Definition
A propeller that continues to rotate after the engine has lost power, driven by the airflow passing through it during flight. The airflow turns the blades like a windmill rather than the engine driving the blades through the air. A windmilling propeller produces significant aerodynamic drag, which steepens the glide and reduces the distance the airplane can travel without power.
Plain English
When the engine quits or is at very low power in flight, the moving air can keep the propeller spinning on its own. A propeller spinning this way acts like a flat disc in the airflow and creates a lot of drag, which makes the airplane sink faster.
Context Anchor
Seen during low-power approaches, power-off approaches, engine-failure practice, and landing discussions where propeller drag affects how steeply the airplane descends.
Derivation
From 'windmill' -- a structure whose blades are turned by the wind. The propeller is doing the same thing in reverse of normal: instead of the engine turning the blades to push air, the air is turning the blades. Knowing this picture makes it obvious why the propeller still spins even when the engine is dead.
Why Pilots Care
A windmilling propeller generates substantial drag that shortens glide distance and steepens the descent, directly affecting the pilot's ability to reach a suitable landing site.
Analogy
Like a pinwheel that spins when you run with it or blow on it, the propeller blades are turned by the wind rather than by any engine force.
Grounding Statement
If the airplane is moving forward and the engine is not producing useful pull, the airflow can still spin the propeller and make it act like a drag-producing disk.
Intuition Check
Do not assume a spinning propeller always means the engine is producing useful power. A windmilling propeller may be spinning mainly because air is pushing it around.
Example Sentence 1
After the engine lost power, the windmilling propeller increased drag and steepened the glide toward the field.
Example Sentence 2
The windmilling propeller increased drag and caused a steeper descent than expected during the simulated engine-out approach.