Definition
A flight path in the airport traffic pattern flown at right angles to the landing runway off its approach end, connecting the downwind leg to the final approach leg.
Plain English
The short cross-wise leg you fly just before turning onto final, sitting at 90 degrees to the runway near its approach end.
Context Anchor
Used during airport traffic pattern flying and radio position reports, especially when a pilot says they are turning base or on base.
Derivation
From the idea of the traffic pattern as a rectangular circuit. 'Base' refers to the bottom side of that rectangle nearest the runway threshold — the leg that forms the base of the pattern before the final straight-in segment.
Why Pilots Care
Flying the base leg at the correct altitude and distance provides proper spacing and alignment for a stabilized final approach and safe landing.
Analogy
Think of the traffic pattern as a rectangle drawn on the ground around the airport; the base leg is one of the two shorter sides of that rectangle.
Intuition Check
Base leg does not mean the lowest part of the flight or the main part of the route. In a landing pattern, it means the crosswise segment between flying beside the runway and lining up with it.
Example Sentence 1
Turning base, the pilot reduced power, extended the flaps, and began the descent toward the runway.
Example Sentence 2
From base leg the pilot maintained altitude while scanning for traffic before turning final.