Definition
An engine whose propeller and internal components are being rotated by airflow passing through the propeller in flight, rather than by the engine producing power itself. The airstream drives the propeller, which in turn keeps the engine turning even though it is not generating useful thrust.
Plain English
An engine that has stopped producing power but is still being spun by the air flowing past the propeller as the airplane moves through the sky.
Context Anchor
Seen in turbine engine and turboprop discussions, especially during power loss, shutdown, restart, and drag-control procedures.
Derivation
From 'windmill' — a device whose blades are turned by the wind. A windmilling engine works the same way in reverse of normal operation: instead of the engine spinning the propeller, the wind (airflow) is spinning the propeller, which spins the engine.
Why Pilots Care
A windmilling propeller creates high drag that steepens the descent and shortens glide distance, often requiring immediate feathering to restore control and performance.
Grounding Statement
Picture the airplane moving forward after an engine loses power: the air keeps striking the propeller and spins it, even though the engine is no longer driving it.
Intuition Check
A windmilling engine is not an engine producing normal power. It is being spun by the outside airflow, much like a windmill, and may add drag rather than thrust.
Example Sentence 1
After the engine quit, the pilot noticed the propeller was still windmilling as the airplane descended.
Example Sentence 2
The checklist called for feathering the propeller to stop the windmilling engine and reduce drag during the forced landing.