation System (LAAS)
LAAS is a ground-based augmentation system which uses
a GPS reference facility located on or in the vicinity of
the airport being serviced. This facility has a reference
receiver that measures GPS satellite pseudo-range and
timing and retransmits the signal. Aircraft landing at
LAAS-equipped airports are able to conduct approaches to
Category I level and above for properly equipped aircraft.
Inertial Navigation System (INS)
Inertial Navigation System (INS) is a system that navigates
precisely without any input from outside of the aircraft. It is
fully self-contained. The INS is initialized by the pilot, who
enters into the system the exact location of the aircraft on the
ground before the fl ight. The INS is also programmed with
WPs along the desired route of fl ight.
7-37
Figure 7-33. LAAS Representation.
INS Components
INS is considered a stand-alone navigation system, especially
when more than one independent unit is onboard. The
airborne equipment consists of an accelerometer to measure
acceleration—which, when integrated with time, gives
velocity—and gyros to measure direction.
Later versions of the INS, called inertial reference systems
(IRS) utilize laser gyros and more powerful computers;
therefore, the accelerometer mountings no longer need to
be kept level and aligned with true north. The computer
system can handle the added workload of dealing with the
computations necessary to correct for gravitational and
directional errors. Consequently, these newer systems are
sometimes called strap down systems, as the accelerometers
and gyros are strapped down to the airframe, rather than being
mounted on a structure that stays fi xed with respect to the
horizon and true north.
INS Errors
The principal error associated with INS is degradation of
position with time. INS computes position by starting with
accurate position input which is changed continuously as
accelerometers and gyros provide speed and direction inputs.
Both accelerometers and gyros are subject to very small
errors; as time passes, those errors probably accumulate.
While the best INS/IRS display errors of 0.1 to 0.4 NM after
fl ights across the North Atlantic of 4 to 6 hours, smaller and
less expensive systems are being built that show errors of 1
to 2 NM per hour. This accuracy is more than suffi cient for
a navigation system that can be combined with and updated
by GPS. The synergy of a navigation system consisting of an
INS/IRS unit in combination with a GPS resolves the errors
and weaknesses of both systems. GPS is accurate all the time
it is working but may be subject to short and periodic outages.
INS is made more accurate because it is continually updated
and continues to function with good accuracy if the GPS has
moments of lost signal.
Instrument Approach Systems
Most navigation systems approved for en route and terminal
operations under IFR, such as VOR, NDB, and GPS, may
also be approved to conduct IAPs. The most common
systems in use in the United States are the ILS, simplifi ed
directional facility (SDF), localizer directional aid (LDA),
and microwave landing system (MLS). These systems
operate independently of other navigation systems. There are
new systems being developed, such as WAAS and LAAS.
Other systems have been developed for special use.
Instrument Landing Systems (ILS)
The ILS system provides both course and altitude guidance
to a specifi c runway. The ILS system is used to execute
a precision instrument approach procedure or precision
approach. The system consists of the following
components:
1. A localizer providing horizontal (left/right) guidance
along the extended centerline of the runway.
2. A glide slope (GS) providing vertical (up/down)
guidance toward the runway touchdown point, usually
at a 3° slope.
3. Marker beacons providing range information along
the approach path.
4. Approach lights assisting in the transition from
instrument to visual fl ight.
The following supplementary elements, though not specifi c
components of the system, may be incorporated to increase
safety and utility:
1. Compass locators providing transition from en route
NAVAIDs to the ILS system and assisting in holding
procedures, tracking the localizer course, identifying
the marker beacon sites, and providing a FAF for ADF
approaches.
2. DME collocated with the GS transmitter providing
positive distance-to-touchdown information or DME
associated with another nearby facility (VOR or standalone),
if specifi ed in the approach procedure.
ILS approaches are categorized into three different types of
approaches based on the equipment at the airport and the
experience level of the pilot. Category I approaches provide
for approach height above touchdown of not less than 200 feet.
Category II approaches provide for approach to a height above
7-38
Figure 7-34. Instrument Landing Systems.
7-39
Figure 7-35. Localizer Coverage Limits.
touchdown of not less than 100 feet. Category III approaches
provide lower minimums for approaches without a decision
height minimum. While pilots need only be instrument rated
and the aircraft be equipped with the appropriate airborne
equipment to execute Category I approaches, Category II
and III approaches require special certifi cation for the pilots,
ground equipment, and airborne equipment.
ILS Components
Ground Components
The ILS uses a number of different ground facilities. These
facilities may be used as a part of the ILS system, as well as
part of another approach. For example, the compass locator
may be used with NDB approaches.
Localizer
The localizer (LOC) ground antenna array is located on the
extended centerline of the instrument runway of an airport,
located at the departure end of the runway to prevent it from
being a collision hazard. This unit radiates a fi eld pattern,
which develops a course down the centerline of the runway
toward the middle markers (MMs) and outer markers
(OMs), and a similar course along the runway centerline in
the opposite direction. These are called the front and back
courses, respectively. The localizer provides course guidance,
transmitted at 108.1 to 111.95 MHz (odd tenths only),
throughout the descent path to the runway threshold from a
distance of 18 NM from the antenna to an altitude of 4,500
feet above the elevation of the antenna site.
The localizer course width is defined as the angular
displacement at any point along the course between a full
“fl y-left” (CDI needle fully defl ected to the left) and a full
“fl y-right” indication (CDI needle fully defl ected to the right).
Each localizer facility is audibly identifi ed by a three-letter
designator, transmitted at frequent regular intervals. The ILS
identifi cation is preceded by the letter “I” (two dots). For
example, the ILS localizer at Springfi eld, Missouri transmits
the identifi er ISGF. The localizer includes a voice feature on
its frequency for use by the associated ATC facility in issuing
approach and landing instructions.
The localizer course is very narrow, normally 5°. This
results in high needle sensitivity. With this course width,
a full-scale defl ection shows when the aircraft is 2.5° to
either side of the centerline. This sensitivity permits accurate
orientation to the landing runway. With no more than onequarter
scale defl ection maintained, the aircraft will be
aligned with the runway.
Glide Slope (GS)
GS describes the systems that generate, receive, and indicate
the ground facility radiation pattern. The glide path is the
straight, sloped line the aircraft should fl y in its descent from
where the GS intersects the altitude used for approaching the
FAF, to the runway touchdown zone. The GS equipment
is housed in a building approximately 750 to 1,250 feet
down the runway from the approach end of the runway, and
between 400 and 600 feet to one side of the centerline.
7-40
Figure 7-36. Localizer receiver indications and aircraft
displacement.
The course projected by the GS equipment is essentially the
same as would be generated by a localizer operating on its
side. The GS projection angle is normally adjusted to 2.5°
to 3.5° above horizontal, so it intersects the MM at about
200 feet and the OM at about 1,400 feet above the runway
elevation. At locations where standard minimum obstruction
clearance cannot be obtained with the normal maximum GS
angle, the GS equipment is displaced farther from the approach
end of the runway if the length of the runway permits; or, the
GS angle may be increased up to 4°.
Unlike the localizer, the GS transmitter radiates signals only
in the direction of the fi nal approach on the front course. The
system provides no vertical guidance for approaches on the
back course. The glide path is normally 1.4° thick. At 10
NM from the point of touchdown, this represents a vertical
distance of approximately 1,500 feet, narrowing to a few feet
at touchdown.
Marker Beacons
Two VHF marker beacons, outer and middle, are normally
used in the ILS system. A third beacon, the
inner, is used where Category II operations are certifi ed. A
marker beacon may also be installed to indicate the FAF on
the ILS back course.
The OM is located on the localizer front course 4–7 miles
from the airport to indicate a position at which an aircraft, at
the appropriate altitude on the localizer course, will intercept
the glide path. The MM is located approximately 3,500 feet
from the landing threshold on the centerline of the localizer
front course at a position where the GS centerline is about 200
feet above the touchdown zone elevation. The inner marker
(IM), where installed, is located on the front course between
the MM and the landing threshold. It indicates the point at
which an aircraft is at the decision height on the glide path
during a Category II ILS approach. The back-course marker,
where installed, indicates the back-course FAF.
Compass Locator
Compass locators are low-powered NDBs and are received
and indicated by the ADF receiver. When used in conjunction
with an ILS front course, the compass locator facilities are
collocated with the outer and/or MM facilities. The coding
identifi cation of the outer locator consists of the fi rst two
letters of the three-letter identifi er of the associated LOC.
For example, the outer locator at Dallas/Love Field (DAL) is
identifi ed as “DA.” The middle locator at DAL is identifi ed
by the last two letters “AL.”
Approach Lighting Systems (ALS)
Normal approach and letdown on the ILS is divided into two
distinct stages: the instrument approach stage using only radio
guidance, and the visual stage, when visual contact with the
ground runway environment is necessary for accuracy and
safety. The most critical period of an instrument approach,
particularly during low ceiling/visibility conditions, is the
point at which the pilot must decide whether to land or
execute a missed approach. As the runway threshold is
approached, the visual glide path will separate into individual
lights. At this point, the approach should be continued by
reference to the runway touchdown zone markers. The ALS
provides lights that will penetrate the atmosphere far enough
from touchdown to give directional, distance, and glide path
information for safe visual transition.
Visual identification of the ALS by the pilot must be
instantaneous, so it is important to know the type of ALS
before the approach is started. Check the instrument approach
chart and the A/FD for the particular type of lighting facilities
at the destination airport before any instrument fl ight. With
reduced visibility, rapid orientation to a strange runway can
be diffi cult, especially during a circling approach to an airport
with minimum lighting facilities, or to a large terminal airport
located in the midst of distracting city and ground facility
lights. Some of the most common ALS systems are shown
in Figure 7-37.
A high-intensity fl asher system, often referred to as “the
rabbit,” is installed at many large airports. The fl ashers consist
of a series of brilliant blue-white bursts of light fl ashing in
sequence along the approach lights, giving the effect of a ball
of light traveling towards the runway. Typically, “the rabbit”
makes two trips toward the runway per second.
Runway end identifi er lights (REIL) are installed for rapid and
positive identifi cation of the approach end of an instrument
runway. The system consists of a pair of synchronized
fl ashing lights placed laterally on each side of the runway
threshold facing the approach area.
7-41
Figure 7-37. Precision and Nonprecision ALS Confi guration.
The visual approach slope indicator (VASI) gives visual
descent guidance information during the approach to a
runway. The standard VASI consists of light bars that
project a visual glide path, which provides safe obstruction
clearance within the approach zone. The normal GS angle
is 3°; however, the angle may be as high as 4.5° for proper
obstacle clearance. On runways served by ILS, the VASI
angle normally coincides with the electronic GS angle.
Visual left/right course guidance is obtained by alignment
with the runway lights. The standard VASI installation
consists of either 2-, 3-, 4-, 6-, 12-, or 16-light units arranged
in downwind and upwind light bars. Some airports serving
long-bodied aircraft have three-bar VASIs which provide two
visual glidepaths to the same runway. The fi rst glide path
encountered is the same as provided by the standard VASI.
The second glide path is about 25 percent higher than the fi rst
and is designed for the use of pilots of long-bodied aircraft.
The basic principle of VASI is that of color differentiation
between red and white. Each light projects a beam having
a white segment in the upper part and a red segment in the
lower part of the beam. From a position above the glide path
the pilot sees both bars as white. Lowering the aircraft with
respect to the glide path, the color of the upwind bars changes
from white to pink to red. When on the proper glide path,
the landing aircraft will overshoot the downwind bars and
undershoot the upwind bars. Thus the downwind (closer)
bars are seen as white and the upwind bars as red. From a
position below the glide path, both light bars are seen as
red. Moving up to the glide path, the color of the downwind
7-42
Figure 7-38. Standard two-bar VASI.
bars changes from red to pink to white. When below the
glide path, as indicated by a distinct all-red signal, a safe
obstruction clearance might not exist. A standard two-bar
VASI is illustrated in Figure 7-38.
ILS Airborne Components
Airborne equipment for the ILS system includes receivers
for the localizer, GS, marker beacons, ADF, DME, and the
respective indicator instruments.
The typical VOR receiver is also a localizer receiver with
common tuning and indicating equipment. Some receivers
have separate function selector switches, but most switch
between VOR and LOC automatically by sensing if odd
tenths between 108 and 111.95 MHz have been selected.
Otherwise, tuning of VOR and localizer frequencies is
accomplished with the same knobs and switches, and the CDI
indicates “on course” as it does on a VOR radial.
Though some GS receivers are tuned separately, in a typical
installation the GS is tuned automatically to the proper
frequency when the localizer is tuned. Each of the 40 localizer
channels in the 108.10 to 111.95 MHz band is paired with a
corresponding GS frequency.
When the localizer indicator also includes a GS needle, the
instrument is often called a cross-pointer indicator. The
crossed horizontal (GS) and vertical (localizer) needles are
free to move through standard fi ve-dot defl ections to indicate
position on the localizer course and glide path.
When the aircraft is on the glide path, the needle is horizontal,
overlying the reference dots. Since the glide path is much
narrower than the localizer course (approximately 1.4° from
full up to full down defl ection), the needle is very sensitive
to displacement of the aircraft from on-path alignment. With
the proper rate of descent established upon GS interception,
very small corrections keep the aircraft aligned.
The localizer and GS warning fl ags disappear from view on
the indicator when suffi cient voltage is received to actuate the
needles. The fl ags show when an unstable signal or receiver
malfunction occurs.
The OM is identifi ed by a low-pitched tone, continuous dashes
at the rate of two per second, and a purple/blue marker beacon
light. The MM is identifi ed by an intermediate tone, alternate
dots and dashes at the rate of 95 dot/dash combinations per
minute, and an amber marker beacon light. The IM, where
installed, is identifi ed by a high-pitched tone, continuous dots
at the rate of six per second, and a white marker beacon light.
The back-course marker (BCM), where installed, is identifi ed
by a high-pitched tone with two dots at a rate of 72 to 75 twodot
combinations per minute, and a white marker beacon light.
Marker beacon receiver sensitivity is selectable as high or low
on many units. The low-sensitivity position gives the sharpest
indication of position and should be used during an approach.
The high-sensitivity position provides an earlier warning that
the aircraft is approaching the marker beacon site.
ILS Function
The localizer needle indicates, by defl ection, whether the
aircraft is right or left of the localizer centerline, regardless of
the position or heading of the aircraft. Rotating the OBS has
no effect on the operation of the localizer needle, although
it is useful to rotate the OBS to put the LOC inbound course
under the course index. When inbound on the front course, or
outbound on the back course, the course indication remains
directional. (See Figure 7-39, aircraft C, D, and E.)
Unless the aircraft has reverse sensing capability and it is in
use, when fl ying inbound on the back course or outbound
on the front course, heading corrections to on-course are
made opposite the needle defl ection. This is commonly
described as “fl ying away from the needle.” (See Figure 7-39,
aircraft A and B.) Back course signals should not be used
for an approach unless a back course approach procedure
is published for that particular runway and the approach is
authorized by ATC.
Once you have reached the localizer centerline, maintain
the inbound heading until the CDI moves off center. Drift
corrections should be small and reduced proportionately as
the course narrows. By the time you reach the OM, your drift
correction should be established accurately enough on a wellexecuted
approach to permit completion of the approach,
with heading corrections no greater then 2°.
The heaviest demand on pilot technique occurs during
descent from the OM to the MM, when you maintain
the localizer course, adjust pitch attitude to maintain the
7-43
Figure 7-39. Localizer Course Indications. To follow indications displayed in the aircraft, start from A and proceed through E.
