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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.
[Figure B-2]
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
　
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