Feathering

Feathering is the act of rotating propeller blades to an edge-on position (approximately 90 degrees blade angle) relative to the airflow, eliminating the drag of a windmilling propeller. Feathering is performed when an engine fails or is shut down in flight.

An unfeathered, windmilling propeller on a failed engine creates enormous drag, severely degrading aircraft performance and controllability. Feathering the propeller is one of the first critical actions in a multi-engine aircraft engine failure checklist.