7-44
Figure 7-40. Illustrates a GS receiver indication and aircraft displacement. An analog system is on the left and the same indication on
the Garmin PFD on the right.
proper rate of descent, and adjust power to maintain proper
airspeed. Simultaneously, the altimeter must be checked
and preparation made for visual transition to land or for a
missed approach. You can appreciate the need for accurate
instrument interpretation and aircraft control within the ILS
as a whole, when you notice the relationship between CDI
and glide path needle indications, and aircraft displacement
from the localizer and glide path centerlines.
Defl ection of the GS needle indicates the position of the
aircraft with respect to the glide path. When the aircraft is
above the glide path, the needle is defl ected downward. When
the aircraft is below the glide path, the needle is defl ected
upward.
ILS Errors
The ILS and its components are subject to certain errors,
which are listed below. Localizer and GS signals are subject to
the same type of bounce from hard objects as space waves.
1. Refl ection. Surface vehicles and even other aircraft
fl ying below 5,000 feet above ground level (AGL)
may disturb the signal for aircraft on the approach.
2. False courses. In addition to the desired course, GS
facilities inherently produce additional courses at
higher vertical angles. The angle of the lowest of these
false courses will occur at approximately 9°–12°. An
aircraft fl ying the LOC/GS course at a constant altitude
would observe gyrations of both the GS needle and GS
warning fl ag as the aircraft passed through the various
false courses. Getting established on one of these
false courses will result in either confusion (reversed
GS needle indications) or in the need for a very high
descent rate. However, if the approach is conducted
at the altitudes specifi ed on the appropriate approach
chart, these false courses will not be encountered.
Marker Beacons
The very low power and directional antenna of the marker
beacon transmitter ensures that the signal will not be received
any distance from the transmitter site. Problems with signal
reception are usually caused by the airborne receiver not
being turned on, or by incorrect receiver sensitivity.
Some marker beacon receivers, to decrease weight and cost,
are designed without their own power supply. These units
utilize a power source from another radio in the avionics
stack, often the ADF. In some aircraft, this requires the
ADF to be turned on in order for the marker beacon receiver
to function, yet no warning placard is required. Another
7-45
source of trouble may be the “High/Low/Off” three-position
switch, which both activates the receiver and selects receiver
sensitivity. Usually, the “test” feature only tests to see if
the light bulbs in the marker beacon lights are working.
Therefore, in some installations, there is no functional way
for the pilot to ascertain the marker beacon receiver is actually
on except to fl y over a marker beacon transmitter, and see if
a signal is received and indicated (e.g., audibly, and visually
via marker beacon lights).
Operational Errors
1. Failure to understand the fundamentals of ILS ground
equipment, particularly the differences in course
dimensions. Since the VOR receiver is used on the
localizer course, the assumption is sometimes made
that interception and tracking techniques are identical
when tracking localizer courses and VOR radials.
Remember that the CDI sensing is sharper and faster
on the localizer course.
2. Disorientation during transition to the ILS due to poor
planning and reliance on one receiver instead of on all
available airborne equipment. Use all the assistance
available; a single receiver may fail.
3. Disorientation on the localizer course, due to the fi rst
error noted above.
4. Incorrect localizer interception angles. A large
interception angle usually results in overshooting,
and possible disorientation. When intercepting,
if possible, turn to the localizer course heading
immediately upon the first indication of needle
movement. An ADF receiver is an excellent aid to
orient you during an ILS approach if there is a locator
or NDB on the inbound course.
5. Chasing the CDI and glide path needles, especially
when you have not suffi ciently studied the approach
before the fl ight.
Simplifi ed Directional Facility (SDF)
The SDF provides a fi nal approach course similar to the ILS
localizer. The SDF course may or may not be aligned with
the runway and the course may be wider than a standard
ILS localizer, resulting in less precision. Usable off-course
indications are limited to 35° either side of the course
centerline. Instrument indications in the area between 35°
and 90° from the course centerline are not controlled and
should be disregarded.
The SDF must provide signals suffi cient to allow satisfactory
operation of a typical aircraft installation within a sector
which extends from the center of the SDF antenna system
to distances of 18 NM covering a sector 10° either side of
centerline up to an angle 7° above the horizontal. The angle
of convergence of the fi nal approach course and the extended
runway centerline must not exceed 30°. Pilots should note
this angle since the approach course originates at the antenna
site, and an approach continued beyond the runway threshold
would lead the aircraft to the SDF offset position rather than
along the runway centerline.
The course width of the SDF signal emitted from the
transmitter is fi xed at either 6° or 12°, as necessary, to provide
maximum fl yability and optimum approach course quality.
A three-letter identifi er is transmitted in code on the SDF
frequency; there is no letter “I” (two dots) transmitted before
the station identifi er, as there is with the LOC. For example,
the identifi er for Lebanon, Missouri, SDF is LBO.
Localizer Type Directional Aid (LDA)
The LDA is of comparable utility and accuracy to a localizer
but is not part of a complete ILS. The LDA course width is
between 3° and 6° and thus provides a more precise approach
course than an SDF installation. Some LDAs are equipped
with a GS. The LDA course is not aligned with the runway,
but straight-in minimums may be published where the angle
between the runway centerline and the LDA course does not
exceed 30°. If this angle exceeds 30°, only circling minimums
are published. The identifi er is three letters preceded by “I”
transmitted in code on the LDA frequency. For example, the
identifi er for Van Nuys, California, LDA is I-BUR.
Microwave Landing System (MLS)
The MLS provides precision navigation guidance for exact
alignment and descent of aircraft on approach to a runway.
It provides azimuth, elevation, and distance. Both lateral
and vertical guidance may be displayed on conventional
course deviation indicators or incorporated into multipurpose
fl ight deck displays. Range information can be displayed
by conventional DME indicators and also incorporated into
multipurpose displays.
The system may be divided into fi ve functions, which are
approach azimuth, back azimuth, approach elevation, range;
and data communications. The standard confi guration of
MLS ground equipment includes an azimuth station to
perform functions as indicated above. In addition to providing
azimuth navigation guidance, the station transmits basic data,
which consists of information associated directly with the
operation of the landing system, as well as advisory data on
the performance of the ground equipment.
Approach Azimuth Guidance
The azimuth station transmits MLS angle and data on one
of 200 channels within the frequency range of 5031 to 5091
7-46
Figure 7-41. MLS Coverage Volumes, 3-D Representation.
MHz. The equipment is normally located about 1,000 feet
beyond the stop end of the runway, but there is considerable
flexibility in selecting sites. For example, for heliport
operations the azimuth transmitter can be collocated with
the elevation transmitter. The azimuth coverage extends
laterally at least 40° on either side of the runway centerline
in a standard confi guration, in elevation up to an angle of 15°
and to at least 20,000 feet, and in range to at least 20 NM.
MLS requires separate airborne equipment to receive and
process the signals from what is normally installed in general
aviation aircraft today. It has data communications capability,
and can provide audible information about the condition
of the transmitting system and other pertinent data such as
weather, runway status, etc. The MLS transmits an audible
identifi er consisting of four letters beginning with the letter
M, in Morse code at a rate of at least six per minute. The
MLS system monitors itself and transmits ground-to-air data
messages about the system’s operational condition. During
periods of routine or emergency maintenance, the coded
identifi cation is missing from the transmissions. At this time
there are only a few systems installed.
Required Navigation Performance
RNP is a navigation system that provides a specifi ed level
of accuracy defi ned by a lateral area of confi ned airspace
in which an RNP-certifi ed aircraft operates. The continuing
growth of aviation places increasing demands on airspace
capacity and emphasizes the need for the best use of the
available airspace. These factors, along with the accuracy of
modern aviation navigation systems and the requirement for
increased operational effi ciency in terms of direct routings
and track-keeping accuracy, have resulted in the concept
of required navigation performance—a statement of the
navigation performance accuracy necessary for operation
within a defi ned airspace. RNP can include both performance
and functional requirements, and is indicated by the RNP type.
These standards are intended for designers, manufacturers,
and installers of avionics equipment, as well as service
providers and users of these systems for global operations.
The minimum aviation system performance specifi cation
(MASPS) provides guidance for the development of airspace
and operational procedures needed to obtain the benefi ts of
improved navigation capability.
The RNP type defi nes the total system error (TSE) that
is allowed in lateral and longitudinal dimensions within
a particular airspace. The TSE, which takes account of
navigation system errors (NSE), computation errors, display
errors and fl ight technical errors (FTE), must not exceed the
specifi ed RNP value for 95 percent of the fl ight time on any
part of any single fl ight. RNP combines the accuracy standards
laid out in the ICAO Manual (Doc 9613) with specific
accuracy requirements, as well as functional and performance
standards, for the RNAV system to realize a system that
can meet future air traffi c management requirements. The
functional criteria for RNP address the need for the fl ight paths
of participating aircraft to be both predictable and repeatable
to the declared levels of accuracy. More information on RNP
is contained in subsequent chapters.
The term RNP is also applied as a descriptor for airspace,
routes, and procedures (including departures, arrivals,
and IAPs). The descriptor can apply to a unique approach
procedure or to a large region of airspace. RNP applies to
navigation performance within a designated airspace, and
includes the capability of both the available infrastructure
(navigation aids) and the aircraft.
RNP type is used to specify navigation requirements for the
airspace. The following are ICAO RNP Types: RNP-1.0,
RNP-4.0, RNP-5.0, and RNP-10.0. The required performance
is obtained through a combination of aircraft capability and
the level of service provided by the corresponding navigation
infrastructure. From a broad perspective:
Aircraft Capability + Level of Service = Access
In this context, aircraft capability refers to the airworthiness
certifi cation and operational approval elements (including
avionics, maintenance, database, human factors, pilot
procedures, training, and other issues). The level of service
element refers to the NAS infrastructure, including published
routes, signal-in-space performance and availability, and air
7-47
Figure 7-42. Required Navigation Performance.
7-48
Figure 7-43. Typical Display and Control Unit(s) in General Aviation. The Universal UNS-1 (left) controls and integrates all other
systems. The Avidyne (center) and Garmin systems (right) illustrate and are typical of completely integrated systems. Although the
Universal CDU is not typically found on smaller general aviation aircraft, the difference in capabilities of the CDUs and stand-alone
sytems is diminishing each year.
traffi c management. When considered collectively, these
elements result in providing access. Access provides the
desired benefi t (airspace, procedures, routes of fl ight, etc.).
RNP levels are actual distances from the centerline of the
fl ight path, which must be maintained for aircraft and obstacle
separation. Although additional FAA-recognized RNP
levels may be used for specifi c operations, the United States
currently supports three standard RNP levels:
• RNP 0.3 – Approach
• RNP 1.0 – Departure, Terminal
• RNP 2.0 – En route
RNP 0.3 represents a distance of 0.3 NM either side of a
specifi ed fl ight path centerline. The specifi c performance
that is required on the final approach segment of an
instrument approach is an example of this RNP level. At
the present time, a 0.3 RNP level is the lowest level used in
normal RNAV operations. Specifi c airlines, using special
procedures, are approved to use RNP levels lower than
RNP 0.3, but those levels are used only in accordance with
their approved operations specifi cations (OpsSpecs). For
aircraft equipment to qualify for a specifi c RNP type, it
must maintain navigational accuracy at least 95 percent of
the total fl ight time.
Flight Management Systems (FMS)
A fl ight management system (FMS) is not a navigation
system in itself. Rather, it is a system that automates the
tasks of managing the onboard navigation systems. FMS may
perform other onboard management tasks, but this discussion
is limited to its navigation function.
FMS is an interface between fl ight crews and fl ight-deck
systems. FMS can be thought of as a computer with a large
database of airport and NAVAID locations and associated
data, aircraft performance data, airways, intersections,
DPs, and STARs. FMS also has the ability to accept and
store numerous user-defi ned WPs, fl ight routes consisting
of departures, WPs, arrivals, approaches, alternates, etc.
FMS can quickly defi ne a desired route from the aircraft’s
current position to any point in the world, perform fl ight
plan computations, and display the total picture of the fl ight
route to the crew.
FMS also has the capability of controlling (selecting) VOR,
DME, and LOC NAVAIDs, and then receiving navigational
data from them. INS, LORAN, and GPS navigational data
may also be accepted by the FMS computer. The FMS may
act as the input/output device for the onboard navigation
systems, so that it becomes the “go-between” for the crew
and the navigation systems.
Function of FMS
At startup, the crew programs the aircraft location, departure
runway, DP (if applicable), WPs defi ning the route, approach
procedure, approach to be used, and routing to alternate. This
may be entered manually, be in the form of a stored fl ight
plan, or be a fl ight plan developed in another computer and
transferred by disk or electronically to the FMS computer.
The crew enters this basic information in the control/display
unit (CDU).
Once airborne, the FMS computer channels the appropriate
NAVAIDs and takes radial/distance information, or
channels two NAVAIDs, taking the more accurate distance
information. FMS then indicates position, track, desired
heading, groundspeed and position relative to desired track.
Position information from the FMS updates the INS. In more
sophisticated aircraft, the FMS provides inputs to the HSI,
RMI, glass fl ight deck navigation displays, head-up display
(HUD), autopilot, and autothrottle systems.
7-49
Figure 7-44. Example of a Head-Up Display (top) and a Head-Down
Display (bottom). The head-up display presents information in front
of the pilot along his/her normal fi eld of view while a head-down
display may present information beyond the normal head-up fi eld
of view.
Head-Up Display (HUD)
The HUD is a display system that provides a projection of
navigation and air data (airspeed in relation to approach
reference speed, altitude, left/right and up/down GS) on a
transparent screen between the pilot and the windshield. Other
information may be displayed, including a runway target in
relation to the nose of the aircraft. This allows the pilot to see
the information necessary to make the approach while also
being able to see out the windshield, which diminishes the
need to shift between looking at the panel to looking outside.
Virtually any information desired can be displayed on the
HUD if it is available in the aircraft’s fl ight computer, and
if the display is user defi nable.
Radar Navigation (Ground Based)
Radar works by transmitting a pulse of RF energy in a specifi c
direction. The return of the echo or bounce of that pulse from
a target is precisely timed. From this, the distance traveled
by the pulse and its echo is determined and displayed on a
radar screen in such a manner that the distance and bearing to
this target can be instantly determined. The radar transmitter
must be capable of delivering extremely high power levels
toward the airspace under surveillance, and the associated
radar receiver must be able to detect extremely small signal
levels of the returning echoes.
The radar display system provides the controller with a maplike
presentation upon which appear all the radar echoes
of aircraft within detection range of the radar facility. By
means of electronically generated range marks and azimuthindicating
devices, the controller can locate each radar target
with respect to the radar facility, or can locate one radar target
with respect to another.
Another device, a video-mapping unit, generates an actual
airway or airport map and presents it on the radar display
equipment. Using the video-mapping feature, the air traffi c
controller not only can view the aircraft targets, but can see
these targets in relation to runways, navigation aids, and
hazardous ground obstructions in the area. Therefore, radar
becomes a NAVAID, as well as the most signifi cant means
of traffi c separation.
In a display presenting perhaps a dozen or more targets, a
primary surveillance radar system cannot identify one specifi c
radar target, and it may have diffi culty “seeing” a small target
at considerable distance—especially if there is a rain shower
or thunderstorm between the radar site and the aircraft. This
problem is solved with the Air Traffi c Control Radar Beacon
System (ATCRBS), sometimes called secondary surveillance
radar (SSR), which utilizes a transponder in the aircraft. The
ground equipment is an interrogating unit, in which the beacon
antenna is mounted so it rotates with the surveillance antenna.
