Definition
The act of rotating a feathered propeller's blades back from their streamlined, edge-into-the-wind position to a normal blade angle so the propeller can again produce thrust. On most light twins this is accomplished by repositioning the propeller control out of the feather detent, which allows engine oil pressure (and on some installations a dedicated unfeathering accumulator or spring) to drive the blades back to a low pitch angle suitable for restart and operation.
Plain English
Turning the propeller blades back to their normal angle after they were stopped and aligned with the airflow, so the engine can run and pull the airplane again.
Context Anchor
You will see this term in multi-engine airplane training, especially when discussing engine shutdown, engine restart, and feathering propeller systems.
Derivation
Built from 'un-' (reverse the action) plus 'feathering.' 'Feather' here refers to turning the blades edge-on to the wind like a bird's feather slicing through air; 'unfeathering' simply reverses that, returning the blades to a working angle.
Why Pilots Care
Required to restart a shutdown engine without the excessive drag a feathered propeller would otherwise create.
Intuition Check
Unfeathering is not the same as simply restarting an engine. It specifically means changing the propeller blade angle out of the feathered position so the propeller can turn and work again.
Example Sentence 1
After the practice engine shutdown, the instructor talked through the unfeathering procedure before moving the propeller control out of the feather detent.
Example Sentence 2
After verifying fuel flow, the crew completed unfeathering so normal thrust returned on the right engine.