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AIM

10/12/17

1−1−26

Navigation Aids

(4) If the receiver does not sequence into

the approach mode or a RAIM failure/status

annunciation occurs prior to the FAWP, the pilot must

not initiate the approach or descend, but instead

proceed to the missed approach waypoint ( MAWP)

via the FAWP, perform a missed approach, and

contact ATC as soon as practical. The GPS receiver

may continue to operate after a RAIM flag/status

annunciation appears, but the navigation information

should be considered advisory only. Refer to the

receiver operating manual for specific indications

and instructions associated with loss of RAIM prior

to the FAF.

(5) If the RAIM flag/status annunciation

appears after the FAWP, the pilot should initiate a

climb and execute the missed approach. The GPS

receiver may continue to operate after a RAIM

flag/status annunciation appears, but the navigation

information should be considered advisory only.

Refer to the receiver operating manual for operating

mode information during a RAIM annunciation.

(i) Waypoints

(1) GPS receivers navigate from one

defined point to another retrieved from the aircraft’s

onboard navigational database. These points are

waypoints (5-letter pronounceable name), existing

VHF intersections, DME fixes with 5−letter

pronounceable names and 3-letter NAVAID IDs.

Each waypoint is a geographical location defined by

a latitude/longitude geographic coordinate. These

5−letter waypoints, VHF intersections, 5−letter

pronounceable DME fixes and 3−letter NAVAID IDs

are published on various FAA aeronautical naviga-

tion products (IFR Enroute Charts, VFR Charts,

Terminal Procedures Publications, etc.).

(2) A Computer Navigation Fix (CNF) is

also a point defined by a latitude/longitude coordinate

and is required to support Performance−Based

Navigation (PBN) operations. The GPS receiver uses

CNFs in conjunction with waypoints to navigate from

point to point. However, CNFs are not recognized by

ATC. ATC does not maintain CNFs in their database

and they do not use CNFs for any air traffic control

purpose. CNFs may or may not be charted on FAA

aeronautical navigation products, are listed in the

chart legends, and are for advisory purposes only.

Pilots are not to use CNFs for point to point

navigation (proceed direct), filing a flight plan, or in

aircraft/ATC communications. CNFs that do appear

on aeronautical charts allow pilots increased

situational awareness by identifying points in the

aircraft database route of flight with points on the

aeronautical chart. CNFs are random five-letter

identifiers, not pronounceable like waypoints and

placed in parenthesis. Eventually, all CNFs will begin

with the letters “CF” followed by three consonants

(for example, CFWBG). This five-letter identifier

will be found next to an “x” on enroute charts and

possibly on an approach chart. On instrument

approach procedures (charts) in the terminal

procedures publication, CNFs may represent un-

named DME fixes, beginning and ending points of

DME arcs, and sensor (ground-based signal i.e.,

VOR, NDB, ILS) final approach fixes on GPS

overlay approaches. These CNFs provide the GPS

with points on the procedure that allow the overlay

approach to mirror the ground-based sensor

approach. These points should only be used by the

GPS system for navigation and should not be used by

pilots for any other purpose on the approach. The

CNF concept has not been adopted or recognized by

the International Civil Aviation Organization

(ICAO).

(3) GPS approaches use fly−over and

fly−by waypoints to join route segments on an

approach. 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.