The interrogating unit transmits a coded pulse sequence that
actuates the aircraft transponder. The transponder answers the
coded sequence by transmitting a preselected coded sequence
back to the ground equipment, providing a strong return signal
and positive aircraft identifi cation, as well as other special data
such as aircraft altitude.
Functions of Radar Navigation
The radar systems used by ATC are air route surveillance
radar (ARSR), airport surveillance radar (ASR), and precision
approach radar (PAR) and airport surface detection equipment
7-50
(ASDE). Surveillance radars scan through 360° of azimuth
and present target information on a radar display located in
a tower or center. This information is used independently or
in conjunction with other navigational aids in the control of
air traffi c.
ARSR is a long-range radar system designed primarily to cover
large areas and provide a display of aircraft while en route
between terminal areas. The ARSR enables air route traffi c
control center (ARTCC) controllers to provide radar service
when the aircraft are within the ARSR coverage. In some
instances, ARSR may enable ARTCC to provide terminal radar
services similar to but usually more limited than those provided
by a radar approach control.
ASR is designed to provide relatively short-range coverage in
the general vicinity of an airport and to serve as an expeditious
means of handling terminal area traffi c through observation
of precise aircraft locations on a radarscope. Nonprecision
instrument approaches are available at airports that have an
approved surveillance radar approach procedure. ASR provides
radar vectors to the fi nal approach course and then azimuth
information to the pilot during the approach. In addition to
range (distance) from the runway, the pilot is advised of MDA,
when to begin descent, and when the aircraft is at the MDA. If
requested, recommended altitudes will be furnished each mile
while on fi nal.
PAR is designed to be used as a landing aid displaying range,
azimuth, and elevation information rather than as an aid for
sequencing and spacing aircraft. PAR equipment may be
used as a primary landing aid, or it may be used to monitor
other types of approaches. Two antennas are used in the PAR
array, one scanning a vertical plane, and the other scanning
horizontally. Since the range is limited to 10 miles, azimuth to
20°, and elevation to 7°, only the fi nal approach area is covered.
The controller’s scope is divided into two parts. The upper half
presents altitude and distance information, and the lower half
presents azimuth and distance.
PAR is a system in which a controller provides highly accurate
navigational guidance in azimuth and elevation to a pilot. Pilots
are given headings to fl y to direct them to and keep their aircraft
aligned with the extended centerline of the landing runway.
They are told to anticipate glide path interception approximately
10–30 seconds before it occurs and when to start descent.
The published decision height (DH) is given only if the pilot
requests it. If the aircraft is observed to deviate above or below
the glide path, the pilot is given the relative amount of deviation
by use of terms “slightly” or “well” and is expected to adjust
the aircraft’s rate of descent/ascent to return to the glide path.
Trend information is also issued with respect to the elevation
of the aircraft and may be modifi ed by the terms “rapidly” and
“slowly” (e.g., “well above glide path, coming down rapidly”).
Range from touchdown is given at least once each mile. If an
aircraft is observed by the controller to proceed outside of
specifi ed safety zone limits in azimuth and/or elevation and
continue to operate outside these prescribed limits, the pilot will
be directed to execute a missed approach or to fl y a specifi ed
course unless the pilot has the runway environment (runway,
approach lights, etc.) in sight. Navigational guidance in azimuth
and elevation is provided to the pilot until the aircraft reaches
the published decision altitude (DA)/DH. Advisory course and
glide path information is furnished by the controller until the
aircraft passes over the landing threshold, at which point the
pilot is advised of any deviation from the runway centerline.
Radar service is automatically terminated upon completion of
the approach.
Airport Surface Detection Equipment
Radar equipment is specifi cally designed to detect all principal
features on the surface of an airport, including aircraft and
vehicular traffi c, and to present the entire image on a radar
indicator console in the control tower. It is used to augment
visual observation by tower personnel of aircraft and/or
vehicular movements on runways and taxiways.
Radar Limitations
1. It is very important for the aviation community to
recognize the fact that there are limitations to radar
service and that ATC controllers may not always be able
to issue traffi c advisories concerning aircraft which are
not under ATC control and cannot be seen on radar.
2. The characteristics of radio waves are such that they
normally travel in a continuous straight line unless
they are “bent” by abnormal atmospheric phenomena
such as temperature inversions; refl ected or attenuated
by dense objects such as heavy clouds, precipitation,
ground obstacles, mountains, etc.; or screened by high
terrain features.
3. Primary radar energy that strikes dense objects will be
refl ected and displayed on the operator’s scope, thereby
blocking out aircraft at the same range and greatly
weakening or completely eliminating the display of
targets at a greater range.
4. Relatively low altitude aircraft will not be seen if they
are screened by mountains or are below the radar beam
due to curvature of the Earth.
5. The amount of refl ective surface of an aircraft will
determine the size of the radar return. Therefore, a small
light airplane or a sleek jet fi ghter will be more diffi cult
to see on primary radar than a large commercial jet or
military bomber.
7-51
6. All ARTCC radar in the conterminous United States and
many ASR have the capability to interrogate Mode C
and display altitude information to the controller from
appropriately equipped aircraft. However, a number of
ASR do not have Mode C display capability; therefore,
altitude information must be obtained from the pilot.
7-52
8-1
Introduction
The National Airspace System (NAS) is the network of
United States airspace: air navigation facilities, equipment,
services, airports or landing areas, aeronautical charts,
information/services, rules, regulations, procedures, technical
information, manpower, and material. Included are system
components shared jointly with the military. The system’s
present confi guration is a refl ection of the technological
advances concerning the speed and altitude capability of jet
aircraft, as well as the complexity of microchip and satellitebased
navigation equipment. To conform to international
aviation standards, the United States adopted the primary
elements of the classification system developed by the
International Civil Aviation Organization (ICAO).
This chapter is a general discussion of airspace classifi cation;
en route, terminal, and approach procedures; and operations
within the NAS. Detailed information on the classifi cation
of airspace, operating procedures, and restrictions is found
in the Aeronautical Information Manual (AIM).
The National
Airspace System
Chapter 8
8-2
Airspace Classifi cation
Airspace in the United States is designated as
follows:
1. Class A. Generally, airspace from 18,000 feet mean
sea level (MSL) up to and including fl ight level (FL)
600, including the airspace overlying the waters
within 12 nautical miles (NM) of the coast of the
48 contiguous states and Alaska. Unless otherwise
authorized, all pilots must operate their aircraft under
instrument fl ight rules (IFR).
2. Class B. Generally, airspace from the surface to 10,000
feet MSL surrounding the nation’s busiest airports in
terms of airport operations or passenger enplanements.
The confi guration of each Class B airspace area is
individually tailored, consists of a surface area and two
or more layers (some Class B airspace areas resemble
upside-down wedding cakes), and is designed to
contain all published instrument procedures once
an aircraft enters the airspace. An air traffi c control
(ATC) clearance is required for all aircraft to operate
in the area, and all aircraft that are so cleared receive
separation services within the airspace.
3. Class C. Generally, airspace from the surface to
4,000 feet above the airport elevation (charted
in MSL) surrounding those airports that have an
operational control tower, are serviced by a radar
approach control, and have a certain number of IFR
operations or passenger enplanements. Although the
confi guration of each Class C area is individually
tailored, the airspace usually consists of a surface
area with a 5 NM radius, an outer circle with a 10 NM
radius that extends from 1,200 feet to 4,000 feet above
the airport elevation and an outer area. Each aircraft
must establish two-way radio communications with
the ATC facility providing air traffi c services prior
to entering the airspace and thereafter maintain those
communications while within the airspace.
4. Class D. Generally, that airspace from the surface
to 2,500 feet above the airport elevation (charted
in MSL) surrounding those airports that have an
operational control tower. The configuration of
each Class D airspace area is individually tailored
and when instrument procedures are published,
the airspace will normally be designed to contain
the procedures. Arrival extensions for instrument
approach procedures (IAPs) may be Class D or Class
E airspace. Unless otherwise authorized, each aircraft
must establish two-way radio communications with
the ATC facility providing air traffi c services prior
to entering the airspace and thereafter maintain those
communications while in the airspace.
5. Class E. Generally, if the airspace is not Class A, B,
C, or D, and is controlled airspace, then it is Class E
airspace. Class E airspace extends upward from either
the surface or a designated altitude to the overlying
or adjacent controlled airspace. When designated as a
surface area, the airspace will be confi gured to contain
all instrument procedures. Also in this class are federal
airways, airspace beginning at either 700 or 1,200 feet
above ground level (AGL) used to transition to and
from the terminal or en route environment, and en
route domestic and offshore airspace areas designated
below 18,000 feet MSL. Unless designated at a lower
altitude, Class E airspace begins at 14,500 MSL over
the United States, including that airspace overlying the
waters within 12 NM of the coast of the 48 contiguous
states and Alaska, up to but not including 18,000 feet
MSL, and the airspace above FL 600.
6. Class G. Airspace not designated as Class A, B, C, D,
or E. Class G airspace is essentially uncontrolled by
ATC except when associated with a temporary control
tower.
Special Use Airspace
Special use airspace is the designation for airspace in which
certain activities must be confi ned, or where limitations
may be imposed on aircraft operations that are not part
of those activities. Certain special use airspace areas can
create limitations on the mixed use of airspace. The special
use airspace depicted on instrument charts includes the
area name or number, effective altitude, time and weather
conditions of operation, the controlling agency, and the chart
panel location. On National Aeronautical Charting Group
(NACG) en route charts, this information is available on one
of the end panels.
Prohibited areas contain airspace of defi ned dimensions
within which the fl ight of aircraft is prohibited. Such areas
are established for security or other reasons associated with
the national welfare. These areas are published in the Federal
Register and are depicted on aeronautical charts. The area is
charted as a “P” followed by a number (e.g., “P-123”).
Restricted areas are areas where operations are hazardous to
nonparticipating aircraft and contain airspace within which
the fl ight of aircraft, while not wholly prohibited, is subject
to restrictions. Activities within these areas must be confi ned
because of their nature, or limitations may be imposed upon
aircraft operations that are not a part of those activities, or
both. Restricted areas denote the existence of unusual, often
invisible, hazards to aircraft (e.g., artillery fi ring, aerial
8-3
Figure 8-1. Airspace Classifi cations.
8-4
gunnery, or guided missiles). IFR fl ights may be authorized
to transit the airspace and are routed accordingly. Penetration
of restricted areas without authorization from the using
or controlling agency may be extremely hazardous to the
aircraft and its occupants. ATC facilities apply the following
procedures when aircraft are operating on an IFR clearance
(including those cleared by ATC to maintain visual fl ight
rules (VFR)-On-Top) via a route that lies within joint-use
restricted airspace:
1. If the restricted area is not active and has been released
to the Federal Aviation Administration (FAA), the
ATC facility will allow the aircraft to operate in the
restricted airspace without issuing specifi c clearance
for it to do so.
2. If the restricted area is active and has not been
released to the FAA, the ATC facility will issue a
clearance which will ensure the aircraft avoids the
restricted airspace.
Restricted areas are charted with an “R” followed by a
number (e.g., “R-5701”) and are depicted on the en route
chart appropriate for use at the altitude or FL being fl own.
Warning areas are similar in nature to restricted areas;
however, the United States government does not have sole
jurisdiction over the airspace. A warning area is airspace of
defi ned dimensions, extending from 12 NM outward from
the coast of the United States, containing activity that may
be hazardous to nonparticipating aircraft. The purpose of
such areas is to warn nonparticipating pilots of the potential
danger. A warning area may be located over domestic or
international waters or both. The airspace is designated with
a “W” followed by a number (e.g., “W-123”).
Military operations areas (MOAs) consist of airspace with
defi ned vertical and lateral limits established for the purpose
of separating certain military training activities from IFR
traffi c. Whenever an MOA is being used, nonparticipating
IFR traffi c may be cleared through an MOA if IFR separation
can be provided by ATC. Otherwise, ATC will reroute or
restrict nonparticipating IFR traffi c. MOAs are depicted on
sectional, VFR terminal area, and en route low altitude charts
and are not numbered (e.g., “Boardman MOA”).
Alert areas are depicted on aeronautical charts with an
“A” followed by a number (e.g., “A-123”) to inform
nonparticipating pilots of areas that may contain a high
volume of pilot training or an unusual type of aerial activity.
Pilots should exercise caution in alert areas. All activity
within an alert area shall be conducted in accordance with
regulations, without waiver, and pilots of participating
aircraft, as well as pilots transiting the area, shall be equally
responsible for collision avoidance.
Military Training Routes (MTRs) are routes used by military
aircraft to maintain profi ciency in tactical fl ying. These routes
are usually established below 10,000 feet MSL for operations
at speeds in excess of 250 knots. Some route segments may
be defi ned at higher altitudes for purposes of route continuity.
Routes are identifi ed as IFR (IR), and VFR (VR), followed by
a number. MTRs with no segment above 1,500 feet AGL are
identifi ed by four number characters (e.g., IR1206, VR1207,
etc.). MTRs that include one or more segments above 1,500
feet AGL are identifi ed by three number characters (e.g.,
IR206, VR207). IFR Low Altitude En Route Charts depict
all IR routes and all VR routes that accommodate operations
above 1,500 feet AGL. IR routes are conducted in accordance
with IFR regardless of weather conditions.
Temporary fl ight restrictions (TFRs) are put into effect
when traffi c in the airspace would endanger or hamper air
or ground activities in the designated area. For example, a
forest fi re, chemical accident, fl ood, or disaster-relief effort
could warrant a TFR, which would be issued as a Notice to
Airmen (NOTAM).
National Security Areas (NSAs) consist of airspace with
defined vertical and lateral dimensions established at
locations where there is a requirement for increased security
and safety of ground facilities. Flight in NSAs may be
temporarily prohibited by regulation under the provisions of
Title 14 of the Code of Federal Regulations (14 CFR) part 99
and prohibitions will be disseminated via NOTAM.
Federal Airways
The primary means for routing aircraft operating under IFR
is the federal airways system.
Each federal airway is based on a centerline that extends from
one NAVAID/waypoint/fi x/intersection to another NAVAID/
waypoint/fi x/intersection specifi ed for that airway. A federal
airway includes the airspace within parallel boundary lines
four NM to each side of the centerline. As in all instrument
fl ight, courses are magnetic, and distances are in NM. The
airspace of a federal airway has a fl oor of 1,200 feet AGL,
unless otherwise specifi ed. A federal airway does not include
the airspace of a prohibited area.
Victor airways include the airspace extending from 1,200 feet
AGL up to, but not including 18,000 feet MSL. The airways
are designated on Sectional and IFR low altitude en route
charts with the letter “V” followed by a number (e.g., “V23”).
Typically, Victor airways are given odd numbers when oriented
north/south and even numbers when oriented east/west. If more
than one airway coincides on a route segment, the numbers are
listed serially (e.g., “V287-495-500”).
8-5
Figure 8-2. Victor Airways and Charted IFR Altitudes.
Jet routes exist only in Class A airspace, from 18,000 feet
MSL to FL 450, and are depicted on high-altitude en route
charts. The letter “J” precedes a number to label the airway
(e.g., J12).
RNAV routes have been established in both the low-altitude
and the high-altitude structures in recent years and are
depicted on the en route low and high chart series. High
altitude RNAV routes are identifi ed with a “Q” prefi x (except
the Q-routes in the Gulf of Mexico) and low altitude RNAV
routes are identifi ed with a “T” prefi x. RNAV routes and data
are depicted in aeronautical blue.
