- 注册时间
- 2008-9-13
- 最后登录
- 1970-1-1
- 在线时间
- 0 小时
- 阅读权限
- 200
- 积分
- 0
- 帖子
- 24482
- 精华
- 4
- UID
- 9
 
|
|
The angle of bank is indicated by the pointer on the banking
scale at the top of the instrument. [Figure 6-5] 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. [Figure 6-6] 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. [Figure 6-7] When power is reduced to
decrease airspeed, the nose pitches down and yaws to the
left. [Figure 6-8] 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. [Figure 7-1] 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. [Figure 7-2] 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.
[Figure 7-7] 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.
[Figure 7-9]
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. [Figure 7-11] 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. [Figure 7-10] 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. [Figure 7-13] 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.)
[Figure 7-14]
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. [Figure 7-23] 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
[Figure 7-25]:
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.
[Figure 7-26]
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. [Figure 7-27] 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. [Figure 7-28]
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). [Figure 7-29]
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. [Figure 7-30]
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. [Figure 7-31] 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.
[Figures 7-32 and 7-33]
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. [Figure 7-34] 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. [Figure 7-35]
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. [Figure 7-36] 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. [Figure 7-40]
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. [Figure 7-41]
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. [Figure 7-42]
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). [Figure 7-43]
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. [Figure 7-44]
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 |
|
IP卡
狗仔卡
发表于 2008-12-9 15:46:16
显身卡