Mechanism

CSDs are mainly used on airliner and military aircraft jet engines to drive the alternating current (AC) electrical generator. In order to produce the proper voltage at a constant AC frequency, usually 3-phase 115 VAC at 400 Hz, a generator needs to spin at a constant specific RPM (typically 6,000 RPM for air-cooled generators) . Since the jet engine gearbox speed varies from idle to full power, this creates the need for the Constant Speed Drive (CSD). The CSD takes the variable speed output of the accessory drive gearbox and hydro-mechanically produce a constant output RPM.

On aircraft such as the Airbus A310, Boeing 747-100 and 777, an Integrated Drive generator (IDG) is used. This unit is simply a CSD and an oil cooled generator inside the same case.

Both the CSD and IDG have a disconnect switch in the cockpit. The flight crew can disconnect a unit when it is overheating. This disconnect mechanism actually withdraws the drive shaft from the accessory drive on the engine by use of a solenoid engaging a threaded drive shaft. Once the unit is disconnected in flight, it cannot be reset. A mechanic on the ground must pull a reset handle to reset the unit. For this reason, the disconnect switch in the cockpit usually has a guarded switch cover to prevent an unintentional disconnect.

Hamilton Sundstrand is an American manufacturer of CSD and IDG units.

Integrated Drive Generator

An Integrated Drive generator (IDG) is simply a CSD and an oil cooled generator inside the same case. Troubleshooting is simplified as this unit is the single line replaceable electrical generation unit on the engine.