featherable
Of a propeller: capable of having its blades streamlined by rotating them perpendicular to the axis of the propeller when the engine is shut down, so that the propeller does not windmill as the aircraft flies.
featherable: propeller blades that can go flat
A featherable propeller is one whose blades can be rotated on their longitudinal axis until they align with the airflow, presenting their edges rather than their broad surfaces to the wind. This action, called feathering, is performed by hydraulic or electric actuators controlled from the flight deck. When feathered, the blades generate no thrust and produce minimal drag, which is essential when an engine has failed or shut down.
The mechanism relies on a pitch-change system: a rotating hub with a helical or cam-follower mechanism that translates axial or radial motion into blade rotation. Most featherable propellers use a constant-speed governor that maintains optimal pitch during normal flight, then allows full feathering when the pilot moves the prop lever fully aft. Some older designs required manual feathering; modern turboprops accomplish it automatically when engine power drops below a threshold.
Without feathering, a windmilling propeller on a dead engine increases drag dramatically and can cause severe vibration. A propeller left in low-pitch (high RPM) position on an inoperative engine acts like a brake, reducing the aircraft's single-engine performance and range. Feathering can reduce the drag penalty of engine failure by 50 percent or more, making it critical for twin-engine aircraft operating over water or terrain where landing options are limited.
Construction and failure modes
Featherable propellers are more complex and heavier than fixed-pitch or simple constant-speed designs. They contain actuators, seals, and a beta valve or equivalent control logic. Common failures include loss of hydraulic pressure, which can trap a propeller in an unsafe pitch position; corrosion of the pitch-change mechanism; and, in older propellers, stiction (static friction) that prevents feathering when needed. Regular functional checks and overhaul at published intervals are mandatory.
Featherable props became standard on twin-engine aircraft from the 1940s onward and remain essential on any multi-engine plane certified for single-engine flight with meaningful performance. The term is less common in modern parlance than in earlier decades, but the capability remains non-negotiable for commercial and high-altitude twin operations. Single-engine aircraft rarely use featherable propellers because the cost and complexity outweigh the benefit.