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.
|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|
|HA, HF, HM|