Definition
In ARINC 424, leg types are standardized two-letter codes that define how an individual segment of an instrument procedure is to be flown, specifying the path geometry and the termination condition for that segment. Common examples include TF (Track to a Fix), CF (Course to a Fix), DF (Direct to a Fix), CA (Course to an Altitude), CI (Course to an Intercept), HM (Holding to a Manual termination), and PI (Procedure Turn). Each leg type tells the flight management system both how to fly the path and what condition ends that path before the next leg begins.
Plain English
Leg types are short codes used in the navigation database to tell the aircraft's flight computer two things about each piece of a procedure: what kind of path to fly, and what marks the end of that piece. They are the building blocks that an FMS uses to assemble a full departure, arrival, or approach.
Context Anchor
Seen when studying how instrument procedures are coded in navigation databases and how RNAV or FMS equipment follows those procedures.
Derivation
From the ordinary navigation use of 'leg' to mean one segment of a route between two defined points. ARINC 424 borrowed the word and added 'type' to mean the rule set that governs that segment.
Why Pilots Care
Different leg types fly differently even when the chart looks similar. A CF leg flies a specific course to a fix; a DF leg flies directly to a fix from wherever the aircraft is; a CA leg ends at an altitude rather than a place. Understanding the leg type explains why the FMS may behave one way on one procedure and differently on another that looks visually alike.
Intuition Check
Do not read “leg types” as just a general description of route parts. In ARINC 424, it means specific coded segment instructions used by the navigation database.
Example Sentence 1
The missed approach began with a CA leg, climbing on runway heading until reaching 2,000 feet before turning toward the holding fix.
Example Sentence 2
An RF leg type allows the aircraft to fly a constant-radius turn around a fix during a curved approach segment.