In addition to the published routes, a random RNAV route may
be fl own under IFR if it is approved by ATC. Random RNAV
routes are direct routes, based on area navigation capability,
between waypoints defi ned in terms of latitude/longitude
coordinates, degree-distance fi xes, or offsets from established
routes/airways at a specifi ed distance and direction.
Radar monitoring by ATC is required on all random
RNAV routes. These routes can only be approved in a radar
environment. Factors that will be considered by ATC in
approving random RNAV routes include the capability to
provide radar monitoring, and compatibility with traffi c
volume and flow. ATC will radar monitor each flight;
however, navigation on the random RNAV route is the
responsibility of the pilot.
Other Routing
Preferred IFR routes have been established between
major terminals to guide pilots in planning their routes of
fl ight, minimizing route changes and aiding in the orderly
management of air traffi c on federal airways. Low and high
8-6
altitude preferred routes are listed in the Airport/Facility
Directory (A/FD). To use a preferred route, reference the
departure and arrival airports; if a routing exists for your
fl ight, then airway instructions will be listed.
Tower En Route Control (TEC) is an ATC program that
uses overlapping approach control radar services to provide
IFR clearances. By using TEC, a pilot is routed by airport
control towers. Some advantages include abbreviated fi ling
procedures and reduced traffi c separation requirements. TEC
is dependent upon the ATC’s workload, and the procedure
varies among locales.
The latest version of Advisory Circular (AC) 90-91, North
American Route Program (NRP), provides guidance to users
of the NAS for participation in the NRP. All fl ights operating
at or above FL 290 within the conterminous United States
and Canada are eligible to participate in the NRP, the primary
purpose of which is to allow operators to plan minimum time/
cost routes that may be off the prescribed route structure. NRP
aircraft are not subject to route-limiting restrictions (e.g.,
published preferred IFR routes) beyond a 200 NM radius of
their point of departure or destination.
IFR En Route Charts
The objective of IFR en route fl ight is to navigate within the
lateral limits of a designated airway at an altitude consistent
with the ATC clearance. Your ability to fl y instruments
safely and competently in the system is greatly enhanced by
understanding the vast array of data available to the pilot on
instrument charts. The NACG maintains and produces the
charts for the United States government.
En route high-altitude charts provide aeronautical information
for en route instrument navigation (IFR) at or above 18,000
feet MSL. Information includes the portrayal of Jet and
RNAV routes, identifi cation and frequencies of radio aids,
selected airports, distances, time zones, special use airspace,
and related information. Established Jet routes from 18,000
feet MSL to FL 450 use NAVAIDs not more than 260 NM
apart. The charts are revised every 56 days.
To effectively depart from one airport and navigate en route
under instrument conditions a pilot needs the appropriate IFR
en route low-altitude chart(s). The IFR low altitude en route
chart is the instrument equivalent of the Sectional chart. When
folded, the cover of the NACG en route chart displays an
index map of the United States showing the coverage areas.
Cities near congested airspace are shown in black type and
their associated area chart is listed in the box in the lower
left-hand corner of the map coverage box. Also noted is an
explanation of the off-route obstruction clearance altitude
(OROCA). The effective date of the chart is printed on the
other side of the folded chart. Information concerning MTRs
is also included on the chart cover. The en route charts are
revised every 56 days.
When the NACG en route chart is unfolded, the legend is
displayed and provides information concerning airports,
NAVAIDs, communications, air traffic services, and
airspace.
Airport Information
Airport information is provided in the legend, and the symbols
used for the airport name, elevation, and runway length are
similar to the sectional chart presentation. Associated city
names are shown for public airports only. FAA identifi ers are
shown for all airports. ICAO identifi ers are also shown for
airports outside of the contiguous United States. Instrument
approaches can be found at airports with blue or green
symbols, while the brown airport symbol denotes airports
that do not have instrument approaches. Stars are used to
indicate the part-time nature of tower operations, ATIS
frequencies, part-time or on request lighting facilities, and
part-time airspace classifi cations. A box after an airport name
with a “C” or “D” inside indicates Class C and D airspace,
respectively, per Figure 8-3.
Charted IFR Altitudes
The minimum en route altitude (MEA) ensures a navigation
signal strong enough for adequate reception by the aircraft
navigation (NAV) receiver and obstacle clearance along the
airway. Communication is not necessarily guaranteed with
MEA compliance. The obstacle clearance, within the limits of
the airway, is typically 1,000 feet in non-mountainous areas
and 2,000 feet in designated mountainous areas. MEAs can
be authorized with breaks in the signal coverage; if this is the
case, the NACG en route chart notes “MEA GAP” parallel
to the affected airway. MEAs are usually bidirectional;
however, they can be single-directional. Arrows are used to
indicate the direction to which the MEA applies.
The minimum obstruction clearance altitude (MOCA), as
the name suggests, provides the same obstruction clearance
as an MEA; however, the NAV signal reception is ensured
only within 22 NM of the closest NAVAID defi ning the route.
The MOCA is listed below the MEA and indicated on NACG
charts by a leading asterisk (e.g., “*3400”—see Figure 8-2,
V287 at bottom left).
The minimum reception altitude (MRA) identifi es the lowest
altitude at which an intersection can be determined from an
off-course NAVAID. If the reception is line-of-sight based,
signal coverage will only extend to the MRA or above.
However, if the aircraft is equipped with distance measuring
equipment (DME) and the chart indicates the intersection can
8-7
Figure 8-3. En Route Airport Legend.
be identifi ed with such equipment, the pilot could defi ne the
fi x without attaining the MRA. On NACG charts, the MRA
is indicated by the symbol and the altitude preceded by
“MRA” (e.g., “MRA 9300”).
The minimum crossing altitude (MCA) will be charted when
a higher MEA route segment is approached. The MCA is
usually indicated when a pilot is approaching steeply rising
terrain, and obstacle clearance and/or signal reception is
compromised. In this case, the pilot is required to initiate
a climb so the MCA is reached by the time the intersection
is crossed. On NACG charts, the MCA is indicated by the
symbol , and the Victor airway number, altitude, and
the direction to which it applies (e.g. “V24 8000 SE”).
The maximum authorized altitude (MAA) is the highest
altitude at which the airway can be fl own with assurance
of receiving adequate navigation signals. Chart depictions
appear as “MAA-15000.”
When an MEA, MOCA, and/or MAA change on a segment
other than at a NAVAID, a sideways “T” is depicted
on the chart. If there is an airway break without the symbol,
one can assume the altitudes have not changed (see the upper
left area of Figure 8-2). When a change of MEA to a higher
MEA is required, the climb may commence at the break,
ensuring obstacle clearance.
Navigation Features
Types of NAVAIDs
Very high frequency omnidirectional ranges (VORs) are the
principal NAVAIDs that support the Victor and Jet airways.
Many other navigation tools are also available to the pilot.
For example, nondirectional beacons (NDBs) can broadcast
signals accurate enough to provide stand-alone approaches,
and DME allows the pilot to pinpoint a reporting point on the
airway. Though primarily navigation tools, these NAVAIDs
can also transmit voice broadcasts.
Tactical air navigation (TACAN) channels are represented
as the two- or three-digit numbers following the three-letter
identifier in the NAVAID boxes. The NACG terminal
procedures provide a frequency-pairing table for the
TACAN-only sites. On NACG charts, very-high frequencies
and ultra-high frequencies (VHF/UHF) NAVAIDs (e.g.,
VORs) are depicted in black, while low frequencies and
medium frequencies (LF/MF) are depicted as brown.
Identifying Intersections
Intersections along the airway route are established by a
variety of NAVAIDs. An open triangle indicates the
location of an ATC reporting point at an intersection. If the
triangle is solid, a report is compulsory.
8-8
Figure 8-4. Legend From En Route Low Attitude Chart, Air Traffi c Services and Airspace Information Section.
8-9
Figure 8-5. Legend From En Route Low Attitude Chart.
8-10
necessary to enable traffi c fl ow. When a holding pattern is
charted, the controller may provide the holding direction and
the statement “as published.”
Boundaries separating the jurisdiction of Air Route Traffi c
Control Centers (ARTCC) are depicted on charts with blue
serrations. The name
of the controlling facility is printed on the
corresponding side of the division line.
ARTCC remote sites are depicted as blue serrated boxes
and contain the center name, sector name, and the sector
frequency.
Weather Information and Communication Features
En route NAVAIDs also provide weather information and
serve communication functions.
When a NAVAID is shown as a
shadowed box, an automated fl ight
service station (AFSS) of the same
name is directly associated with the facility. If an AFSS is
located without an associated NAVAID, the shadowed box is
smaller and contains only the name and identifi er. The AFSS
frequencies are provided above the
box. (Frequencies 122.2 and 255.4,
and emergency frequencies 121.5 and
243.0 are not listed.)
A Remote Communications Outlet (RCO) associated with
a NAVAID is designated by a thinlined
box with the controlling AFSS
frequency above the box, and the
name under the box. Without an
associated facility, the thin-lined
RCO box contains the AFSS name
and remote frequency.
Automated Surface Observing Station (ASOS), Automated
Weather Observing Station (AWOS), Hazardous Infl ight
Weather Advisory Service
(HIWAS) and Transcribed
Weather Broadcast (TWEB) are
continuously transmitted over
selected NAVAIDs and depicted in the NAVAID box. ASOS/
AWOS are depicted by a white “A”, HIWAS by a “H” and
TWEB broadcasts by a “T” in a solid black circle in the upper
right or left corner.
New Technologies
Technological advances have made multifunction displays
and moving maps more common in newer aircraft. Even older
aircraft are being retrofi tted to include “glass” in the fl ight
deck. Moving maps improve pilot situational
awareness by providing a picture of aircraft location in
NDBs, localizers, and off-route VORs are used to establish
intersections. NDBs are sometimes collocated with
intersections, in which case passage of the NDB would mark
the intersection. A bearing to an off-route NDB also can
provide intersection identifi cation. A localizer course used to
identify an intersection is depicted by a feathered arrowhead
symbol on the en route chart.
If feathered markings appear on the left-hand side of the
arrowhead, a back course (BC)
signal is transmitted. On NACG en route charts, the localizer
symbol is only depicted to identify an intersection.
Off-route VORs remain the most common means of
identifying intersections when traveling on an airway. Arrows
depicted next to the intersection indicate the NAVAID to
be used for identifi cation. Another means of identifying an
intersection is with the use of DME. A hollow arrowhead
indicates DME is authorized for intersection identifi cation. If
the DME mileage at the intersection is a cumulative distance
of route segments, the mileage is totaled and indicated by
a D-shaped symbol with a mileage number inside.
Approved IFR GPS units can also be used to
report intersections.
Other Route Information
DME and GPS provide valuable route information concerning
such factors as mileage, position, and groundspeed. Even
without this equipment, information is provided on the
charts for making the necessary calculations using time and
distance. The en route chart depicts point-to-point distances
on the airway system. Distances from VOR to VOR are
charted with a number inside of a box. To differentiate
distances when two airways coincide, the word “TO” with the
three-letter VOR identifi er appear to the left of the distance
boxes.
VOR changeover points (COPs) are depicted on the charts by
this symbol: The numbers indicate the distance at which
to change the VOR frequency. The frequency change might
be required due to signal reception or confl icting frequencies.
If a COP does not appear on an airway, the frequency should
be changed midway between the facilities. A COP at an
intersection may indicate a course change.
Occasionally an “x” will appear at a separated segment of
an airway that is not an intersection. The “x” is a mileage
breakdown or computer navigation fi x and may indicate a
course change.
Today’s computerized system of ATC has greatly reduced
the need for holding en route. However,
published holding patterns are still found on
charts at junctures where ATC has deemed it Holding Pattern
NAME
Name
000.0 000.0
8-11
Figure 8-6. Moving Map Display.
Figure 8-7. Example of an Electronic Flight Bag.
relation to NAVAIDS, waypoints, airspace, terrain, and
hazardous weather. GPS systems can be certifi ed for terminal
area and en route use as well as approach guidance.
Additional breakthroughs in display technology are the new
electronic chart systems or electronic fl ight bags that facilitate
the use of electronic documents in the general aviation
fl ight deck. An electronic chart or fl ight bag
is a self-powered electronic library that stores and displays
en route charts and other essential documents on a screen.
These electronic devices can store the digitized United States
terminal procedures, en route charts, the complete airport
facility directory, in addition to Title 14 of the Code of Federal
Regulations (14 CFR) and the AIM. Full touch-screen based
computers allow pilots to view airport approach and area
charts electronically while fl ying. It replaces paper charts as
well as other paper materials including minimum equipment
lists (MELs), standard operating procedures (SOPs), standard
instrument departures (SIDs), standard terminal arrival
routes (STARs), checklists, and fl ight deck manuals. As with
paper fl ight publications, the electronic database needs to be
current to provide accurate information regarding NAVAIDS,
waypoints, and terminal procedures. Databases are updated
every 28 days and are available from various commercial
vendors. Pilots should be familiar with equipment operation,
capabilities, and limitations prior to use.
8-12
Terminal Procedures Publications
While the en route charts provide the information necessary
to safely transit broad regions of airspace, the United States
Terminal Procedures Publication (TPP) enables pilots to
guide their aircraft in the airport area. Whether departing or
arriving, these procedures exist to make the controllers’ and
pilots’ jobs safer and more effi cient. Available in booklets
by region (published by NACG), the TPP includes approach
procedures, STARs, Departure Procedures (DPs), and airport
diagrams.
Departure Procedures (DPs)
There are two types of DPs, Obstacle Departure Procedures
(ODP) and SIDs. Both types of DPs provide
obstacle clearance protection to aircraft in instrument
meteorological conditions (IMC), while reducing
communications and departure delays. DPs are published in
text and/or charted graphic form. Regardless of the format, all
DPs provide a way to depart the airport and transition to the
en route structure safely. When possible, pilots are strongly
encouraged to fi le and fl y a DP at night, during marginal
visual meteorological conditions (VMC) and IMC.
All DPs provide obstacle clearance provided the aircraft
crosses the end of the runway at least 35 feet AGL; climbs
to 400 feet above airport elevation before turning; and climbs
at least 200 feet per nautical mile (FPNM), unless a higher
climb gradient is specifi ed to the assigned altitude. ATC may
vector an aircraft off a previously assigned DP; however, the
200 FPNM or the FPNM specifi ed in the DP is required.
Textual ODPs are listed by city and airport in the IFR
Take-Off Minimums and DPs Section of the TPP. SIDs are
depicted in the TPP following the approach procedures for
the airport.
Standard Terminal Arrival Routes (STARs)
STARs depict prescribed routes to transition the instrument
pilot from the en route structure to a fi x in the terminal area
from which an instrument approach can be conducted. If a
pilot does not have the appropriate STAR, write “No STAR”
in the fl ight plan. However, if the controller is busy, the pilot
might be cleared along the same route and, if necessary,
the controller will have the pilot copy the entire text of the
procedure.
STARs are listed alphabetically at the beginning of the
NACG booklet. Figure 8-9 shows an example of a STAR, and
the legend for STARs and DPs printed in NACG booklets.
Instrument Approach Procedure (IAP)
Charts
The IAP chart provides the method to descend and land safely
in low visibility conditions. The FAA establishes an IAP
after thorough analyses of obstructions, terrain features, and
navigational facilities. Maneuvers, including altitude changes,
course corrections, and other limitations, are prescribed in the
IAP. The approach charts refl ect the criteria associated with
the United States Standard for Terminal Instrument Approach
Procedures (TERPs), which prescribes standardized methods
for use in designing instrument fl ight procedures.
