What is a defining feature of a controllable pitch propeller?

A controllable pitch propeller is designed to allow the pilot to adjust the angle of the propeller blades while the aircraft is in flight. This capability enables the pilot to optimize the propeller's performance under varying flight conditions, enhancing efficiency, climb performance, and overall aircraft handling. By changing the blade angle, the pilot can increase thrust during takeoff and climb or adjust for cruise flight, improving fuel economy and performance.

The other options do not accurately describe a controllable pitch propeller. A fixed blade angle is a feature of fixed-pitch propellers, which cannot be altered during flight. While controllable pitch propellers do require maintenance, option C implies a lack of need for regular servicing, which is misleading. Lastly, option D incorrectly implies that controllable pitch propellers only function under manual control, whereas many modern systems include automatic adjustments based on flight parameters, allowing for enhanced performance without constant manual input.