Aeronautical Information Manual (AIM), page 79

Index   78 -- Page 79 -- 80

(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

(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.

(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
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

Navigation Aids 1-1-25

Page 79 of the Aeronautical Information Manual (AIM.pdf)
AIM: Official Guide to Basic Flight Information and ATC Procedures

Index   78 -- Page 79 -- 80