In addition to the NACG, other governmental and corporate
entities produce approach procedures. The United States
military IAPs are established and published by the
Department of Defense and are available to the public
upon request. Special IAPs are approved by the FAA for
individual operators and are not available to the general
public. Foreign country standard IAPs are established and
published according to the individual country’s publication
procedures. The information presented in the following
sections will highlight features of the United States Terminal
Procedures Publications.
The instrument approach chart is divided into six main
sections, which include the margin identification, pilot
briefing (and notes), plan view, profile view, landing
minimums, and airport diagram. An
examination of each section follows.
Margin Identifi cation
The margin identifi cation, at the top and bottom of the chart,
depicts the airport location and procedure identifi cation.
The civil approach plates are organized by city, then airport
name and state. For example, Orlando Executive in Orlando,
Florida is alphabetically listed under “O” for Orlando.
Military approaches are organized by airport name fi rst.
The chart’s amendment status appears below the city and state
in the bottom margin. The amendment number is followed
by the fi ve-digit julian-date of the last chart change.“05300”
is read, “the 300th day of 2005”. At the center of the top
margin is the FAA chart reference number and the approving
authority. At the bottom center, the airport’s latitude and
longitude coordinates are provided.
8-13
Figure 8-8. Obstacle Departure Procedures (ODP) and Standard Instrument Departures (SID).
8-14
Figure 8-9. DP Chart Legend and STAR.
8-15
Figure 8-10. Instrument Approach Chart.
8-16
The procedure chart title (top and bottom margin area of
Figure 8-10) is derived from the type of navigational facility
providing fi nal approach course guidance. A runway number
is listed when the approach course is aligned within 30º of the
runway centerline. This type of approach allows a straightin
landing under the right conditions. The type of approach
followed by a letter identifi es approaches that do not have
straight-in landing minimums. Examples include procedure
titles at the same airport, which have only circling minimums.
The fi rst approach of this type created at the airport will be
labeled with the letter A, and the lettering will continue in
alphabetical order (e.g., “VOR-A or “LDA-B”). The letter
designation signifi es the expectation is for the procedure to
culminate in a circling approach to land. As a general rule,
circling-only approaches are designed for one of the two
following reasons:
• The fi nal approach course alignment with the runway
centerline exceeds 30º.
• The descent gradient is greater than 400 feet per
NM from the FAF to the threshold crossing height
(TCH). When this maximum gradient is exceeded, the
circling-only approach procedure may be designed to
meet the gradient criteria limits.
Further information on this topic can be found in the
Instrument Procedures Handbook, Chapter 5, under Approach
Naming Conventions.
To distinguish between the left, right, and center runways, an
“L,” “R,” or “C” follows the runway number (e.g., “ILS RWY
16R”). In some cases, an airport might have more than one
circling approach, shown as VOR-A, VOR/DME-B, etc.
More than one navigational system separated by a slash
indicates more than one type of equipment is required to
execute the fi nal approach (e.g., VOR/DME RWY 31). More
than one navigational system separated by “or” indicates either
type of equipment may be used to execute the fi nal approach
(e.g., VOR or GPS RWY 15). Multiple approaches of the same
type, to the same runway and using the same guidance, have
an additional letter from the end of the alphabet, number, or
term in the title (e.g., ILS Z RWY 28, SILVER ILS RWY
28, or ILS 2 RWY 28). VOR/DME RNAV approaches are
identifi ed as VOR/DME RNAV RWY (runway number).
Helicopters have special IAPs, designated with COPTER in
the procedure identifi cation (e.g., COPTER LOC/DME 25L).
Other types of navigation systems may be required to execute
other portions of the approach prior to intercepting the fi nal
approach segment or during the missed approach.
The Pilot Briefi ng
The pilot briefi ng is located at the top of the chart and
provides the pilot with information required to complete the
published approach procedure. Included in the pilot briefi ng
are the NAVAID providing approach guidance, its frequency,
the fi nal approach course, and runway information. A notes
section contains additional procedural information. For
example, a procedural note might indicate restrictions for
circling maneuvers. Some other notes might concern a local
altimeter setting and the resulting change in the minimums.
The use of RADAR may also be noted in this section.
Additional notes may be found in the plan view.
When a triangle containing a “T” ( ) appears in the notes
section, it signifi es the airport has nonstandard IFR takeoff
minimums. Pilots should refer to the DPs section of the TPP
to determine takeoff minimums.
When a triangle containing an “A” ( ) appears in the notes
section, it signifi es the airport has nonstandard IFR alternate
minimums. Civil pilots should refer to the Alternate Minimums
Section of the TPP to determine alternate minimums. Military
pilots should refer to appropriate regulations.
When a triangle containing an “A” NA ( ) appears in
the notes area, it signifi es that Alternate Minimums are Not
Authorized due to unmonitored facility or the absence of
weather reporting service.
Communication frequencies are listed in the order in which
they would be used during the approach. Frequencies for
weather and related facilities are included, where applicable,
such as automatic terminal information service (ATIS),
automated surface observing system (ASOS), automated
weather observation system (AWOS), and AFSSs.
The Plan View
The plan view provides a graphical overhead view of the
procedure, and depicts the routes that guide the pilot from
the en route segments to the initial approach fi x (IAF).
During the initial approach, the aircraft has
departed the en route phase of fl ight and is maneuvering
to enter an intermediate or fi nal segment of the instrument
approach. An initial approach can be made along prescribed
routes within the terminal area, which may be along an arc,
radial, course, heading, radar vector, or a combination thereof.
Procedure turns and high altitude teardrop penetrations
are initial approach segments. Features of the plan view,
including the procedure turn, obstacle elevation, minimum
safe altitude (MSA), and procedure track, are depicted in
Figure 8-11. Terrain will be depicted in the plan view portion
of all IAPs if the terrain within the plan view exceeds 4,000
feet above the airport elevation, or if within a 6 nautical mile
radius of the airport reference point the terrain rises at least
2,000 feet above the airport elevation.
8-17
Figure 8-11. IAP Plan View and Symbol Legends.
8-18
Some NACG charts contain a reference or distance circle
with a specifi ed radius (10 NM is most common). Normally,
approach features within the plan view are shown to scale;
however, only the data within the reference circle is always
drawn to scale.
Concentric dashed circles, or concentric rings around the
distance circle, are used when the information necessary to
the procedure will not fi t to scale within the limits of the plan
view area. They serve as a means to systematically arrange
this information in its relative position outside and beyond
the reference circle. These concentric rings are labeled en
route facilities and feeder facilities.
The primary airport depicted in the plan view is drawn with
enough detail to show the runway orientation and fi nal
approach course alignment. Airports other than the primary
approach airport are not normally depicted in the NACG
plan view.
Known spot elevations are indicated on the plan view with a
dot in MSL altitude. The largest dot and number combination
indicates the highest elevation. An inverted “V” with a dot
in the center depicts an obstacle. The highest obstacle is
indicated with a bolder, larger version of the same symbol.
The MSA circle appears in the plan view, except in approaches
for which the Terminal Arrival Area (TAA) format is used or
appropriate NAVAIDs (e.g., VOR or NDB)
are unavailable. The MSA is provided for
emergency purposes only and guarantees
1,000 feet obstruction clearance in the sector
indicated with reference to the bearings in the
circle. For conventional navigation systems,
the MSA is normally based on the primary omnidirectional
facility (NAVAID) on which the IAP is predicated. The MSA
depiction on the approach chart contains the facility identifi er
of the NAVAID used to determine the MSA altitudes. For
RNAV approaches, the MSA is based on the runway waypoint
for straight-in approaches, or the airport waypoint for circling
approaches. For GPS approaches, the MSA center header will
be the missed approach waypoint. The MSL altitudes appear
in boxes within the circle, which is typically a 25 NM radius
unless otherwise indicated. The MSA circle header refers to
the letter identifi er of the NAVAID or waypoint that describes
the center of the circle.
NAVAIDs necessary for the completion of the instrument
procedure include the facility name, letter identifi er, and
Morse code sequence. They may also furnish the frequency,
Morse code, and channel. A heavy-lined NAVAID box depicts
the primary NAVAID used for the approach. An “I” in front
of the NAVAID identifi er (in Figure 8-11, “I-AVL”) listed in
the NAVAID box indicates a localizer. The requirement for
an ADF, DME, or RADAR in the approach is noted in the
plan view.
Intersections, fi xes, radials, and course lines describe route
and approach sequencing information. The main procedure
or fi nal approach course is a thick, solid line. A
DME arc, which is part of the main procedure course, is
also represented as a thick, solid line. A feeder
route is depicted with a medium line and provides
heading, altitude, and distance information. (All three
components must be designated on the chart to provide a
navigable course.) Radials, such as lead radials, are shown
by thin lines. The missed approach track is drawn
using a thin, hash marked line with a directional arrow.
A visual fl ight path segment
appears as a thick dashed line with a directional arrow.
IAFs are charted IAF when associated with
a NAVAID or when freestanding.
The missed approach holding pattern track is represented with
a thin-dashed line. When collocated, the missed approach
holding pattern and procedure turn holding pattern are
indicated as a solid, black line. Arrival holding patterns are
depicted as thin, solid lines.
Terminal Arrival Area (TAA)
The design objective of the TAA procedure is to provide
a transition method for arriving aircraft with GPS/RNAV
equipment. TAAs will also eliminate or reduce the need
for feeder routes, departure extensions, and procedure
turns or course reversal. The TAA is controlled airspace
established in conjunction with the standard or modifi ed
RNAV approach confi gurations.
The standard TAA has three areas: straight-in, left base, and
right base. The arc boundaries of the three areas of the TAA
are published portions of the approach and allow aircraft to
transition from the en route structure direct to the nearest
IAF. When crossing the boundary of each of these areas or
when released by ATC within the area, the pilot is expected
to proceed direct to the appropriate waypoint IAF for the
approach area being fl own. A pilot has the option in all areas
of proceeding directly to the holding pattern.
The TAA has a “T” structure that normally provides a NoPT
for aircraft using the approach. The TAA
provides the pilot and air traffi c controller with an effi cient
method for routing traffi c from the en route to the terminal
structure. The basic “T” contained in the TAA normally
aligns the procedure on runway centerline, with the missed
8-19
Figure 8-12. Basic “T” Design of Terminal Arrival Area (TAA) and Legend.
8-20
Figure 8-13. 45° Procedure Turn. Figure 8-14. Holding in Lieu of Procedure Turn.
approach point (MAP) located at the threshold, the FAF 5
NM from the threshold, and the intermediate fi x (IF) 5 NM
from the FAF.
In order to accommodate descent from a high en route altitude
to the initial segment altitude, a hold in lieu of a procedure
turn provides the aircraft with an extended distance for the
necessary descent gradient. The holding pattern constructed
for this purpose is always established on the center IAF
waypoint. Other modifi cations may be required for parallel
runways, or special operational requirements. When
published, the RNAV chart will depict the TAA through
the use of icons representing each TAA associated with the
RNAV procedure. These icons are depicted in the plan view
of the approach, generally arranged on the chart in accordance
with their position relative to the aircraft’s arrival from the
en route structure.
Course Reversal Elements in Plan View and
Profi le View
Course reversals included in an IAP are depicted in one of
three different ways: a 45°/180° procedure turn, a holding
pattern in lieu of procedure turn, or a teardrop procedure.
The maneuvers are required when it is necessary to reverse
direction to establish the aircraft inbound on an intermediate
or final approach course. Components of the required
procedure are depicted in the plan view and the profi le view.
The maneuver must be completed within the distance and
at the minimum altitude specifi ed in the profi le view. Pilots
should coordinate with the appropriate ATC facility relating
to course reversal during the IAP.
Procedure Turns
A procedure turn barbed arrow indicates
the direction or side of the outbound course on which
the procedure turn is made. Headings are
provided for course reversal using the 45° procedure turn.
However, the point at which the turn may be commenced,
and the type and rate of turn is left to the discretion of the
pilot. Some of the options are the 45° procedure turn, the
racetrack pattern, the teardrop procedure turn, or the 80°/260°
course reversal. The absence of the procedure turn barbed
arrow in the plan view indicates that a procedure turn is
not authorized for that procedure. A maximum procedure
turn speed of not greater than 200 knots indicated airspeed
(KIAS) should be observed when turning outbound over the
IAF and throughout the procedure turn maneuver to ensure
staying within the obstruction clearance area. The normal
procedure turn distance is 10 NM. This may be reduced to
a minimum of 5 NM where only Category A or helicopter
aircraft are operated, or increased to as much as 15 NM to
accommodate high performance aircraft. Descent below the
procedure turn altitude begins after the aircraft is established
on the inbound course.
The procedure turn is not required when the symbol “NoPT”
appears, when radar vectoring to the final approach is
provided, when conducting a timed approach, or when the
procedure turn is not authorized. Pilots should contact the
appropriate ATC facility when in doubt if a procedure turn
is required.
Holding in Lieu of Procedure Turn
A holding pattern in lieu of a procedure turn may be specifi ed
for course reversal in some procedures. In such
cases, the holding pattern is established over an intermediate
fi x or a fi nal approach fi x (FAF). The holding pattern distance
8-21
Figure 8-15. Teardrop Procedure.
or time specifi ed in the profi le view must be observed.
Maximum holding airspeed limitations as set forth for all
holding patterns apply. The holding pattern maneuver is
completed when the aircraft is established on the inbound
course after executing the appropriate entry. If cleared for
the approach prior to returning to the holding fi x and the
aircraft is at the prescribed altitude, additional circuits of the
holding pattern are neither necessary nor expected by ATC.
If pilots elect to make additional circuits to lose excessive
altitude or to become better established on course, it is their
responsibility to advise ATC upon receipt of their approach
clearance. When holding in lieu of a procedure turn, the
holding pattern must be followed, except when RADAR
VECTORING to the fi nal approach course is provided or
when NoPT is shown on the approach course.
Teardrop Procedure
When a teardrop procedure turn is depicted and a course
reversal is required, unless otherwise authorized by ATC,
this type of procedure must be executed. The
teardrop procedure consists of departure from an IAF on the
published outbound course followed by a turn toward and
intercepting the inbound course at or prior to the intermediate
fi x or point. Its purpose is to permit an aircraft to reverse
direction and lose considerable altitude within reasonably
limited airspace. Where no fi x is available to mark the
beginning of the intermediate segment, it shall be assumed
to commence at a point 10 NM prior to the FAF. When the
facility is located on the airport, an aircraft is considered to
be on fi nal approach upon completion of the penetration turn.
However, the fi nal approach segment begins on the fi nal
approach course 10 NM from the facility.
The Profi le View
The profi le view is a depiction of the procedure from the side
and illustrates the vertical approach path altitudes, headings,
distances, and fi xes. The
view includes the minimum altitude and the maximum
distance for the procedure turn, altitudes over prescribed
fi xes, distances between fi xes, and the missed approach
procedure. The profi le view aids in the pilot’s interpretation
of the IAP. The profile view is not drawn to scale.
The precision approach glide slope (GS) intercept altitude
is a minimum altitude for GS interception after completion
of the procedure turn, illustrated by an altitude number and
“zigzag” line. It applies to precision approaches, and except
where otherwise prescribed, also applies as a minimum
altitude for crossing the FAF when the GS is inoperative
or not used. Precision approach profi les also depict the GS
angle of descent, threshold-crossing height (TCH), and GS
altitude at the outer marker (OM).
