Definition
Small, flat, fin-like vertical strips mounted on the upper surface of a wing, running chordwise (front to back), used to control the spanwise flow of air and prevent the stall from spreading from one part of the wing to another. By blocking air from sliding sideways along the wing, they help the wing stall progressively rather than all at once, preserving aileron effectiveness at high angles of attack.
Plain English
Thin metal strips standing up on top of the wing, running front to back, that stop air from sliding sideways across the wing. This makes the wing lose lift in a controlled way during a stall instead of all at once, so the pilot keeps better control of the airplane.
Context Anchor
Seen during preflight inspection of the wing and in discussions of how a wing behaves near a stall.
Derivation
The word 'fence' is used here in its everyday sense: a barrier that stops something from crossing. In this case, the barrier stops airflow from drifting sideways along the wing. Knowing that helps the picture stick: it's literally a small fence on top of the wing.
Why Pilots Care
They reduce the chance of an abrupt wing drop or loss of aileron control by ensuring the wing stalls first at the root rather than the tip.
Grounding Statement
Picture air trying to slide sideways across the wing near a stall; the stall fence helps keep that air moving more nearly front-to-back over the wing.
Intuition Check
Do not read “stall” here as the engine quitting. In this term, “stall” refers to the wing losing smooth airflow, and the “fences” are small airflow-control plates, not large barriers.
Example Sentence 1
The swept-wing trainer has stall fences on each wing to help the inboard section stall before the outboard section.
Example Sentence 2
On the swept-wing trainer the stall fences prevented the tips from stalling first and causing a sudden roll.