Staying Within Protected Airspace
B-1At higher altitudes, protected airspace helps to maintain
separation between aircraft. At lower altitudes, protected
airspace also provides separation from terrain or
obstructions. But, what does it mean to be established
on course? How wide is the protected airspace of a particular route? How can you tell from the cockpit whether
your aircraft is nearing the limits of protected airspace?
The intent of this appendix is to answer these questions
and explain the general limits of protected airspace by
means of typical instrument indications.
Some pilots assume that flying to the tolerances set out
in the FAA Instrument Practical Test Standards (PTS)
(http://www.faa.gov/education_research/testing/airmen/test_standards/) will keep them within protected
airspace. As a result, it is important to observe the last
sentence of the following note in the PTS:
“The tolerances stated in this standard are intended to be
used as a measurement of the applicant's ability to operate in the instrument environment. They provide guidance for examiners to use in judging the applicant's
qualifications. The regulations governing the tolerances
for operation under Instrument Flight Rules (IFR) are
established in 14 CFR Part 91.”
The in-flight presentation of course data can vary widely
based upon the selection and distance from a
Navigational Aid (NAVAID) or airfield. Consequently,
you need to understand that in some cases, flying to the
same standards required during your instrument rating
flight test does not necessarily ensure that your aircraft
will remain within protected airspace during IFR operations or that your aircraft will be in a position from
which descent to a landing can be made using normal
maneuvers.
For example, the PTS requires tracking a selected
course, radial, or bearing within 3/4 of full-scale deflection (FSD) of the course deviation indicator (CDI).
Since very high frequency omnidirectional ranges
(VORs) use angular cross track deviation, the 3/4 scale
deflection equates to 7.5 degrees, and means that the aircraft could be as much as 6.7 NM from the centerline
when 51 NM from the VOR station. A VOR receiver is
acceptable for IFR use if it indicates within four degrees
of the reference when checked at a VOR test facility. If
the maximum receiver tolerance is added to the allowable off-course indication, an aircraft could be 11.5
degrees from the centerline, or about 10.4 NM off the
course centerline at 51 NM from the station. The primary protected airspace normally extends only 4 NM to
each side of the centerline of published airways. (This
example does not take into account any misalignment of
the signals transmitted by the VOR.)
Primary Protected
Airspace
11.5 degrees
20 N.M.
Figure B-1. With 3/4 scale CDI deflection, the aircraft could leave primary protected airspace when 20 NM from the
station, assuming the transmitter is accurate and the receiver has a four degree error.
B-2
Lateral guidance is more intuitive with Area Navigation
(RNAV) systems. For basic GPS, the CDI scale uses
linear cross track deviation indications. During
approach operations, a Wide Area Augmentation
System (WAAS) navigation receiver combines the best
of linear and angular deviations resulting in reduced
Flight Technical Error (FTE). For departures, en route,
and terminal operations, WAAS uses a linear deviation
with varying scales. With linear scaling, if the CDI
scaling is at 1 NM, a half scale deflection indicates that
the aircraft is 1/2 NM off the course centerline, regardless of how far the aircraft is from the waypoints of the
route segment. You need to be familiar with the distance and approach parameters that change the CDI
scaling, and monitor the navigation unit to be sure the
CDI scaling is appropriate for the route segment and
phase of flight, e.g., GPS C129 – Class C1 equipment
used with a flight management system (FMS), unlike a
C129A receiver, normally remains at the terminal scale
of ±1 NM FSD during the approach (instead of ramping down to ±0.3 NM scaling beginning at 2 NM from
the FAF). For this class of equipment, if a deviation of
±3/4 FSD is made from centerline during the approach,
the aircraft will exceed the primary protected airspace
width of ±0.5 NM by 1/4 NM.