For nonprecision approaches, a fi nal descent is initiated and
the fi nal segment begins at either the FAF or the fi nal approach
point (FAP). The FAF is identifi ed by use of the Maltese cross
symbol in the profi le view. When no FAF
is depicted, the fi nal approach point is the point at which the
aircraft is established inbound on the fi nal approach course.
Stepdown fi xes in nonprecision procedures are provided
between the FAF and the airport for authorizing a lower
minimum descent altitude (MDA) after passing an
obstruction. Stepdown fi xes can be identifi ed by NAVAID,
NAVAID fi x, waypoint or radar, and are depicted by a hash
marked line. Normally, there is only one stepdown fi x
between the FAF and the MAP, but there can be several.
If the stepdown fi x cannot be identifi ed for any reason, the
minimum altitude at the stepdown fi x becomes the MDA for
the approach. However, circling minimums apply if they are
higher than the stepdown fi x minimum altitude, and a circling
approach is required.
The visual descent point (VDP) is a defined point on
the final approach course of a nonprecision straight-in
approach procedure. A normal descent from the MDA to
the runway touchdown point may be commenced, provided
visual reference is established. The VDP is identifi ed on
the profi le view of the approach chart by the symbol “V.”
The MAP varies depending upon the approach fl own. For
the ILS, the MAP is at the decision altitude/decision height
(DA/DH). For nonprecision procedures, the pilot determines
8-22
Figure 8-16. More IAP Profi le View Features.
8-23
Figure 8-17. Vertical Descent Angle (VDA).
the MAP by timing from FAF when the approach aid is away
from the airport, by a fi x or NAVAID when the navigation
facility is located on the fi eld, or by waypoints as defi ned
by GPS or VOR/DME RNAV. The pilot may execute the
MAP early, but pilots should, unless otherwise cleared by
ATC, fl y the IAP as specifi ed on the approach plate to the
MAP at or above the MDA or DA/DH before executing a
turning maneuver.
A complete description of the missed approach procedure
appears in the pilot briefi ng section. Icons
indicating what is to be accomplished at the MAP are located
in the profi le view. When initiating a missed approach, the
pilot will be directed to climb straight ahead (e.g., “Climb to
2,000”) or commence a turning climb to a specifi ed altitude
(e.g., “Climbing right turn to 2,000”). In some cases, the
procedure will direct the pilot to climb straight ahead to
an initial altitude, then turn or enter a climbing turn to the
holding altitude (e.g., “Climb to 900, then climbing right turn
to 2,500 direct ABC VOR and hold”).
When the missed approach procedure specifi es holding at
a facility or fi x, the pilot proceeds according to the missed
approach track and pattern depicted on the plan view. An
alternate missed approach procedure may also be issued by
ATC. The textual description will also specify the NAVAID(s)
or radials that identify the holding fi x.
The profile view also depicts minimum, maximum,
recommended, and mandatory block altitudes used in
approaches. The minimum altitude is depicted with the altitude
underscored. On fi nal approach, aircraft are required
to maintain an altitude at or above the depicted altitude until
reaching the subsequent fi x. The maximum altitude will be
depicted with the altitude overscored, and aircraft
must remain at or below the depicted altitude. Mandatory
altitudes will be depicted with the altitude both underscored
and overscored, and altitude is to be maintained at the
depicted value. Recommended altitudes are advisory altitudes
and are neither over- nor underscored. When an over- or
underscore spans two numbers, a mandatory block altitude is
indicated, and aircraft are required to maintain altitude within
the range of the two numbers.
The Vertical Descent Angle (VDA) found on nonprecision
approach charts provides the pilot with information required
to establish a stabilized approach descent from the FAF
or stepdown fi x to the threshold crossing height (TCH).
Pilots can use the published angle and
estimated or actual ground speed to fi nd a target rate of descent
using the rate of descent table in the back of the TPP.
Landing Minimums
The minimums section sets forth the lowest altitude and
visibility requirements for the approach, whether precision
or nonprecision, straight-in or circling, or radar vectored.
When a fi x is incorporated in a nonprecision fi nal segment,
two sets of minimums may be published, depending upon
how the fi x can be identifi ed. Two sets of minimums may
also be published when a second altimeter source is used
in the procedure. The minimums ensure that fi nal approach
obstacle clearance is provided from the start of the fi nal
segment to the runway or MAP, whichever occurs last. The
same minimums apply to both day and night operations unless
different minimums are specifi ed in the Notes section of the
pilot briefi ng. Published circling minimums provide obstacle
clearance when pilots remain within the appropriate area of
protection.
Minimums are specified for various aircraft approach
categories based upon a value 1.3 times the stalling speed
of the aircraft in the landing confi guration at maximum
certifi ed gross landing weight. If it is necessary to maneuver
at speeds in excess of the upper limit of a speed range for a
category, the minimums for the next higher category should
be used. For example, an aircraft that falls into category A,
but is circling to land at a speed in excess of 91 knots, should
use approach category B minimums when circling to land.
The minimums for straight-in and circling appear directly
under each aircraft category. When there is
no solid division line between minimums for each category
on the rows for straight-in or circling, the minimums apply
to the two or more categories.
The terms used to describe the minimum approach altitudes
differ between precision and nonprecision approaches.
8-24
Figure 8-18. IAP Profi le Legend.
8-25
Figure 8-19. Descent Rate Table.
8-26
Figure 8-20. Terms/Landing Minima Data.
8-27
Precision approaches use decision height (DH), which
is referenced to the height above threshold elevation
(HAT). Nonprecision approaches use MDA, referenced
to “feet MSL.” The MDA is also referenced to HAT for
straight-in approaches, or height above airport (HAA) for
circling approaches. On NACG charts, the fi gures listed
parenthetically are for military operations and are not used
in civil aviation.
Visibility fi gures are provided in statute miles or runway
visual range (RVR), which is reported in hundreds of feet.
RVR is measured by a transmissometer, which represents the
horizontal distance measured at points along the runway. It
is based on the sighting of either high intensity runway lights
or on the visual contrast of other targets, whichever yields
the greater visual range. RVR is horizontal visual range, not
slant visual range, and is used in lieu of prevailing visibility
in determining minimums for a particular runway. It is
illustrated in hundreds of feet if less than a mile (i.e., “24”
is an RVR of 2,400 feet).
Visibility fi gures are depicted after the DA/DH or MDA in the
minimums section. If visibility in statute miles is indicated,
an altitude number, hyphen, and a whole or fractional
number appear; for example, 530-1, which indicates “530
feet MSL” and 1 statute mile visibility. This is the descent
minimum for the approach. The RVR value is separated
from the minimum altitude with a slash, such as “1065/24,”
which indicates 1,065 feet MSL and an RVR of 2,400 feet.
If RVR is prescribed for the procedure, but not available, a
conversion table is used to provide the equivalent visibility
in this case, of 1/2 statute mile visibility. The
conversion table is also available in the TPP.
When an alternate airport is required, standard IFR alternate
minimums apply. For aircraft other than helicopters, precision
approach procedures require a 600-feet ceiling and two
statute miles visibility; nonprecision approaches require an
800-feet ceiling and two statute miles visibility. Helicopter
alternate minimums are a ceiling that is 200 feet above the
minimum for the approach to be fl own and visibility of at
least one statute mile, but not less than the minimum visibility
for the approach to be fl own. When a black triangle with a
white “A” appears in the notes section of the pilot briefi ng,
it indicates non-standard IFR alternate minimums exist for
the airport. If an “NA” appears after the “A,” then
alternate minimums are not authorized. This information is
found in the beginning of the TPP.
In addition to the COPTER approaches, instrument-equipped
helicopters may fly standard approach procedures. The
required visibility minimum may be reduced to one-half the
published visibility minimum for category A aircraft, but
in no case may it be reduced to less than 1/4 mile or 1,200
feet RVR.
Two terms are specifi c to helicopters. Height above landing
(HAL) means height above a designated helicopter landing
area used for helicopter IAPs. “Point in space approach”
refers to a helicopter IAP to a MAP more than 2,600 feet
from an associated helicopter landing area.
Airport Sketch /Airport Diagram
The airport sketch, located on the bottom right side of the
chart, includes many helpful features. IAPs for some of the
larger airports devote an entire page to an airport diagram.
Airport sketch information concerning runway orientation,
lighting, final approach bearings, airport beacon, and
obstacles all serve to guide the pilot in the fi nal phases of
fl ight. See Figure 8-21 for a legend of airport diagram/airport
sketch features (see also Figure 8-10 for an example of an
airport diagram).
The airport elevation is indicated in a separate box at the
top left of the airport sketch. The touchdown zone elevation
(TDZE), which is the highest elevation within the fi rst 3,000
feet of the runway, is designated at the approach end of the
procedure’s runway.
Beneath the airport sketch is a time and speed table when
applicable. The table provides the distance and the amount
of time required to transit the distance from the FAF to the
MAP for selected groundspeeds.
The approach lighting systems and the visual approach lights
are depicted on the airport sketch. White on black symbols
are used for identifying pilot-controlled lighting (PCL).
Runway lighting aids are also noted (e.g., REIL, HIRL), as
is the runway centerline lighting (RCL).
The airport diagram shows the paved runway confi guration
in solid black, while the taxiways and aprons are shaded
gray. Other runway environment features are shown, such
as the runway identifi cation, dimensions, magnetic heading,
displaced threshold, arresting gear, usable length, and slope.
Inoperative Components
Certain procedures can be fl own with inoperative components.
According to the Inoperative Components Table, for
example, an ILS approach with a malfunctioning Medium
Intensity Approach Lighting System with Runway Alignment
Indicator Lights (MALSR = MALS with RAIL) can be
fl own if the minimum visibility is increased by 1/4 mile.
A note in this section might read, “Inoperative
Table does not apply to ALS or HIRL Runway 13L.”
8-28
Figure 8-21. Airport Legend and Diagram.
8-29
Figure 8-22. Approach Lighting Legend.
8-30
Figure 8-23. IAP Inoperative Components Table.
8-31
Figure 8-24. RNAV Instrument Approach Charts.
8-32
RNAV Instrument Approach Charts
To avoid unnecessary duplication and proliferation of
approach charts, approach minimums for unaugmented
GPS, Wide Area Augmentation System (WAAS), Local
Area Augmentation System (LAAS), will be published
on the same approach chart as lateral navigation/vertical
navigation (LNAV/VNAV). Other types of equipment may
be authorized to conduct the approach based on the minima
notes in the front of the TPP approach chart books. Approach
charts titled “RNAV RWY XX” may be used by aircraft
with navigation systems that meet the required navigational
performance (RNP) values for each segment of the approach.
The chart may contain as many as four lines of approach
minimums: global landing system (GLS), WAAS and LAAS,
LNAV/VNAV, LNAV, and circling. LNAV/VNAV is an
instrument approach with lateral and vertical guidance with
integrity limits similar to barometric vertical navigation
(BARO VNAV).
RNAV procedures that incorporate a fi nal approach stepdown
fi x may be published without vertical navigation on a separate
chart also titled RNAV. During a transition period when GPS
procedures are undergoing revision to a new title, both RNAV
and GPS approach charts and formats will be published. ATC
clearance for the RNAV procedure will authorize a properly
certifi cated pilot to utilize any landing minimums for which
the aircraft is certifi ed.
Chart terminology will change slightly to support the new
procedure types:
1. DA replaces the term DH. DA conforms to the
international convention where altitudes relate to
MSL and heights relate to AGL. DA will eventually
be published for other types of IAPs with vertical
guidance, as well. DA indicates to the pilot that the
published descent profi le is fl own to the DA (MSL),
where a missed approach will be initiated if visual
references for landing are not established. Obstacle
clearance is provided to allow a momentary descent
below DA while transitioning from the fi nal approach to
the missed approach. The aircraft is expected to follow
the missed approach instructions while continuing
along the published fi nal approach course to at least
the published runway threshold waypoint or MAP (if
not at the threshold) before executing any turns.
2. MDA will continue to be used only for the LNAV and
circling procedures.
3. Threshold crossing height (TCH) has been traditionally
used in precision approaches as the height of the GS
above threshold. With publication of LNAV/VNAV
minimums and RNAV descent angles, including
graphically depicted descent profiles, TCH also
applies to the height of the “descent angle,” or glide
path, at the threshold. Unless otherwise required for
larger type aircraft, which may be using the IAP, the
typical TCH will be 30 to 50 feet.
The minima format changes slightly:
1. Each line of minima on the RNAV IAP will be titled
to refl ect the RNAV system applicable (e.g., GLS,
LNAV/VNAV, and LNAV). Circling minima will
also be provided.
2. The minima title box will also indicate the nature of
the minimum altitude for the IAP. For example: DA
will be published next to the minima line title for
minimums supporting vertical guidance, and MDA
will be published where the minima line supports only
lateral guidance. During an approach where an MDA
is used, descent below MDA is not authorized.
3. Where two or more systems share the same minima,
each line of minima will be displayed separately.
For more information concerning government charts, the
NACG can be contacted by telephone, or via their internet
address at:
National Aeronautical Charting Group
Telephone 800-626-3677
http://naco.faa.gov/
9-1
The Air Traffi c
Control System
Introduction
This chapter covers the communication equipment,
communication procedures, and air traffi c control (ATC)
facilities and services available for a fl ight under instrument
fl ight rules (IFR) in the National Airspace System (NAS).
Chapter 9
9-2
Figure 9-2. Audio Panel.
Figure 9-1. Typical NAV/COM Installation.
Communication Equipment
Navigation/Communication (NAV/COM)
Equipment
Civilian pilots communicate with ATC on frequencies in
the very high frequency (VHF) range between 118.000 and
136.975 MHz. To derive full benefi t from the ATC system,
radios capable of 25 kHz spacing are required (e.g., 134.500,
134.575, 134.600). If ATC assigns a frequency that cannot
be selected, ask for an alternative frequency.
Figure 9-1 illustrates a typical radio panel installation,
consisting of a communications transceiver on the left and a
navigational receiver on the right. Many radios allow the pilot
to have one or more frequencies stored in memory and one
frequency active for transmitting and receiving (called simplex
operation). It is possible to communicate with some automated
fl ight service stations (AFSS) by transmitting on 122.1 MHz
(selected on the communication radio) and receiving on a
VHF omnidirectional range (VOR) frequency (selected on
the navigation radio). This is called duplex operation.
An audio panel allows a pilot to adjust the volume of the
selected receiver(s) and to select the desired transmitter.
The audio panel has two positions for receiver
selection, cabin speaker, and headphone (some units might
have a center “off” position). Use of a hand-held microphone
and the cabin speaker introduces the distraction of reaching
for and hanging up the microphone. A headset with a boom
microphone is recommended for clear communications. The
microphone should be positioned close to the lips to reduce
9-3
Figure 9-3. Boom Microphone, Headset, and Push-To-Talk
Switch.
Figure 9-4. Combination GPS-Com Unit.
the possibility of ambient fl ight deck noise interfering with
transmissions to the controller. Headphones deliver the
received signal directly to the ears; therefore, ambient noise
does not interfere with the pilot’s ability to understand the
transmission.
Switching the transmitter selector between COM1 and
COM2 changes both transmitter and receiver frequencies.
It is necessary only when a pilot wants to monitor one
frequency while transmitting on another. One example is
listening to automatic terminal information service (ATIS)
on one receiver while communicating with ATC on the
other. Monitoring a navigation receiver to check for proper
identifi cation is another reason to use the switch panel.
Most audio switch panels also include a marker beacon
receiver. All marker beacons transmit on 75 MHz, so there
is no frequency selector.
