Definition
A path terminator leg used in instrument procedure coding that defines a course reversal flown on the outbound side of a fix, allowing the aircraft to lose altitude and/or reverse direction to become established inbound on the final approach course. The PI leg specifies an outbound course from the fix, a turn direction, a maximum distance from the fix, and ends when the aircraft is established inbound on the next leg. The pilot may fly the maneuver as charted (typically a 45/180 or 80/260 reversal) or any acceptable variation, provided the aircraft remains within the protected airspace.
Plain English
It is a coded instruction in an instrument approach that says: fly outbound from this fix, turn around, and come back established on the inbound course. You have some freedom in how you fly the turn, but you must stay within the distance limit shown on the chart.
Context Anchor
Seen in RNAV and instrument procedure coding, flight management system displays, and approach procedure design discussions.
Derivation
The 'PI' code comes from ARINC 424, the standard that defines how navigation databases store flight procedures. Each leg type has a two-letter code; PI stands for the Procedure turn leg that ends when the aircraft is established Inbound. Knowing this helps pilots recognize PI in FMS leg-page displays as the same maneuver they see drawn on the approach plate.
Why Pilots Care
Allows safe alignment with the final approach course without radar vectors and ensures obstacle clearance during the reversal.
Analogy
It is like following a marked U-turn in a road route: the turn is not a detour, it is the planned way to get lined up in the correct direction.
Intuition Check
Do not read “procedure turn” as just any turn made during a procedure. Here it means a specific published course-reversal segment used to get the aircraft established inbound.
Example Sentence 1
The approach required a procedure turn at CENTR, so we flew the outbound course, performed the 45/180 reversal within ten miles, and rolled out established inbound.
Example Sentence 2
After completing the PI leg the aircraft was established inbound on the localizer with the approach clearance in hand.