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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,