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AIM

10/12/17

1

−1−23

Navigation Aids

(e) Procedures for Accomplishing GPS

Approaches

(1)

An RNAV (GPS) procedure may be

associated with a Terminal Arrival Area (TAA). The
basic design of the RNAV procedure is the “T” design
or a modification of the “T” (See Paragraph 5-4-5d,
Terminal Arrival Area (TAA), for complete informa-
tion).

(2)

Pilots cleared by ATC for an RNAV

(GPS) approach should fly the full approach from an
Initial Approach Waypoint (IAWP) or feeder fix.
Randomly joining an approach at an intermediate fix
does not assure terrain clearance.

(3)

When an approach has been loaded in

the navigation system, GPS receivers will give an
“arm” annunciation 30 NM straight line distance
from the airport/heliport reference point. Pilots
should arm the approach mode at this time if not
already armed (some receivers arm automatically).
Without arming, the receiver will not change from
en route CDI and RAIM sensitivity of ±5 NM either
side of centerline to ±1 NM terminal sensitivity.
Where the IAWP is inside this 30 mile point, a CDI
sensitivity change will occur once the approach mode
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
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 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
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, 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,