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prior). Failure to sequence may be an indication of the
detection of a satellite anomaly, failure to arm the
receiver (if required), or other problems which
preclude completing the approach.
4. If the receiver does not sequence into the
approach mode or a RAIM failure/status annunciation
occurs prior to the FAWP, the pilot should not
descend to Minimum Descent Altitude (MDA), but
should proceed to the missed approach waypoint
(MAWP) via the FAWP, perform a missed
approach, and contact ATC as soon as practical. Refer
to the receiver operating manual for specific
AIM 2/19/04
1−1−36 Air Navigation Aids
indications and instructions associated with loss of
RAIM prior to the FAF.
5. If a RAIM failure occurs after the FAWP, the
receiver is allowed to continue operating without an
annunciation for up to 5 minutes to allow completion
of the approach (see receiver operating manual). If
the RAIM flag/status annunciation appears after
the FAWP, the missed approach should be
executed immediately.
k. Waypoints
1. GPS approaches make use of both fly−over
and fly−by waypoints. Fly−by waypoints are used
when an aircraft should begin a turn to the next course
prior to reaching the waypoint separating the two
route segments. This is known as turn anticipation
and is compensated for in the airspace and terrain
clearances. Approach waypoints, except for the
MAWP and the missed approach holding waypoint
(MAHWP), are normally fly−by waypoints. Fly−
over waypoints are used when the aircraft must fly
over the point prior to starting a turn. New approach
charts depict fly−over waypoints as a circled
waypoint symbol. Overlay approach charts and some
early stand alone GPS approach charts may not
reflect this convention.
2. Since GPS receivers are basically “To−To”
navigators, they must always be navigating to a
defined point. On overlay approaches, if no
pronounceable five−character name is published for
an approach waypoint or fix, it was given a database
identifier consisting of letters and numbers. These
points will appear in the list of waypoints in the
approach procedure database, but may not appear on
the approach chart. A point used for the purpose of
defining the navigation track for an airborne
computer system (i.e., GPS or FMS) is called a
Computer Navigation Fix (CNF). CNFs include
unnamed DME fixes, beginning and ending points of
DME arcs and sensor final approach fixes (FAFs) on
some GPS overlay approaches. To aid in the approach
chart/database correlation process, the FAA has
begun a program to assign five−letter names to CNFs
and to chart CNFs on various National Oceanic
Service aeronautical products. These CNFs are not to
be used for any air traffic control (ATC) application,
such as holding for which the fix has not already been
assessed. CNFs will be charted to distinguish them
from conventional reporting points, fixes, intersections,
and waypoints. The CNF name will be enclosed
in parenthesis, e.g., (MABEE), and the name will be
placed next to the CNF it defines. If the CNF is not at
an existing point defined by means such as crossing
radials or radial/DME, the point will be indicated by
an “X.” The CNF name will not be used in filing a
flight plan or in aircraft/ATC communications. Use
current phraseology, e.g., facility name, radial,
distance, to describe these fixes.
3. Unnamed waypoints in the database will be
uniquely identified for each airport but may be
repeated for another airport (e.g., RW36 will be used
at each airport with a runway 36 but will be at the
same location for all approaches at a given airport).
4. The runway threshold waypoint, which is
normally the MAWP, may have a five letter identifier
(e.g., SNEEZ) or be coded as RW## (e.g., RW36,
RW36L). Those thresholds which are coded as five
letter identifiers are being changed to the RW##
designation. This may cause the approach chart and
database to differ until all changes are complete. The
runway threshold waypoint is also used as the center
of the Minimum Safe Altitude (MSA) on most GPS
approaches. MAWPs not located at the threshold will
have a five letter identifier.
l. Position Orientation
As with most RNAV systems, pilots should pay
particular attention to position orientation while
using GPS. Distance and track information are
provided to the next active waypoint, not to a fixed
navigation aid. Receivers may sequence when the
pilot is not flying along an active route, such as when
　
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