The Garrett Auxilary Power Unit (APU) is fitted in the extreme rear fuselage of the BAC 1-11 and Trident Airliners.
This is a single Combustion Chamber Gas Turbine Engine running at 40,000rpm. It's Fuel consumption is 40 imperial Gallons (400 pounds) per hour.
It's function is to supply Compressed Air for engine starting and Air Conditioning and Electrical Power for Aircraft Services. In certain circumstances it can be used in the air for an emergency electrical supply should there be a failure of the normal Aircraft Electrical System.