Likewise, if a Category (CAT) I ILS is flown with ±3/4
FSD it can preclude an aircraft from safely transitioning to a landing on the runway. At a decision altitude
(DA) point located 3,000 feet from the threshold with
3/4 FSD from centerline and above glidepath, the aircraft will be approximately 400 feet from centerline
and 36 feet above the glidepath. If the aircraft were
operating at 130 knots it would require two track
changes within the 14-second transit time from the DA
point to the threshold to align the aircraft with the runway. This may not allow landing within the touchdown
zone (typically the first 3000 feet of a runway) when
combined with strong crosswinds or Category C, D, or
E airplane approach speeds.
Staying within protected airspace depends primarily on
five factors:
• Accurate flying
• Accurate navigation equipment in the aircraft
• Accurate navigation signals from ground and
space-based transmitters
• Accurate direction by air traffic control (ATC)
• Accurate (current) charts and publications
Incorporated within these factors are other related
items, for example, flying accurately includes using the
navigation equipment correctly, and accurate navigation equipment includes the altimeter.
• It is important for pilots to understand that the
altimeter is a barometric device that measures
pressure, not altitude. Some pilots may think of
the altimeter as a true “altitude indicator,” without error. In fact, the pressure altimeter is a
barometer that measures changes in atmospheric
pressure, and through a series of mechanisms
and/or computer algorithms, converts these
changes, and displays an altitude. This conversion process assumes standard atmospheric
conditions, but since we fly in weather conditions other than standard, errors will result.
Also, certain procedures may be annotated
“NA” below a given temperature.
• The Instrument Flying Handbook (FAA-H-8083-
15), Chapter 3, and the Aeronautical Information
Manual (AIM), Chapter 7, include detailed discussions about altimeters and associated errors.
Each includes the International Civil Aviation
Organization (ICAO) Cold Temperature Error
Table for altitude corrections when operating
with an outside air temperature (OAT) below +10
degrees C.
The design of protected airspace is a very detailed and
complex process, combining the professional skills of
many different experts. Terrain elevations and contours,
runway configurations, traffic considerations, prevailing winds and weather patterns, and the performance
capabilities of the aircraft that will use the procedures
must be balanced to create airspace that combines functionality with safety. Although it is not necessary for
pilots to have an in-depth knowledge of how airspace is
protected, it is useful to understand some of the terms
used.
Required Obstacle Clearance (ROC) is the minimum
vertical clearance required between the aircraft and
ground obstructions over a specific point in an instrument procedure. Procedure designers apply the ROC
when designing instrument approach procedures. On
the initial segment, the ROC is approximately 1,000
feet, and it is at least 500 feet on the intermediate segment. Obviously, an imaginary surface 1,000 feet
above the actual terrain and obstacles would be as
rough and irregular as the surface below, so for practical reasons, airspace planners create smooth planes
above the highest ground features and obstructions.
These are called obstacle clearance surfaces (OCSs).
Procedure designers use both level and sloping obstacle clearance surfaces when designing approaches.
Fix Displacement Area (FDA) is an area created by
combining the permissible angular errors from the two
VOR or nondirectional beacon (NDB) NAVAIDs that
define the fix. When the NAVAIDs are close together
and the angle that defines the fix is near 90 degrees, the
B-3
FDA is relatively small. At greater distances or less
favorable angles, the FDA is larger. Airspace planners
use the FDA to define the limits of protected airspace.
Fix Displacement Tolerance (FDT) is an area that
applies to area navigation (RNAV) and equates to a
FDA for VOR or NDB NAVAIDs. The FDT has an
Along Track (ATRK) tolerance and a Cross Track
(XTRK) tolerance.
Flight Technical Error (FTE) is the measure of the pilot
or autopilot’s ability to control the aircraft so that its
indicated position matches the desired position. For
example, FTE increases as the CDI swings further from
center. If the cockpit instruments show the airplane to
be exactly where you want it, the FTE is essentially
zero.
Navigation System Error (NSE) is the error attributable
to the navigation system in use. It includes the navigation sensor error, receiver error, and path definition
error. NSE combines with FTE to produce the Total
System Error (TSE). TSE is the difference between
true position of the aircraft and the desired position.
It combines the flight technical errors and the navigation system tracking errors.
