Definition

A constant-speed drive (CSD) is a component that allows a generator on the accessory drive of a turbojet engine to run at a constant speed as engine RPM varies.

Description

To provide useable electrical power, an aircraft generator must furnish that power at a specific frequency. Most modern aircraft need three-phase alternating current at 400 hertz.

If a generator were geared directly to the accessory drive of a jet engine, its speed would vary with engine RPM. For that reason, generators are connected to a CSD. The CSD works by taking the rotational output from the accessory drive and hydro-mechanically converting that output into a constant speed.

In some aircraft, the CSD and generator are combined inside the same housing. This unit is called an integrated drive generator (IDG).

If a generator malfunctions, it must be disconnected from its CSD. Pilots normally accomplish this, under guidance from a quick reference handbook (QRH) or other manual, by moving the generator switch to the disconnect position and holding it there momentarily. Once a generator is disconnected, it cannot be reconnected in flight.