background image

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.