Actual navigation performance (ANP) is an estimate of
confidence in the current navigation system’s performance. ANP computations consider accuracy, availability, continuity, and integrity of navigation performance
at a given moment in time. Required Navigation
Performance (RNP) necessitates the aircraft navigation
system monitor the ANP and ensures the ANP does not
exceed the RNP value required for the operation. The
navigation system must also provide the pilot an alert
in the primary field of view when ANP exceeds RNP.
While you may have thought of protected airspace as
static and existing at all times whether aircraft are present or not, protection from conflicts with other aircraft
is dynamic and constantly changing as aircraft move
through the airspace. With continuous increases in air
traffic, some routes have become extremely congested.
Fortunately, the accuracy and integrity of aircraft navigation systems has also increased, making it possible to
reduce the separation between aircraft routes without
compromising safety. RNP is a standard for the navigation performance necessary to accurately keep an
aircraft within a specific block of airspace.
Containment is a term central to the basic concept of
RNP. This is the idea that the aircraft will remain within
a certain distance of its intended position (the stated
RNP value) at least 95 percent of the time on any flight.
The FDA is smallest when
the NAVAIDs are close to
the fix and the angle
defining the fix is
90 degrees.
At greater distances
from the NAVAIDs,
the FDA is larger.
At less favorable angles,
the FDA is larger.
Fix Displacement Area (FDA)
Facility
Facility
Facility
Figure B-2. The size of the protected airspace depends on where the terrestrial NAVAIDs that define it are located.
B-4
This is a very high percentage, but it would
not be enough to ensure the required level of
safety without another layer of protection outside the basic containment area. This larger
area has dimensions that are twice the RNP
value, giving the aircraft two times the lateral
area of the primary RNP area. Aircraft are
expected to be contained within this larger
boundary 99.999 percent of the time, which
achieves the required level of confidence for
safety.
Figure B-5 on pages B-6 through B-9 helps
explain the cockpit indications and tolerances that will comply with criteria to keep
you within protected airspace. The tolerances are predicated on zero instrument error
unless noted otherwise. Special Aircraft and
Figure B-3. An alerting system in the pilot’s primary view must warn if ANP
exceeds RNP. This alerting system is comparable to an “OFF” flag for a VOR
or ILS.
Twice RNP Value
RNP Value
Aircraft must remain
within this area 95
percent of the flight.
Aircraft remains within this
area 99.999 percent of the time.
Figure B-4. RNP Containment.
B-5
Aircrew Authorization Required (SAAAR) routes are
not covered in this table.
For approaches, it is not enough to just stay within protected airspace. For nonprecision approaches, you must
also establish a rate of descent and a track that will
ensure arrival at the MDA prior to reaching the MAP
with the aircraft continuously in a position from which a
descent to a landing on the intended runway can be made
at a normal rate using normal maneuvers. For precision
approaches or approaches with vertical guidance, a transition to a normal landing is made only when the aircraft
is in a position from which a descent to a landing on the
runway can be made at a normal rate of descent using
normal maneuvering.
For a pilot, remaining within protected airspace is
largely a matter of staying as close as possible to the
centerline of the intended course. There are formal definitions of what it means to be established on course, and
these are important in practice as well as theory, since
controllers often issue clearances contingent on your
being established on a course.
You must be established on course before a descent is
started on any route or approach segment. The ICAO
Procedures for Air Navigation Services – Aircraft
Operations (PANS-OPS) Volume I Flight Procedures,
specifies, “Descent shall not be started until the aircraft
is established on the inbound track,” and that an aircraft
is considered established when it is “within half full
scale deflection for the ILS and VOR; or within ±5
degrees of the required bearing for the NDB.”
In the AIM “established” is defined as “to be stable or
fixed on a route, route segment, altitude, heading, etc.”
The “on course” concept for IFR is spelled out in Part
91.181, which states that the course to be flown on an
airway is the centerline of the airway, and on any other
route, along the direct course between the NAVAIDS or
fixes defining that route.
