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.
There are 23 leg types that have been created to translate into computer language (FMS), procedure designed for clock & compass manual flight. 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.
Track to a Fix or TF Leg defines a great circle track over ground between two known databases fixes
Preferred method for specification of straight legs (course or heading can be mentioned on charts, but designer should ensure TF leg is used for coding).
RF legs:
Constant Radius Arc or RF Leg defines a constant radius turn between two database fixes, lines tangent to the arc and a center fix.
CF legs:
Course to a Fix or CF Leg defines a specified course to a specific database fix. TF legs should be used instead of CF whenever possible to avoid magnetic.
DF legs:
Direct to a Fix or DF Leg defines an unspecified track starting from an undefined position to a specified fix. Procedure designers should take into account the FMS flight path depends on initial aircraft heading aswell.
FA legs:
Fix to an Altitude or FA Leg defines a specified track over ground from a database fix to a specified altitude at an unspecified position.
FC legs:
Track from a Fix from a Distance or FC Leg defines a specified track over ground from a database fix for a specific distance.
FD legs:
Track from a Fix to a DME Distance or FD Leg defines a specified track over ground from a database fix to a specific DME Distance which is from a specific database DME Navaid.
FM legs:
From a Fix to a Manual termination or FM Leg defines a specified track over ground from a database fix until Manual termination of the leg.
CA legs:
Course to an Altitude or CA Leg defines a specified course to a specific altitude at an unspecified position.
CD legs:
Course to a DME Distance or CD Leg defines a specified course to a specific DME Distance which is from a specific database DME Navaid.
CI legs:
Course to an Interceptor CI Leg defines a specified course to intercept a subsequent leg
CR legs:
Course to a Radial termination or CR Leg defines a course to a specified Radial from a specific database VOR Navaid.
AF legs:
Arc to a Fix or AF Leg defines a track over ground at specified constant distance from a database DME Navaid.
VA legs:
Heading to an Altitude termination or VA Leg defines a specified heading to a specific Altitude termination at an unspecified position.
VD legs:
Heading to a DME Distance termination or VD Leg defines a specified heading terminating at a specified DME Distance from a specific database DME Navaid.
VI legs:
Heading to an Interceptor VI Leg defines a specified heading to intercept the subsequent leg at an unspecified position.
VM legs:
Heading to a Manual termination or VM Leg defines a specified heading until a Manual termination.
VR legs:
Heading to a Radial termination or VR Leg defines a specified heading to a specified radial from a specific database VOR Navaid.
PI legs:
Procedure Turn or PI Leg defines a course reversal starting at a specific database fix, includes Outbound Leg followed by a left or right turn and 180 degree course reversal to intercept the next leg.
HA, HF, HM leg types:
Racetrack Course Reversal or HA, HF and HM Leg Types define racetrack pattern or course reversals at a specified database fix