Definition
A hydraulic-mechanical transmission installed between an aircraft engine and an AC generator that holds the generator's input shaft at a fixed rotational speed regardless of changes in engine RPM. By keeping the generator turning at a constant speed, the unit ensures the AC electrical output stays at the required frequency.
Plain English
A device that sits between the engine and the generator and keeps the generator spinning at one steady speed, even when the engine speeds up or slows down. This keeps the aircraft's AC electrical power at a steady frequency.
Context Anchor
Seen in aircraft electrical system discussions, especially on turbine aircraft that use engine-driven AC generators.
Derivation
Constant-speed' simply means 'unchanging speed,' and 'drive' here means a mechanical unit that transmits power from one shaft to another. The name describes exactly what the device does: it drives the generator at a constant speed.
Why Pilots Care
Maintains stable electrical frequency and voltage output for reliable avionics and system operation.
Intuition Check
Do not read “constant-speed” as aircraft speed or propeller speed. Here it means the drive keeps the generator turning at a steady speed even when engine speed changes.
Example Sentence 1
After the constant-speed drive failed, the crew shut down the affected AC generator and continued the flight using power from the remaining generator.
Example Sentence 2
During the preflight inspection the mechanic confirmed the constant-speed drive was supplying consistent drive speed to the alternator.