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AIM

10/12/17

1−1−27

Navigation Aids

(4) Unnamed waypoints for each airport

will be uniquely identified in the database. Although

the identifier may be used at different airports (for

example, RW36 will be the identifier at each airport

with a runway 36), the actual point, at each airport, is

defined by a specific latitude/longitude coordinate.

(5) The runway threshold waypoint, nor-

mally the MAWP, may have a five−letter identifier

(for example, SNEEZ) or be coded as RW## (for

example, RW36, RW36L). MAWPs located at the

runway threshold are being changed to the RW##

identifier, while MAWPs not located at the threshold

will have a five−letter identifier. This may cause the

approach chart to differ from the aircraft database

until all changes are complete. The runway threshold

waypoint is also used as the center of the Minimum

Safe Altitude (MSA) on most GPS approaches.

(j) Position Orientation.

Pilots should pay particular attention to position

orientation while using GPS. Distance and track

information are provided to the next active

waypoint, not to a fixed navigation aid. Receivers

may sequence when the pilot is not flying along an

active route, such as when being vectored or

deviating for weather, due to the proximity to another

waypoint in the route. This can be prevented by

placing the receiver in the non-sequencing mode.

When the receiver is in the non-sequencing mode,

bearing and distance are provided to the selected

waypoint and the receiver will not sequence to the

next waypoint in the route until placed back in the

auto sequence mode or the pilot selects a different

waypoint. The pilot may have to compute the ATD

to stepdown fixes and other points on overlay

approaches, due to the receiver showing ATD to the

next waypoint rather than DME to the VOR or ILS

ground station.

(k) Impact of Magnetic Variation on PBN

Systems

(1) Differences may exist between PBN

systems and the charted magnetic courses on

ground−based NAVAID instrument flight procedures

(IFP), enroute charts, approach charts, and Standard

Instrument Departure/Standard Terminal Arrival

(SID/STAR) charts. These differences are due to the

magnetic variance used to calculate the magnetic

course. Every leg of an instrument procedure is first

computed along a desired ground track with reference

to true north. A magnetic variation correction is then

applied to the true course in order to calculate a

magnetic course for publication. The type of

procedure will determine what magnetic variation

value is added to the true course. A ground−based

NAVAID IFP applies the facility magnetic variation

of record to the true course to get the charted magnetic

course. Magnetic courses on PBN procedures are

calculated two different ways. SID/STAR procedures

use the airport magnetic variation of record, while

IFR enroute charts use magnetic reference bearing.

PBN systems make a correction to true north by

adding a magnetic variation calculated with an

algorithm based on aircraft position, or by adding the

magnetic variation coded in their navigational

database. This may result in the PBN system and the

procedure designer using a different magnetic

variation, which causes the magnetic course

displayed by the PBN system and the magnetic course

charted on the IFP plate to be different. It is important

to understand, however, that PBN systems, (with the

exception of VOR/DME RNAV equipment) navigate

by reference to true north and display magnetic

course only for pilot reference. As such, a properly

functioning PBN system, containing a current and

accurate navigational database, should fly the

correct ground track for any loaded instrument

procedure, despite differences in displayed magnetic

course that may be attributed to magnetic variation

application. Should significant differences between

the approach chart and the PBN system avionics’

application of the navigation database arise, the

published approach chart, supplemented by NOT-

AMs, 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 PBN

system avionics’ computation of geodesic paths,

distance between waypoints, and differences in

magnetic variation application.  Variations in

distances may also occur since PBN system

distance−to−waypoint values are ATDs 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.