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until the aircraft is within 30 NM even if the approach is
armed earlier. Feeder route obstacle clearance is predicated
on the receiver CDI and RAIM being in terminal CDI
sensitivity within 30 NM of the airport/heliport reference
point; therefore, the receiver should always be armed no later
than the 30 NM annunciation.
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 WPs by the receiver
and to resume automatic GPS navigation sequencing once
the maneuver is complete. The same WP may appear in the
route of flight more than once and consecutively (e.g., IAWP,
final approach waypoint (FAWP), missed approach waypoint
(MAWP) on a procedure turn). Care must be exercised to
ensure the receiver is sequenced to the appropriate WP for
the segment of the procedure being flown, especially if one
or more fly-over WPs are skipped (e.g., FAWP rather than
IAWP if the procedure turn is not flown). The pilot may need
to sequence past one or more fly-overs of the same WP in
order to start GPS automatic sequencing at the proper place
in the sequence of WPs.
When receiving vectors to final, most receiver operating
manuals suggest placing the receiver in the nonsequencing
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 WPs
outside the FAWP or step-down fixes must be considered.
Calculating the distance to the FAWP may be required in
order to descend at the proper location.
When within 2 NM of the FAWP with the approach mode
armed, the approach mode will switch to active, which results
in RAIM and CDI sensitivity changing to the approach
mode. Beginning 2 NM prior to the FAWP, the full scale
CDI sensitivity will change smoothly from ±1 NM to ±0.3
NM at the FAWP. As sensitivity changes from ±1 NM to
±0.3 NM approaching the FAWP, and the CDI not centered,
the corresponding increase in CDI displacement may give
the impression the aircraft is moving further away from the
intended course even though it is on an acceptable intercept
heading. If digital track displacement information (cross-track
error) is available in the approach mode, it 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. Requesting
or accepting vectors, which will cause the aircraft to intercept
the final approach course within 2 NM of the FAWP, is not
recommended.
Incorrect inputs into the GPS receiver are especially critical
during approaches. In some cases, an incorrect entry can
cause the receiver to leave the approach mode. 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 minimum descent altitude (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.
A GPS missed approach requires pilot action to sequence the
receiver past the MAWP to the missed approach portion of
the procedure. The pilot must be thoroughly familiar with
the activation procedure for the particular GPS receiver
installed in the aircraft and must initiate appropriate action
after the MAWP. Activating the missed approach prior to the
MAWP will cause CDI sensitivity to change immediately to
terminal (±1 NM) sensitivity, and the receiver will continue
to navigate to the MAWP. The receiver will not sequence past
the MAWP. Turns should not begin prior to the MAWP. If
the missed approach is not activated, the GPS receiver will
display an extension of the inbound final approach course and
the along track distance (ATD) will increase from the MAWP
until it is manually sequenced after crossing the MAWP.
　
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