Figure 9-4 illustrates an increasingly popular form of
NAV/COM radio; it contains a global positioning system
(GPS) receiver and a communications transceiver. Using its
navigational capability, this unit can determine when a fl ight
crosses an airspace boundary or fi x and can automatically
select the appropriate communications frequency for that
location in the communications radio.
Radar and Transponders
ATC radars have a limited ability to display primary returns,
which is energy refl ected from an aircraft’s metallic structure.
Their ability to display secondary returns (transponder replies
to ground interrogation signals) makes possible the many
advantages of automation.
A transponder is a radar beacon transmitter/receiver installed
in the instrument panel. ATC beacon transmitters send out
interrogation signals continuously as the radar antenna
rotates. When an interrogation is received by a transponder, a
coded reply is sent to the ground station where it is displayed
on the controller’s scope. A reply light on the transponder
panel fl ickers every time it receives and replies to a radar
interrogation. Transponder codes are assigned by ATC.
When a controller asks a pilot to “ident” and the ident button
is pushed, the return on the controller’s scope is intensifi ed for
precise identifi cation of a fl ight. When requested, briefl y push
the ident button to activate this feature. It is good practice
for pilots to verbally confi rm that they have changed codes
or pushed the ident button.
Mode C (Altitude Reporting)
Primary radar returns indicate only range and bearing from
the radar antenna to the target; secondary radar returns can
display altitude, Mode C, on the control scope if the aircraft
is equipped with an encoding altimeter or blind encoder. In
either case, when the transponder’s function switch is in the
ALT position the aircraft’s pressure altitude is sent to the
controller. Adjusting the altimeter’s Kollsman window has
no effect on the altitude read by the controller.
Transponders, when installed, must be ON at all times when
operating in controlled airspace; altitude reporting is required
by regulation in Class B and Class C airspace and inside a
30-mile circle surrounding the primary airport in Class B
airspace. Altitude reporting should also be ON at all times.
9-4
Figure 9-5. Phonetic Pronunciation Guide.
Communication Procedures
Clarity in communication is essential for a safe instrument
fl ight. This requires pilots and controllers to use terms that
are understood by both—the Pilot/Controller Glossary in the
Aeronautical Information Manual (AIM) is the best source of
terms and defi nitions. The AIM is revised twice a year and
new defi nitions are added, so the glossary should be reviewed
frequently. Because clearances and instructions are comprised
largely of letters and numbers, a phonetic pronunciation guide
has been developed for both.
ATCs must follow the guidance of the Air Traffi c Control
Manual when communicating with pilots. The manual
presents the controller with different situations and prescribes
precise terminology that must be used. This is advantageous
for pilots because once they have recognized a pattern
or format they can expect future controller transmissions
to follow that format. Controllers are faced with a wide
variety of communication styles based on pilot experience,
profi ciency, and professionalism.
Pilots should study the examples in the AIM, listen to
other pilots communicate, and apply the lessons learned
to their own communications with ATC. Pilots should ask
for clarifi cation of a clearance or instruction. If necessary,
use plain English to ensure understanding, and expect the
controller to reply in the same way. A safe instrument fl ight
is the result of cooperation between controller and pilot.
Communication Facilities
The controller’s primary responsibility is separation of
aircraft operating under IFR. This is accomplished with ATC
facilities which include the AFSS, airport traffi c control tower
(ATCT), terminal radar approach control (TRACON), and
air route traffi c control center (ARTCC).
Automated Flight Service Stations (AFSS)
A pilot’s fi rst contact with ATC is usually through AFSS,
either by radio or telephone. AFSSs provide pilot briefi ngs,
receive and process fl ight plans, relay ATC clearances,
originate Notices to Airmen (NOTAMs), and broadcast
aviation weather. Some facilities provide En Route Flight
Advisory Service (EFAS), take weather observations,
and advise United States Customs and Immigration of
international fl ights.
Telephone contact with Flight Service can be obtained
by dialing 1-800-WX-BRIEF. This number can be used
anywhere in the United States and connects to the nearest
AFSS based on the area code from which the call originates.
There are a variety of methods of making radio contact:
direct transmission, remote communication outlets (RCOs),
ground communication outlets (GCOs), and by using duplex
transmissions through navigational aids (NAVAIDs). The
best source of information on frequency usage is the Airport/
Facility Directory (A/FD) and the legend panel on sectional
charts also contains contact information.
9-5
Figure 9-6. Flight Strip.
The briefer sends a flight plan to the host computer at
the ARTCC (Center). After processing the flight plan,
the computer will send fl ight strips to the tower, to the
radar facility that will handle the departure route, and to
the Center controller whose sector the fl ight fi rst enters.
Figure 9-6 shows a typical strip. These strips are delivered
approximately 30 minutes prior to the proposed departure
time. Strips are delivered to en route facilities 30 minutes
before the fl ight is expected to enter their airspace. If a
fl ight plan is not opened, it will “time out” 2 hours after the
proposed departure time.
When departing an airport in Class G airspace, a pilot receives
an IFR clearance from the AFSS by radio or telephone. It
contains either a clearance void time, in which case an aircraft
must be airborne prior to that time, or a release time. Pilots
should not take-off prior to the release time. Pilots can help
the controller by stating how soon they expect to be airborne.
If the void time is, for example, 10 minutes past the hour and
an aircraft is airborne at exactly 10 minutes past the hour,
the clearance is void—a pilot must take off prior to the void
time. A specifi c void time may be requested when fi ling a
fl ight plan.
ATC Towers
Several controllers in the tower cab are involved in handling
an instrument fl ight. Where there is a dedicated clearance
delivery position, that frequency is found in the A/FD and
on the instrument approach chart for the departure airport.
Where there is no clearance delivery position, the ground
controller performs this function. At the busiest airports, pretaxi
clearance is required; the frequency for pre-taxi clearance
can be found in the A/FD. Taxi clearance should be requested
not more than 10 minutes before proposed taxi time.
It is recommended that pilots read their IFR clearance back to
the clearance delivery controller. Instrument clearances can
be overwhelming when attempting to copy them verbatim,
but they follow a format that allows a pilot to be prepared
when responding “Ready to copy.” The format is: clearance
limit (usually the destination airport); route, including any
departure procedure; initial altitude; frequency (for departure
control); and transponder code. With the exception of the
transponder code, a pilot knows most of these items before
engine start. One technique for clearance copying is writing
C-R-A-F-T.
Assume an IFR fl ight plan has been fi led from Seattle,
Washington to Sacramento, California via V-23 at 7,000
feet. Traffi c is taking off to the north from Seattle-Tacoma
(Sea-Tac) airport and, by monitoring the clearance delivery
frequency, a pilot can determine the departure procedure
being assigned to southbound fl ights. The clearance limit
is the destination airport, so write “SAC” after the letter C.
Write “SEATTLE TWO – V23” after R for Route, because
departure control issued this departure to other fl ights. Write
“7” after the A, the departure control frequency printed on
the approach charts for Sea-Tac after F, and leave the space
after the letter T blank—the transponder code is generated by
computer and can seldom be determined in advance. Then,
call clearance delivery and report “Ready to copy.”
As the controller reads the clearance, check it against what
is already written down; if there is a change, draw a line
through that item and write in the changed item. Chances
are the changes are minimal, and most of the clearance is
copied before keying the microphone. Still, it is worthwhile
to develop clearance shorthand to decrease the verbiage that
must be copied (see Appendix 1).
Pilots are required to have either the text of a departure
procedure (DP) or a graphic representation (if one is
available), and should review it before accepting a clearance.
This is another reason to fi nd out ahead of time which DP is
in use. If the DP includes an altitude or a departure control
frequency, those items are not included in the clearance.
The last clearance received supersedes all previous clearances.
For example, if the DP says “Climb and maintain 2,000 feet,
expect higher in 6 miles,” but upon contacting the departure
controller a new clearance is received: “Climb and maintain
8,000 feet,” the 2,000 feet restriction has been canceled. This
rule applies in both terminal and Center airspace.
9-6
Figure 9-7. Combined Radar and Beacon Antenna. Figure 9-8. Minimum Vectoring Altitude Chart.
When reporting ready to copy an IFR clearance before the
strip has been received from the Center computer, pilots
are advised “clearance on request.” The controller initiates
contact when it has been received. This time can be used for
taxi and pre-takeoff checks.
The local controller is responsible for operations in the Class
D airspace and on the active runways. At some towers,
designated as IFR towers, the local controller has vectoring
authority. At visual fl ight rules (VFR) towers, the local
controller accepts inbound IFR fl ights from the terminal radar
facility and cannot provide vectors. The local controller also
coordinates fl ights in the local area with radar controllers.
Although Class D airspace normally extends 2,500 feet above
fi eld elevation, towers frequently release the top 500 feet to
the radar controllers to facilitate overfl ights. Accordingly,
when a fl ight is vectored over an airport at an altitude that
appears to enter the tower controller’s airspace, there is no
need to contact the tower controller—all coordination is
handled by ATC.
The departure radar controller may be in the same building
as the control tower, but it is more likely that the departure
radar position is remotely located. The tower controller will
not issue a takeoff clearance until the departure controller
issues a release.
Terminal Radar Approach Control (TRACON)
TRACONs are considered terminal facilities because they
provide the link between the departure airport and the en route
structure of the NAS. Terminal airspace normally extends 30
nautical miles (NM) from the facility, with a vertical extent
of 10,000 feet; however, dimensions vary widely. Class B
and Class C airspace dimensions are provided on aeronautical
charts. At terminal radar facilities the airspace is divided
into sectors, each with one or more controllers, and each
sector is assigned a discrete radio frequency. All terminal
facilities are approach controls and should be addressed
as “Approach” except when directed to do otherwise (e.g.,
“Contact departure on 120.4”).
Terminal radar antennas are located on or adjacent to the
airport. Figure 9-7 shows a typical confi guration. Terminal
controllers can assign altitudes lower than published
procedural altitudes called minimum vectoring altitudes
(MVAs). These altitudes are not published or accessible to
pilots, but are displayed at the controller’s position, as shown
in Figure 9-8. However, when pilots are assigned an altitude
that seems to be too low, they should query the controller
before descending.
When a pilot accepts a clearance and reports ready for takeoff,
a controller in the tower contacts the TRACON for a release.
An aircraft is not cleared for takeoff until the departure
controller can fi t the fl ight into the departure fl ow. A pilot may
have to hold for release. When takeoff clearance is received,
the departure controller is aware of the fl ight and is waiting
for a call. All of the information the controller needs is on
the departure strip or the computer screen there is no need to
repeat any portion of the clearance to that controller. Simply
establish contact with the facility when instructed to do so by
the tower controller. The terminal facility computer picks up
the transponder and initiates tracking as soon as it detects the
9-7
assigned code. For this reason, the transponder should remain
on standby until takeoff clearance has been received.
The aircraft appears on the controller’s radar display as a
target with an associated data block that moves as the aircraft
moves through the airspace. The data block includes aircraft
identifi cation, aircraft type, altitude, and airspeed.
A TRACON controller uses Airport Surveillance Radar
(ASR) to detect primary targets and Automated Radar
Terminal Systems (ARTS) to receive transponder signals; the
two are combined on the controller’s scope.
At facilities with ASR-3 equipment, radar returns from
precipitation are not displayed as varying levels of intensity,
and controllers must rely on pilot reports and experience
to provide weather avoidance information. With ASR-9
equipment, the controller can select up to six levels of
intensity. Light precipitation does not require avoidance
tactics but precipitation levels of moderate, heavy or
extreme should cause pilots to plan accordingly. Along
with precipitation the pilot must additionally consider the
temperature, which if between -20° and +5° C will cause icing
even during light precipitation. The returns from higher levels
of intensity may obscure aircraft data blocks, and controllers
may select the higher levels only on pilot request. When
uncertainty exists about the weather ahead, ask the controller
if the facility can display intensity levels—pilots of small
aircraft should avoid intensity levels 3 or higher.
Tower En Route Control (TEC)
At many locations, instrument fl ights can be conducted
entirely in terminal airspace. These TEC routes are generally
for aircraft operating below 10,000 feet, and they can be
found in the A/FD. Pilots desiring to use TEC should include
that designation in the remarks section of the fl ight plan.
Pilots are not limited to the major airports at the city pairs
listed in the A/FD. For example, a tower en route fl ight from
an airport in New York (NYC) airspace could terminate
at any airport within approximately 30 miles of Bradley
International (BDL) airspace, such as Hartford (HFD).
A valuable service provided by the automated radar
equipment at terminal radar facilities is the Minimum Safe
Altitude Warnings (MSAW). This equipment predicts an
aircraft’s position in 2 minutes based on present path of
fl ight—the controller issues a safety alert if the projected
path encounters terrain or an obstruction. An unusually
rapid descent rate on a nonprecision approach can trigger
such an alert.
Air Route Traffi c Control Center (ARTCC)
ARTCC facilities are responsible for maintaining separation
between IFR fl ights in the en route structure. Center radars
(Air Route Surveillance Radar (ARSR)) acquire and track
transponder returns using the same basic technology as
terminal radars.
Earlier Center radars display weather as an area of slashes
(light precipitation) and Hs (moderate rainfall), as illustrated
in Figure 9-12. Because the controller cannot detect higher
levels of precipitation, pilots should be wary of areas showing
moderate rainfall. Newer radar displays show weather as
three levels of blue. Controllers can select the level of weather
to be displayed. Weather displays of higher levels of intensity
can make it diffi cult for controllers to see aircraft data blocks,
so pilots should not expect ATC to keep weather displayed
continuously.
Center airspace is divided into sectors in the same manner
as terminal airspace; additionally, most Center airspace is
divided by altitudes into high and low sectors. Each sector
has a dedicated team of controllers and a selection of radio
frequencies, because each Center has a network of remote
transmitter/receiver sites. All Center frequencies can be found
in the back of the A/FD in the format shown in Figure 9-13;
they are also found on en route charts.
Each ARTCC’s area of responsibility covers several states;
when fl ying from the vicinity of one remote communication
site toward another, expect to hear the same controller on
different frequencies.
Center Approach/Departure Control
The majority of airports with instrument approaches do not
lie within terminal radar airspace, and when operating to
or from these airports pilots communicate directly with the
Center controller. Departing from a tower-controlled airport,
the tower controller provides instructions for contacting the
appropriate Center controller. When departing an airport
without an operating control tower, the clearance includes
instructions such as “Upon entering controlled airspace,
contact Houston Center on 126.5.” Pilots are responsible
for terrain clearance until reaching the controller’s MVA.
Simply hearing “Radar contact” does not relieve a pilot of
this responsibility.
If obstacles in the departure path require a steeper-thanstandard
climb gradient (200 FPNM), then the controller
advises the pilot. However, it is the pilot’s responsibility to
check the departure airport listing in the A/FD to determine if
there are trees or wires in the departure path. When in doubt,
ask the controller for the required climb gradient.
9-8
Figure 9-9. The top image is a display as seen by controllers in an Air Traffi c Facility. The one illustrated is an ARTS III (Automated
Radar Terminal System). The display shown provides an explanation of the symbols in the graphic. The lower fi gure is an example of
the Digital Bright Radar Indicator Tower Equipment (DBRITE) screen as seen by tower personnel. It provides tower controllers with
a visual display of the airport surveillance radar, beacon signals, and data received from ARTS III. The display shown provides an
explanation of the symbols in the graphic.
9-9
Figure 9-10. A Portion of the New York Area Tower En Route List. (From the A/FD)
9-10
Figure 9-11. Center Radar Displays. Figure 9-12. A Center Controller’s Scope.
Figure 9-13. Center Symbology.
