Definition
A movable, hinged panel on the engine cowling that the pilot opens or closes to control the volume of cooling air flowing across the engine cylinders. Opening the cowl flaps increases airflow and cooling at the cost of additional drag; closing them reduces airflow to keep the engine warm during low-power, high-airspeed, or cold-weather operations.
Plain English
A small door on the engine cover that the pilot can open or close to let more or less air flow over the engine. Open it when the engine needs more cooling, close it when it doesn't.
Context Anchor
You may see cowl flaps in engine temperature procedures, abnormal engine instrument indications, and checklists for airplanes equipped with engine cooling controls.
Derivation
Cowl comes from the Latin cucullus, meaning hood or covering. The engine cowling is the hood over the engine, and the cowl flap is the adjustable flap built into that hood.
Why Pilots Care
Correct operation prevents overheating while keeping drag low, protecting the engine and improving performance and fuel economy.
Intuition Check
A cowl flap is not a wing flap and does not control lift. It controls cooling airflow around the engine.
Example Sentence 1
During the climb, the pilot opened the cowl flaps to keep the cylinder head temperature within limits.
Example Sentence 2
After reaching cruise altitude the pilot closed the cowl flaps to reduce drag and improve fuel efficiency.