As new navigational systems are developed with the
capability of flying routes and approaches with
increased resolution, increased navigation precision
and pilot situational awareness is required. For safety,
deviations from altitudes or course centerline should
be communicated to ATC promptly. This is increasingly
important when flights are in close proximity to
restricted airspace. Whether you are a high time corporate pilot flying an aircraft that is equipped with state of
the art avionics or a relatively new general aviation pilot
that ventures into the NAS with only a VOR for navigation, adhering to the tolerances in Figure B-5 will help
facilitate your remaining within protected airspace when
conducting flights under IFR.
B-6
Phase of Flight
NAVAID DEPARTURE EN ROUTE TERMINAL
NDB
VOR
ILS
RMI ±5 degrees.
For departures, the climb area
protected airspace initially splays at
15 degees from the ±500-foot width at
the departure end of runway (DER) to
2 NM from the DER. The initial climb
area width at 2 NM is ±3,756 feet from
centerline. After the initial splay, the
splay is 4.76 degrees until reaching an
en route fix.
RMI ± 5 degrees. Because of angular cross
track deviation, the NDB needle becomes
less sensitive as you fly away from the NDB
and more sensitive as you approach the
station. The airway primary width is 4.34
NM either side of centerline to 49.6 NM.
From 49.6 NM to the maximum standard
service volume of 75 NM, the primary
protected airspace splays at 5 degrees.
RMI ± 5 degrees.
The maximum standard service
volume for a compass locator is
15 NM. The feeder route width is
±4.34 NM.
CDI ±3/10 FSD (scale ±10 degrees).
Same as NDB except after the initial
splay, the splay is 2.86 degrees until
reaching an en route fix.
CDI ±1/2 FSD up to 51 NM and beyond 51
NM CDI ±2/5 FSD (scale ±10 degrees).
Like the NDB, the farther you are from the
VOR, the more the signal diverges. The
airway primary width is 4 NM either side of
centerline to 51 NM. From 51 NM to the
maximum standard service volume of 130
NM, the primary protected airspace splays
at 4.5 degrees.
CDI ±1/2 FSD (scale ±10 degrees).
The maximum standard service
volume for a T VOR is 25 NM. The
feeder route width is ±4 NM.
N/A N/A CDI ± 3/4 FSD for both lateral and
vertical.
The standard service volume for a
localizer is 18 NM. The localizer total
width at 18 NM is ±2.78 NM from
centerline and tapers to approximately ±5,000 feet from centerline at
the FAF.
The standard service volume for the
glide slope is 10 NM.
Figure B-5. Cockpit Indications and Tolerances to Keep You Within Protected Airspace. (Continued on Pages B-8 and B-9)
B-7
Phase of Flight
FINAL APPROACH MISSED APPROACH HOLDING
RMI ±10 degrees. If flown “FROM” the NDB, RMI ±5 degrees at
the visual descent point (VDP) or equivalent for a normal landing.
The course width for an approach with a FAF may be as small as
2.5 NM at the NDB and as wide as 5 NM at 15 NM from the NDB.
For an on-airport facility, no FAF approach, the course width
tapers from 6 NM (10 NM from the NDB) to 2.5 NM at the
MAP/NDB.
RMI ±10 degrees.
The course width widens to ±4
NM at 15 NM from the MAP.
RMI ±5 degrees.
Intersections – the size of
protected airspace varies
with the distance from the
NAVAID. See Figure 3-27
on page 3-23.
CDI ±3/4 FSD. If flown “FROM” the VOR, CDI ±1/2 FSD at the
VDP or equivalent for a normal landing (scale ±10 degrees).
The course width for an approach with a FAF may be as small as
2.0 NM at the VOR and as wide as 5 NM at 30 NM from the
VOR. For an on airport facility no FAF approach, the course width
tapers from 6 NM (10 NM from the VOR) to 2.0 NM at the
MAP/VOR.
CDI ±3/4 FSD (scale ±10
degrees).
