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AIM

4/20/23

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29

Navigation Aids

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 annunciation.

(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 information 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
further away from the intended course even though it 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, requesting 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 Supplement 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 published approach procedure. The proper RAIM 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, IAWP, FAWP, MAHWP on a procedure turn). Care
must be exercised to ensure that the receiver is sequenced to the appropriate waypoint for the segment of the
procedure being flown, especially if one or more fly

overs are skipped (for example, FAWP rather than IAWP