AIM

4/3/14

1−1−29

Navigation Aids

differences between the approach chart and the

RNAV system avionics’ application of the navigation

database arise, the published approach chart,

supplemented by NOTAMs, holds precedence.

2.

The course into a waypoint may not always be

180 degrees different from the course leaving the

previous waypoint, due to the RNAV system

avionics’ computation of geodesic paths, distance

between waypoints and differences in magnetic

variation application. Variations in distances may

also occur since RNAV system distance−to−waypoint

values are along−track distances (ATD) computed to

the next waypoint and the DME values published on

underlying procedures are slant−range distances

measured to the station. This difference increases

with aircraft altitude and proximity to the NAVAID.

m. Departures and Instrument Departure

Procedures (DPs)

The GPS receiver must be set to terminal (

±1 NM)

CDI sensitivity and the navigation routes contained in

the database in order to fly published IFR charted

departures and DPs. Terminal RAIM should be

automatically provided by the receiver. (Terminal

RAIM for departure may not be available unless the

waypoints are part of the active flight plan rather than

proceeding direct to the first destination.) Certain

segments of a DP may require some manual

intervention by the pilot, especially when radar

vectored to a course or required to intercept a specific

course to a waypoint. The database may not contain

all of the transitions or departures from all runways

and some GPS receivers do not contain DPs in the

database. It is necessary that helicopter procedures be

flown at 70 knots or less since helicopter departure

procedures and missed approaches use a

20:1 obstacle clearance surface (OCS), which is

double the fixed−wing OCS, and turning areas are

based on this speed as well.

n. Flying GPS Approaches

1.

Determining which area of the TAA the

aircraft will enter when flying a “T” with a TAA must

be accomplished using the bearing and distance to the

IF(IAF). This is most critical when entering the TAA

in the vicinity of the extended runway centerline and

determining whether you will be entering the right or

left base area. Once inside the TAA, all sectors and

stepdowns are based on the bearing and distance to

the IAF for that area, which the aircraft should be

proceeding direct to at that time, unless on vectors.

(See FIG 5−4−3 and FIG 5−4−4.)

2.

Pilots should fly the full approach from an

Initial Approach Waypoint (IAWP) or feeder fix

unless specifically cleared otherwise. Randomly

joining an approach at an intermediate fix does not

assure terrain clearance.

3.

When an approach has been loaded in the

flight plan, 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 it has not already been

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, CDI sensitivity will not change until the

aircraft is within 30 miles of the airport/heliport

reference point even if the approach is armed earlier.

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 FAWP with the

approach mode armed, the approach mode will

switch to active, which results in RAIM changing to

approach sensitivity and a change in CDI 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