Definition
A path terminator leg type used in RNAV procedure coding that defines a procedure turn. The PI leg directs the aircraft to fly outbound from a fix on a specified course for a specified distance or time, then make a turn (typically 180 degrees) in a specified direction to intercept an inbound course back to the fix or onto the next leg of the procedure.
Plain English
A coded instruction in an RNAV flight plan that tells the aircraft's navigation system how to fly a procedure turn — fly out, turn around, and come back inbound on the correct course.
Context Anchor
Seen in instrument procedure design and in discussions of how GPS or flight management systems code and fly approach legs.
Derivation
PI stands for Procedure turn. The two-letter code is part of the ARINC 424 path-terminator system, where each leg of a procedure is given a short code that tells the navigation database how to fly that segment.
Why Pilots Care
Correct execution keeps the aircraft within protected airspace and positions it on the proper course for the final approach.
Grounding Statement
Picture reaching a published point on an approach, turning around as shown on the chart, and then lining up with the inbound course.
Intuition Check
PI does not mean “pilot initiated” here. It is a procedure-coding label for a procedure turn that ends by intercepting the inbound course.
Example Sentence 1
Reviewing the approach in the FMS, the crew confirmed the PI leg was loaded so the aircraft would fly the published procedure turn.
Example Sentence 2
After crossing the fix, the procedure called for a PI leg before turning inbound.