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A-15
2 RNP 2 RNP
ROC
RNP navigation enables the geometry of instrument approach procedure design to be very
flexible, and allows the incorporation of radius-to-fix (RF) legs enabling the FMS/autopilot to
follow curved flight tracks. The constant radius arc RF leg defines a constant radius turn between
two database fixes, lines tangent to the arc, and a center fix. While the arc initial point, arc ending
point, and arc center point are available as database fixes, implementation of this leg type may
not require the arc center point to be available as a fix.
A-16
B-1
At 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.) [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
　
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