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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.) [Figure B-1]
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
　
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