Aeronautical Information Manual (AIM), page 76

Index   75 -- Page 76 -- 77

is armed and the aircraft is inside 30 NM. Where the
IAWP is beyond 30 NM from the airport/heliport
reference point and the approach is armed, the CDI
sensitivity will not change until the aircraft is within
30 miles of the airport/heliport reference point.
Feeder route obstacle clearance is predicated on the
receiver being in terminal (±1 NM) CDI sensitivity
and RAIM within 30 NM of the airport/heliport
reference point; therefore, the receiver should always
be armed (if required) not later than the 30 NM
(4) The pilot must be aware of what bank
angle/turn rate the particular receiver uses to compute
turn anticipation, and whether wind and airspeed are
included in the receiver's calculations. This informa-
tion should be in the receiver operating manual. Over
or under banking the turn onto the final approach
course may significantly delay getting on course and
may result in high descent rates to achieve the next
segment altitude.

(5) When within 2 NM of the Final
Approach Waypoint (FAWP) with the approach
mode armed, the approach mode will switch to
active, which results in RAIM and CDI changing to
approach sensitivity. Beginning 2 NM prior to the
FAWP, the full scale CDI sensitivity will smoothly
change from ±1 NM to ±0.3 NM at the FAWP. As
sensitivity changes from ±1 NM to ±0.3 NM
approaching the FAWP, with the CDI not centered,
the corresponding increase in CDI displacement
may give the impression that the aircraft is moving
is on an acceptable intercept heading. Referencing the
digital track displacement information (cross track
error), if it is available in the approach mode, may
help the pilot remain position oriented in this
situation. Being established on the final approach
course prior to the beginning of the sensitivity change
at 2 NM will help prevent problems in interpreting the
CDI display during ramp down. Therefore, request-
ing or accepting vectors which will cause the aircraft
to intercept the final approach course within 2 NM of
the FAWP is not recommended.

(6) When receiving vectors to final, most
receiver operating manuals suggest placing the
receiver in the non-sequencing mode on the FAWP
and manually setting the course. This provides an
extended final approach course in cases where the
aircraft is vectored onto the final approach course

outside of any existing segment which is aligned with
the runway. Assigned altitudes must be maintained
until established on a published segment of the
approach. Required altitudes at waypoints outside the
FAWP or stepdown fixes must be considered.
Calculating the distance to the FAWP may be
required in order to descend at the proper location.
(7) Overriding an automatically selected
sensitivity during an approach will cancel the
approach mode annunciation. If the approach mode
is not armed by 2 NM prior to the FAWP, the approach
mode will not become active at 2 NM prior to the
FAWP, and the equipment will flag. In these
conditions, the RAIM and CDI sensitivity will not
ramp down, and the pilot should not descend to MDA,
but fly to the MAWP and execute a missed approach.
The approach active annunciator and/or the receiver
should be checked to ensure the approach mode is
active prior to the FAWP.
(8) Do not attempt to fly an approach unless
the procedure in the onboard database is current and
identified as "GPS" on the approach chart. The
navigation database may contain information about
non-overlay approach procedures that enhances
position orientation generally by providing a map,
while flying these approaches using conventional
NAVAIDs. This approach information should not be
confused with a GPS overlay approach (see the
receiver operating manual, AFM, or AFM Supple-
ment for details on how to identify these procedures
in the navigation database). Flying point to point on
the approach does not assure compliance with the
sensitivity will not be available and the CDI
sensitivity will not automatically change to ±0.3
NM. Manually setting CDI sensitivity does not
automatically change the RAIM sensitivity on some
receivers. Some existing non-precision approach
procedures cannot be coded for use with GPS and will
not be available as overlays.

(9) Pilots should pay particular attention
to the exact operation of their GPS receivers for
performing holding patterns and in the case of
overlay approaches, operations such as procedure
turns. These procedures may require manual
intervention by the pilot to stop the sequencing of
waypoints by the receiver and to resume automatic
GPS navigation sequencing once the maneuver is
complete. The same waypoint may appear in the route
of flight more than once consecutively (for example,

1-1-22 Navigation Aids

Page 76 of the Aeronautical Information Manual (AIM.pdf)
AIM: Official Guide to Basic Flight Information and ATC Procedures

Index   75 -- Page 76 -- 77