Definition
The condition in which a propeller continues to rotate after engine power is lost, driven by the airflow passing through the propeller disc rather than by the engine. A windmilling propeller produces high aerodynamic drag and, in a multi-engine airplane, significantly degrades single-engine climb performance and controllability.
Plain English
When the engine quits or is shut down in flight, the air flowing past the propeller can keep it spinning, like wind turning a windmill. The spinning blades create a lot of drag on the airplane.
Context Anchor
Encountered in feathering-propeller discussions, especially after an engine failure or shutdown in a multi-engine airplane.
Derivation
From 'windmill' -- a device whose blades are turned by the wind. The propeller behaves the same way: instead of the engine driving the blades, the airflow drives them.
Why Pilots Care
A windmilling propeller produces high drag that reduces glide performance; feathering aligns the blades to stop rotation and minimize that drag.
Analogy
It is like a pinwheel held out of a car window. The pinwheel spins because the air is hitting it, not because it has its own power.
Grounding Statement
If the airplane is still moving forward after an engine loses power, the airflow can keep the propeller spinning.
Intuition Check
Windmilling does not mean the propeller is helping pull the airplane forward. It means the airflow is driving the propeller, often creating drag instead of useful pull.
Example Sentence 1
After the right engine failed, the pilot feathered the propeller to stop it from windmilling and reduce drag.
Example Sentence 2
The pilot pulled the propeller control to feather and halt the windmilling action.