The Honeywell GTCP660-4 is an auxiliary power unit that was installed in early model Boeing 747 airliners manufactured between 1968 and 1990.  Auxiliary Power Units are small engines that are run when an aircraft’s main engines are not running to supply power to the aircraft.

These small engines, called Auxiliary Power Units (or APUs), are now commonly used in all large aircraft where power is needed for their internal electrical systems when the main engines are not running.  They also generate the power to start the main engines.  Some APUs also power an aircraft’s hydraulic systems.

APU’s began appearing in large aircraft such as the Boeing B-29 bomber during World War 2.  They were usually small piston engines.  The first jet aircraft to have a gas turbine APU was the Boeing 727 airliner in 1963. The APU can be identified on many modern airliners by an exhaust pipe at the aircraft’s tail. This APU was originally designed and manufactured by Garrett Air Turbine Company (hence the designation) that was later acquired by Honeywell.  It generated up to 25.4kW of energy that was used to drive the electrical generator built into the unit.