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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
i n f o r m a t i on a r e 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.