Definition
A mechanical or hydromechanical transmission mounted between an aircraft engine and its AC generator that converts the variable rotational speed of the engine into a constant output speed. This constant output is required so the generator produces alternating current at a fixed frequency (typically 400 Hz) regardless of engine RPM.
Plain English
A gearbox-like unit between the engine and the generator that keeps the generator spinning at the same speed even when the engine's speed changes. This keeps the electricity it produces at a steady frequency.
Context Anchor
Seen in aircraft electrical power system descriptions, maintenance checks, and generator drive fault indications.
Derivation
"Constant-speed" describes the unchanging output rotational speed; "drive" is the mechanical assembly that delivers power from one shaft to another. Together: a drive unit whose output stays at one steady speed.
Why Pilots Care
Stable generator output prevents electrical frequency fluctuations that could damage avionics or cause system failures in flight.
Intuition Check
A constant-speed drive does not keep the airplane or the engine at a constant speed. It keeps the generator turning at a steady speed so the electrical output stays usable.
Example Sentence 1
The technician inspected the CSD for oil leaks during the engine accessory check.
Example Sentence 2
If the CSD fails, the generator may produce unstable power and require the crew to switch to an alternate source.