When the power being produced by the engine drops to the point where it is not contributing to thrust, the propeller will go into a feathered mode to reduce drag.
[1] The auto feather system also allows pilots to reduce the drag of the propellers during an engine failure, therefore, allowing the plane to glide for a longer period of time.
An automatic feathering system was first introduced on the Martin 4-0-4 aircraft, a piston engine airplane.
The system was designed to automatically feather an engine that failed during takeoff or initial climb.
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