The Constant Speed Drive (CSD) is a mainly mechanical device, which is positioned between the aeroplane engine and the brushless AC generator. The CSD is designed to keep the generator running at a constant speed.

Excessive Constant Speed Drive (CSD) oil temperature, low pressures and low oil levels indicate an impending failure of the CSD. If any of these occur, there will be an indication on the flight deck to alert the flight crew. To prevent damage to the accessory gearbox and engine, the pilot is required to  disconnect the CSD in flight through a "dog clutch" remotely operated from the flight deck. It can be reconnected on the ground.