What does the feather and autofeather do exactly?

Question

When it comes to Turboprops

Answer 1

Feathering a propeller, whether manually or automatically (called "autofeather) moves the blades so that they are parallel to the airflow and offer little or no resistance: i.e. they no longer create drag. The autofeather mechanism senses when something is terribly wrong, typically when the power drops to the point where it is not contributing to thrust, and feathers the prop automatically without the pilot having to do anything, so long as the system was armed to begin with.  To feather a prop, oil pressure to the propeller dome is released and the blades move toward the full-feather position through the centrifugal action of counterweights.

Answer 2

"Feather" means to move the propeller of a failed engine to an angle where the relative wind does not make it move ("windmill").  A windmilling propeller creates a lot of drag.  This drag is greatly reduced (to almost nothing!) when the propeller is feathered.

An autofeather system automatically and immediately feathers a propeller if the engine fails without the need for pilot input.  If a plane has an autofeather system installed, it is normally armed before takeoff so if an engine fails at low altitude, the pilot doesn't need to take any action other than flying the plane.