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AIM 

4/20/23

 

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Navigation Aids

out on course to the next waypoint. This is known as turn anticipation and is compensated for in the airspace and
terrain clearances. The missed approach waypoint (MAWP) will always be a fly

over waypoint. A holding

waypoint will always be designed as a fly

over waypoint in the navigational database but may be charted as a

fly

by event unless the holding waypoint is used for another purpose in the procedure and both events require

the waypoint to be a fly

over event. Some waypoints may have dual use; for example, as a fly

by waypoint when

used as an IF for a NoPT route and as a fly

over waypoint when the same waypoint is also used as an IAF/IF

hold

in

lieu of PT. Since the waypoint can only be charted one way, when this situation occurs, the fly

by

waypoint symbol will be charted in all uses of the waypoint.

(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, normally 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.

(i) 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.

(j) 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 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 PBN system avionics’ computation of geodesic paths, distance between