Flight Legs

Flight plan allows you to see your course on the map, provides autopilot guidance to track it automatically, and organizes its waypoint database for quick information retrieval. Basically, you create a flight plan by making a list of waypoints from start to end. A leg is a segment between two waypoints. There are 23 leg types that have been created to translate into FMS language.

Type Name Type Name
VA Heading to Altitude CA Crs to Altitude
VI Heading to Intercept CI Crs to Intercept
VR Heading to Radial CR Crs to Radial
VD Heading to DME dist CD Crs to DME dist
VM Heading to Manual Termination (Vector) CF Crs to Fix, or VTF
IF Initial Fix (point leg) PI Procedure Turn
TF Track to Fix HM Hold to Manual Termination
DF Direct to Fix (present position to fix) HF Hold to Fix (Once hold)
FA Fix to Altitude HA Hold to Altitude
FC Fix to a Distance AF Arc to Fix (DME arc)
FD Fix to a DME distance RF Radius to Fix
FM Fix to Manual termination (Vector)

Note that only 6 of them (IF, TF, DF, CF, FM, HM) can be entered by pilots and others are stored in Navigation database (or NDB) as Procedures which can only be loaded.

In Navigation database(ARINC 424), The Path and Terminator concept is a means to permit coding of Terminal Area Procedures, SIDs, STARs and Approach Procedures. Charted procedure are translated into a sequence of ARINC 424 legs in the Navigation Database. Flight plans are entered into the FMS by using procedures from the navigation database and chaining them together.

It’s high time to implement RNAV, using only DO236 preferred leg types: IF, TF, RF which are fixed and without possible interpretation. The leg type is specified at the end point : “path terminator concept” .

The Initial Fix or IF Leg defines a database fix as a point in space. It is only required to define the beginning of a route or procedure. An interesting thing is that we always use SF instead of IF in coding. Guess what? “IF” is a RESERVED KEY WORD in most programming language, compiler cannot tell what IF leg_type == IF means.

Track to a Fix or TF Leg defines a great circle track over ground between two known databases fixes.


Leg Type Example Path
IF IF
TF IF
CF IF
DF IF
FA IF
FC IF
FD IF
FM IF
CA IF
CD IF
CI IF
CR IF
RF IF
AF IF
VA IF
VD IF
VI IF
VM IF
VR IF
PI IF
HA, HF, HM IF