A common clearance in these situations is “When able,
proceed direct to the Astoria VOR…” The words “when able”
mean to proceed to the waypoint, intersection, or NAVAID
when the pilot is able to navigate directly to that point using
onboard available systems providing proper guidance, usable
signal, etc. If provided such guidance while fl ying VFR, the
pilot remains responsible for terrain and obstacle clearance.
Using the standard climb gradient, an aircraft is 2 miles
from the departure end of the runway before it is safe to
turn (400 feet above ground level (AGL)). When a Center
controller issues a heading, a direct route, or says “direct
when able,” the controller becomes responsible for terrain
and obstruction clearance.
Another common Center clearance is “Leaving (altitude)
fl y (heading) or proceed direct when able.” This keeps the
terrain/obstruction clearance responsibility in the fl ight deck
until above the minimum IFR altitude. A controller cannot
issue an IFR clearance until an aircraft is above the minimum
IFR altitude unless it is able to climb in VFR conditions.
On a Center controller’s scope, 1 NM is about 1/28 of an inch.
When a Center controller is providing Approach/Departure
control services at an airport many miles from the radar
antenna, estimating headings and distances is very diffi cult.
Controllers providing vectors to fi nal must set the range on
their scopes to not more than 125 NM to provide the greatest
possible accuracy for intercept headings. Accordingly, at
locations more distant from a Center radar antenna, pilots
should expect a minimum of vectoring.
9-11
ATC radar systems cannot detect turbulence. Generally,
turbulence can be expected to occur as the rate of rainfall or
intensity of precipitation increases. Turbulence associated
with greater rates of rainfall/precipitation is normally more
severe than any associated with lesser rates of rainfall/
precipitation. Turbulence should be expected to occur near
convective activity, even in clear air. Thunderstorms are a
form of convective activity that implies severe or greater
turbulence. Operation within 20 miles of thunderstorms
should be approached with great caution, as the severity of
turbulence can be markedly greater than the precipitation
intensity might indicate.
Weather Avoidance Assistance
ATC’s fi rst duty priority is to separate aircraft and issue
safety alerts. ATC provides additional services to the extent
possible, contingent upon higher priority duties and other
factors including limitations of radar, volume of traffi c,
frequency congestion, and workload. Subject to the above
factors/limitations, controllers issue pertinent information
on weather or chaff areas; and if requested, assist pilots, to
the extent possible, in avoiding areas of precipitation. Pilots
should respond to a weather advisory by acknowledging the
advisory and, if desired, requesting an alternate course of
action, such as:
1. Request to deviate off course by stating the direction
and number of degrees or miles needed to deviate from
the original course;
2. Request a change of altitude; or
3. Request routing assistance to avoid the affected
area. Because ATC radar systems cannot detect the
presence or absence of clouds and turbulence, such
assistance conveys no guarantee that the pilot will not
encounter hazards associated with convective activity.
Pilots wishing to circumnavigate precipitation areas
by a specific distance should make their desires
clearly known to ATC at the time of the request for
services. Pilots must advise ATC when they can
resume normal navigation.
IFR pilots shall not deviate from their assigned course or
altitude without an ATC clearance. Plan ahead for possible
course deviations because hazardous convective conditions
can develop quite rapidly. This is important to consider
because the precipitation data displayed on ARTCC radar
scopes can be up to 6 minutes old and thunderstorms can
develop at rates exceeding 6,000 feet per minute (fpm). When
encountering weather conditions that threaten the safety of
the aircraft, the pilot may exercise emergency authority as
ATC Infl ight Weather Avoidance
Assistance
ATC Radar Weather Displays
ATC radar systems are able to display areas of precipitation
by sending out a beam of radio energy that is refl ected back to
the radar antenna when it strikes an object or moisture which
may be in the form of rain drops, hail, or snow. The larger
the object, or the denser its refl ective surface, the stronger the
return will be. Radar weather processors indicate the intensity
of refl ective returns in terms of decibels with respect to the
radar refl ectively factor (dBZ).
ATC systems cannot detect the presence or absence of
clouds. ATC radar systems can often determine the intensity
of a precipitation area, but the specifi c character of that area
(snow, rain, hail, VIRGA, etc.) cannot be determined. For
this reason, ATC refers to all weather areas displayed on
ATC radar scopes as “precipitation.”
All ATC facilities using radar weather processors with the
ability to determine precipitation intensity describes the
intensity to pilots as:
1. “LIGHT” (< 30 dBZ)
2. “MODERATE” (30 to 40 dBZ)
3. “HEAVY” (>40 to 50 dBZ)
4. “EXTREME” (>50 dBZ)
ARTCC controllers do not use the term “LIGHT” because
their systems do not display “LIGHT” precipitation
intensities. ATC facilities that, due to equipment limitations,
cannot display the intensity levels of precipitation, will
describe the location of the precipitation area by geographic
position, or position relative to the aircraft. Since the intensity
level is not available, the controller states, “INTENSITY
UNKNOWN.”
ARTCC facilities normally use a Weather and Radar
Processor (WARP) to display a mosaic of data obtained from
multiple NEXRAD sites. The WARP processor is only used
in ARTCC facilities.
There is a time delay between actual conditions and those
displayed to the controller. For example, the precipitation
data on the ARTCC controller’s display could be up to 6
minutes old. When the WARP is not available, a secondary
system, the narrowband ARSR is utilized. The ARSR system
can display two distinct levels of precipitation intensity that
is described to pilots as “MODERATE” (30 to 40 dBZ) and
“HEAVY to EXTREME” (>40 dBZ).
9-12
Figure 9-14. High Resolution ATC Displays Used in PRM.
stated in 14 CFR part 91, section 91.3 should an immediate
deviation from the assigned clearance be necessary and time
does not permit approval by ATC.
Generally, when weather disrupts the fl ow of air traffi c,
greater workload demands are placed on the controller.
Requests for deviations from course and other services
should be made as far in advance as possible to better assure
the controller’s ability to approve these requests promptly.
When requesting approval to detour around weather activity,
include the following information to facilitate the request:
1. The proposed point where detour commences;
2. The proposed route and extent of detour (direction
and distance);
3. The point where original route will be resumed;
4. Flight conditions (IMC or VMC);
5. Whether the aircraft is equipped with functioning
airborne radar; and
6. Any further deviation that may become necessary.
To a large degree, the assistance that might be rendered
by ATC depends upon the weather information available
to controllers. Due to the extremely transitory nature of
hazardous weather, the controller’s displayed precipitation
information may be of limited value.
Obtaining IFR clearance or approval to circumnavigate
hazardous weather can often be accommodated more readily
in the en route areas away from terminals because there
is usually less congestion and, therefore, greater freedom
of action. In terminal areas, the problem is more acute
because of traffi c density, ATC coordination requirements,
complex departure and arrival routes, and adjacent airports.
As a consequence, controllers are less likely to be able to
accommodate all requests for weather detours in a terminal
area. Nevertheless, pilots should not hesitate to advise
controllers of any observed hazardous weather and should
specifi cally advise controllers if they desire circumnavigation
of observed weather.
Pilot reports (PIREPs) of fl ight conditions help defi ne the
nature and extent of weather conditions in a particular area.
These reports are disseminated by radio and electronic means
to other pilots. Provide PIREP information to ATC regarding
pertinent fl ight conditions, such as:
1. Turbulence;
2. Visibility;
3. Cloud tops and bases; and
4. The presence of hazards such as ice, hail, and
lightning.
Approach Control Facility
An approach control facility is a terminal ATC facility
that provides approach control service in the terminal area.
Services are provided for arriving and departing VFR and
IFR aircraft and, on occasion, en route aircraft. In addition,
for airports with parallel runways with ILS or LDA
approaches, the approach control facility provides monitoring
of the approaches.
Approach Control Advances
Precision Runway Monitor (PRM)
Over the past few years, a new technology has been installed
at airports that permits a decreased separation distance
between parallel runways. The system is called a Precision
Runway Monitor (PRM) and is comprised of high-update
radar, high-resolution ATC displays, and PRM-certifi ed
controllers.
Precision Runway Monitor (PRM) Radar
The PRM uses a Monopulse Secondary Surveillance Radar
(MSSR) that employs electronically scanned antennas.
Because the PRM has no scan rate restrictions, it is capable
of providing a faster update rate (up to 0.5 second) over
conventional systems, thereby providing better target
presentation in terms of accuracy, resolution, and track
prediction. The system is designed to search, track, process,
and display SSR-equipped aircraft within airspace of over
30 miles in range and over 15,000 feet in elevation. Visual
and audible alerts are generated to warn controllers to take
corrective actions.
9-13
Figure 9-15. Aircraft Management Using PRM. (Note the no transgression zone (NTZ) and how the aircraft are separated.)
PRM Benefi ts
Typically, PRM is used with dual approaches with centerlines
separated less than 4,300 feet but not less than 3,000 feet
(under most conditions). Separating the two
fi nal approach courses is a No Transgression Zone (NTZ)
with surveillance of that zone provided by two controllers,
one for each active approach. The system tracking software
provides PRM monitor controllers with aircraft identifi cation,
position, speed, projected position, as well as visual and
aural alerts.
Control Sequence
The IFR system is fl exible and accommodating if pilots do
their homework, have as many frequencies as possible written
down before they are needed, and have an alternate in mind
if the fl ight cannot be completed as planned. Pilots should
familiarize themselves with all the facilities and services
available along the planned route of fl ight.
Always know where the nearest VFR conditions can be
found, and be prepared to head in that direction if the situation
deteriorates.
A typical IFR fl ight, with departure and arrival at airports
with control towers, would use the ATC facilities and services
in the following sequence:
1. AFSS: Obtain a weather briefi ng for a departure,
destination and alternate airports, and en route
conditions, and then file a flight plan by calling
1-800-WX-BRIEF.
2. ATIS: Prefl ight complete, listen for present conditions
and the approach in use.
3. Clearance Delivery: Prior to taxiing, obtain a departure
clearance.
4. Ground Control: Noting that the fl ight is IFR, receive
taxi instructions.
5. Tower: Pre-takeoff checks complete, receive clearance
to takeoff.
6. Departure Control: Once the transponder “tags up”
with the ARTS, the tower controller instructs the pilot
to contact Departure to establish radar contact.
9-14
7. ARTCC: After departing the departure controller’s
airspace, aircraft is handed off to Center, who
coordinates the flight while en route. Pilots may
be in contact with multiple ARTCC facilities; they
coordinate the hand-offs.
8. EFAS/HIWAS: Coordinate with ATC before
leaving their frequency to obtain infl ight weather
information.
9. ATIS: Coordinate with ATC before leaving their
frequency to obtain ATIS information.
10. Approach Control: Center hands off to approach
control where pilots receive additional information
and clearances.
11. Tower: Once cleared for the approach, pilots are
instructed to contact tower control; the fl ight plan is
canceled by the tower controller upon landing.
A typical IFR fl ight, with departure and arrival at airports
without operating control towers, would use the ATC
facilities and services in the following sequence:
1. AFSS: Obtain a weather briefing for departure,
destination, and alternate airports, and en route
conditions, and then file a flight plan by calling
1-800-WX-BRIEF. Provide the latitude/longitude
description for small airports to ensure that Center is
able to locate departure and arrival locations.
2. AFSS or UNICOM: ATC clearances can be fi led and
received on the UNICOM frequency if the licensee
has made arrangements with the controlling ARTCC;
otherwise, fi le with AFSS via telephone. Be sure all
prefl ight preparations are complete before fi ling. The
clearance includes a clearance void time. Pilots must
be airborne prior to the void time.
3. ARTCC: After takeoff, establish contact with Center.
During the flight, pilots may be in contact with
multiple ARTCC facilities; ATC coordinates the handoffs.
4. EFAS/HIWAS: Coordinate with ATC before
leaving their frequency to obtain in-fl ight weather
information.
5. Approach Control: Center hands off to approach
control where pilots receive additional information and
clearances. If a landing under visual meteorological
conditions (VMC) is possible, pilots may cancel their
IFR clearance before landing.
Letters of Agreement (LOA)
The ATC system is indeed a system, and very little happens
by chance. As a fl ight progresses, controllers in adjoining
sectors or adjoining Centers coordinate its handling by
telephone or by computer. Where there is a boundary between
the airspace controlled by different facilities, the location and
altitude for hand-off is determined by Letters of Agreement
(LOA) negotiated between the two facility managers. This
information is not available to pilots in any Federal Aviation
Administration (FAA) publication. For this reason, it is good
practice to note on the en route chart the points at which handoffs
occur. Each time a fl ight is handed-off to a different
facility, the controller knows the altitude and location—this
was part of the hand-off procedure.
9-15
Figure 9-16. ATC Facilities, Services, and Radio Call Signs.
9-16
10-1
Introduction
This chapter is a discussion of conducting a fl ight under
instrument fl ight rules (IFR). It also explains the sources for
fl ight planning, the conditions associated with instrument
fl ight, and the procedures used for each phase of IFR fl ight:
departure, en route, and approach. The chapter concludes
with an example of an IFR fl ight which applies many of the
procedures discussed in the chapter.
IFR Flight
Chapter 10
10-2
Sources of Flight Planning Information
The following resources are available for a pilot planning a
fl ight conducted under instrument fl ight rules (IFR).
National Aeronautical Charting Group (NACG)
publications:
• IFR en route charts
• area charts
• United States (U.S.) Terminal Procedures Publications
(TPP)
The Federal Aviation Administration (FAA) publications:
• AIM
• Airport/Facility Directory (A/FD)
• Notices to Airmen Publication (NTAP) for fl ight
planning in the National Airspace System (NAS)
Pilots should also consult the Pilot’s Operating Handbook/
Airplane Flight Manual (POH/AFM) for fl ight planning
information pertinent to the aircraft to be fl own.
A review of the contents of all the listed publications will help
determine which material should be referenced for each fl ight.
As a pilot becomes more familiar with these publications, the
fl ight planning process becomes quicker and easier.
Aeronautical Information Manual (AIM)
The AIM provides the aviation community with basic
fl ight information and air traffi c control (ATC) procedures
used in the United States NAS. An international version
called the Aeronautical Information Publication contains
parallel information, as well as specifi c information on the
international airports used by the international community.
Airport/Facility Directory (A/FD)
The A/FD contains information on airports, communications,
and navigation aids pertinent to IFR fl ight. It also includes
very-high frequency omnidirectional range (VOR) receiver
checkpoints, automated fl ight service station (AFSS), weather
service telephone numbers, and air route traffi c control center
(ARTCC) frequencies. Various special notices essential
to fl ight are also included, such as land-and-hold-short
operations (LAHSO) data, the civil use of military fi elds,
continuous power facilities, and special fl ight procedures.
In the major terminal and en route environments, preferred
routes have been established to guide pilots in planning their
routes of fl ight, to minimize route changes, and to aid in the
orderly management of air traffi c using the federal airways.
The A/FD lists both high and low altitude preferred routes.
Notices to Airmen Publication (NTAP)
The NTAP is a publication containing current Notices to
Airmen (NOTAMs) which are essential to the safety of fl ight,
as well as supplemental data affecting the other operational
publications listed. It also includes current Flight Data Center
(FDC) NOTAMs, which are regulatory in nature, issued to
establish restrictions to fl ight or to amend charts or published
instrument approach procedures (IAPs).
POH/AFM
The POH/AFM contain operating limitations, performance,
normal and emergency procedures, and a variety of other
operational information for the respective aircraft. Aircraft
manufacturers have done considerable testing to gather and
substantiate the information in the aircraft manual. Pilots should
refer to it for information relevant to a proposed fl ight.
IFR Flight Plan
