Definition
A turn executed by an aircraft during the initial approach segment between the end of the outbound track and the beginning of the intermediate or final approach track. The tracks are not reciprocal, and the base turn is specified in terms of the outbound track and timing or the outbound track and a DME distance from the facility.
Plain English
A turn flown during an instrument approach that connects the outbound leg to the leg leading to final approach. Unlike a procedure turn, the outbound and inbound paths are at an angle to each other, not exactly opposite, so a single turn at the right point lines you up for the final approach.
Context Anchor
Seen on instrument approach or departure procedure descriptions, especially where the procedure needs to turn the aircraft onto the next course in a controlled way.
Derivation
Base' here borrows from the traffic pattern term 'base leg' — the segment flown roughly across the runway extension before turning final. A base turn does the same job in instrument procedures: it positions the aircraft to roll out on the final approach course.
Why Pilots Care
A base turn must be flown as charted — the outbound track and the timing or DME fix determine where the turn begins. Starting the turn early or late puts the aircraft off the protected airspace and may result in a poor intercept of the final approach course.
Intuition Check
Do not read “base turn” as the traffic-pattern turn from downwind to base. Here it means a published instrument-procedure turn used to get established on the next course.
Example Sentence 1
After crossing the IAF outbound, the pilot timed the leg and then began the base turn to intercept the final approach course.
Example Sentence 2
After completing the base turn, the aircraft was established on the final approach segment with the runway in sight.