For both FAF and no FAF
approaches, the course width
widens to 4 NM at 15 NM from
the MAP.
CDI ±1/2 FSD (scale ±10
degrees).
Intersections – the size of
protected airspace varies with
the distance from the NAVAIDs
that form the holding fix. See
Figure 3-27 on page 3-23.
CAT I CDI ±3/4 FSD for localizer and glidepath at the glide slope
intercept. CDI ±1/2 FSD at the DA point for a normal landing.
(scale total width may vary from 3 to 6 degrees).
The normal length of final is 5 NM from the threshold. The final
approach obstacle clearance area width at the FAF is approximately ±5,000 feet from centerline and tapers to as small as
±500 feet from centerline at 200 feet from the runway threshold.
The CAT I final approach OCS can be as small as 500 feet below
glidepath at the FAF. At a DA point located 3,000 feet from the
threshold, the OCS may be as close as 114 feet below the
glidepath.
Decision range for airplane CAT II CDI ±1/6 FSD for localizer and
±1/4 FSD for glidepath and for helicopter ±1/4 FSD for localizer
and glidepath. The tracking performance parameters within the
decision range (that portion of the approach between 300 feet
AGL and DH) are maximums, with no sustained oscillations
about the localizer or glidepath. If the tracking performance is
outside of these parameters while within the decision region,
execute a go-around since the overall tracking performance is
not sufficient to ensure that the aircraft will arrive at the DH on a
flight path that permits the landing to be safely completed.
N/A N/A
B-8
Phase of Flight
CDI centered when departing the runway to
the IDF with a maximum of ±3/4 FSD upon
reaching the terminal route (CDI scale
±1NM).
The HAL in the terminal mode is 1 NM.
NAVAID DEPARTURE EN ROUTE TERMINAL
GPS
(C-129A)
WAAS
LPV
CDI centered when departing the runway
with a maximum of ±3/4 FSD upon reaching
the terminal route (scale ± 1NM).
For departures, the climb area protected
airspace initially splays at 15 degrees from
the ±500-foot width either side of centerline
at the DER to a nominal distance of 2 NM
from the DER. The initial climb area width at
2 NM is ± 3,756 feet from centerline. After the
initial splay to the Initial Departure Fix (IDF) a
smaller splay continues until reaching a
terminal width as small as 2 NM at 10.89 NM
from the DER.
The horizontal alarm limit (HAL) is ±1 NM
within 30 NM of the airport reference point
(ARP).
CDI ±1/2 FSD (scale ±5 NM).
The airway primary width is ±4 NM
from centerline at 30 NM from the
airport reference point (ARP).
The HAL is ±2 NM for distances
greater than 30 NM from the ARP.
CDI ±3/4 FSD (scale ±1 NM
within 30 NM of the ARP).
For arrivals, the terminal primary
width is ±2 NM from centerline at
approximately 30 NM from the
ARP.
The HAL is ±1 NM within 30 NM
of the ARP.
CDI ±3/4 FSD (scale ±2 NM).
The airway primary width is ±4 NM
from centerline (equivalent to 2
RNP) at approximately 30 NM from
the ARP.
The HAL in the en route mode is 2
NM.
CDI ±3/4 FSD (scale ±1 NM
within 30 NM of the ARP).
For arrivals the terminal primary
width is ±2 NM from centerline at
approximately 30 NM from the
ARP.
The HAL in the terminal mode is 1
NM. The terminal mode begins at
30 NM from the ARP or at the
initial approach fix (IAF) when
more than 30 NM from the ARP.
Figure B-5. Continued
B-9
Phase of Flight
FINAL APPROACH MISSED APPROACH HOLDING
CDI ±2/3 FSD (scale ±0.3 NM).
For conventional GPS approaches the primary width is ±1.0 NM
from centerline at the FAF and tapers to ±0.5 NM at the MAP.
CDI ±1/3 FSD for “Copter” approaches (scale ±0.3 NM).
For “Copter” approaches the primary width is ± 0.55 NM from
centerline at the FAF and tapers to ±0.4 NM at the MAP.
