AIM

4/3/14

1−1−28

Navigation Aids

Fly−by waypoints connect the two segments by

allowing the aircraft to turn prior to the current

waypoint in order to roll out on course to the next

waypoint. This is known as turn anticipation and is

compensated for in the airspace and terrain

clearances. The MAWP and the missed approach

holding waypoint (MAHWP) are normally the only

two waypoints on the approach that are not fly−by

waypoints. Fly−over waypoints are used when the

aircraft must overfly the waypoint prior to starting a

turn to the new course. The symbol for a fly-over

waypoint is a circled waypoint.  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. When this occurs, the less

restrictive (fly-by) symbology will be charted.

Overlay approach charts and some early stand-alone

GPS approach charts may not reflect this convention.

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.

k. Position Orientation

As with most RNAV systems, 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

nonsequencing mode. When the receiver is in the

nonsequencing 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. On overlay

approaches, the pilot may have to compute the

along−track distance to stepdown fixes and other

points due to the receiver showing along−track

distance to the next waypoint rather than DME to the

VOR or ILS ground station.

l. Impact of Magnetic Variation on RNAV

Systems

1.

Differences may exist between charted

magnetic courses on ground-based navigational aid

(NAVAID) instrument flight procedures (IFP), area

navigation (RNAV) procedures, and RNAV systems

on 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 RNAV procedures are

calculated two different ways. SID/STAR procedures

use the airport magnetic variation of record, while

IFR enroute charts use magnetic reference bearing.

RNAV 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 RNAV system and the

procedure designer using a different  magnetic

variation, which causes the magnetic course
displayed 

by the RNAV system and the magnetic

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

important to understand, however, that RNAV

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 RNAV system,

containing a current and accurate navigational
database

, should still fly the correct ground track for

any loaded instrument procedure, despite any

differences in magnetic course that may be attributed

to magnetic variation application. Should significant