The HAL is ±0.3 NM on the final approach segment (FAS).
NOTE: GPS C129 – Class C1 (FMS equipped) Flight
Director/Autopilot required since 1.0 NM scaling on the CDI is used.
For Airplane approaches CDI ±1/5 FSD and for Copter approaches
CDI ± 1/10 FSD (scale ±1.0 NM).
CDI ±3/4 FSD (scale ±1.0 NM).
For missed approaches the
primary width at the MAP is 0.5
NM and splays to ±4.0 NM from
centerline at 15 NM from the MAP.
For Copter approaches CDI ±1/2
FSD (scale ±1.0 NM). For missed
approaches the primary width at
the MAP is ±0.4 NM and splays to
±1.5 NM from centerline at 7.5 NM
from the MAP.
The HAL is ±1.0 NM within 30 NM
of the ARP.
Terminal (within 30 NM of the
ARP). CDI ±3/4 FSD (scale
±1.0 NM).
En route (more than 30 NM
from the ARP) CDI ±1/2 FSD
(scale ±5.0 NM).
The HAL for terminal holding is
±1.0 NM within 30 NM of the
ARP and ±2 NM when more
than 30 NM from the ARP.
CDI ±3/4 FSD lateral and vertical (LPV scale is ±2 degrees or ±0.3 NM
FSD at the FAF whichever is less. Nonprecision scale is ±0.3 NM).
LPV/LNAV approaches are similar to ILS/LOC approaches.
LNAV (nonprecision): The CDI scaling for not vectored to final
(VTF) approaches starts out with a linear width of ±1 NM FSD on
the intermediate segment. At 2 NM prior to the FAF the scaling
begins a change to either an angular ±2 degrees taper or ±0.3 NM
FSD whichever is smaller. This change must be completed at the
FAF. At the landing threshold point (LTP) the angular scale then
becomes linear again with a width of approximately ±350 feet from
centerline. For VTF approaches the CDI scaling starts out linear at
±1 NM FSD and changes to a ±2 degrees taper FSD and then
becomes linear again with a width of approximately 350 feet from
centerline at the LTP.
Approaching the runway, a LPV nominal 3 degrees glidepath starts
out linear (±150 M FSD) and then approximately 6 NM from the
landing threshold becomes angular at a width of ±0.75 degrees
and then becomes linear again as early as approximately 1.9 NM
from the GPI for a ±45 M FSD or as small as a ±15 M FSD at a
distance of approximately 0.6 NM from the landing threshold
(depending on the manufacturer).
The normal length of final is 5.0 NM from the threshold. The final
approach obstacle clearance area width at the FAF is approximately ±4,000 feet from centerline and tapers to ±700 feet from
centerline 200 feet from the runway threshold.
The final approach OCS can be as small as 500 feet below
glidepath at the FAF. At a DA point located 3,000 feet from the
threshold, the OCS may be as small as 118 feet below the
glidepath.
The HAL for LNAV is 0.3 NM. The HAL for LPV is 40 M and the
vertical alarm limit (VAL) starts out at 150 M and may be as large
as 45 M near the LTP.
CDI ±1/2 FSD (From the LTP to
the DER the scale is approximately ±350 feet wide and then
changes ±0.3 NM at the DER)
The primary width at the DA point
for missed approaches (aligned
within 3 degrees of the final
approach course) is approximately
±1,000 feet from centerline and
splays outward for 8,341 feet until
reaching a width of ±3,038 feet
from centerline.
The HAL for missed approaches
aligned within 3 degrees of the
final approach course is ±0.3 NM
at the DER and then changes to a
HAL of ±1 NM at the turn initiation
point for the first waypoint in the
missed approach.
CDI ±3/4 FSD for terminal or
en route holding (scale ±1.0
NM terminal and ±2.0 NM en
route).
The HAL is 1.0 NM when within
30 NM of the ARP and 2.0 NM
beyond 30 NM of the ARP.
B-10
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