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Auxiliary power units, or APUs as they are more commonly known, are devices that supply essential electrical power, pneumatics or hydraulics to a vehicle when the regular operational sources are not available. APUs are found on aircraft, boats, trucks and trains, and may be diesel, electrical or turbine powered. By far, the most common application of the auxiliary power unit is in the aviation industry.
Most commercial turbo prop and jet engine aircraft have APUs that supply electric power, compressed air and hydraulic pressure to the aircraft systems. These are needed to supply essential resources during periods when the aircraft is undergoing maintenance or is being prepared for a flight prior to the main engines being started. The primary source of in-flight electric power, pneumatic and hydraulic pressure and air conditioning on commercial aircraft comes from generators and pumps which run through auxiliary gearboxes on the main engines. During ground operations, the main engines are very seldom started, although power on the aircraft systems is very often essential for testing, maintenance, pre-flight cabin and flight deck preparation and air-conditioning.
An aircraft's auxiliary power unit is typically a small jet engine situated in the tail section of the fuselage behind the rear cabin bulkhead. This engine drives an electric generator, as well as pneumatic and hydraulic pumps that supply the aircraft with enough power, air and oil pressure to run all essential systems. These engines typically draw their fuel from the aircraft's main fuel tanks and have a self-contained electric start system.
Once the aircraft's main engines are running and supplying essential power, the auxiliary power unit is stopped, but it can be started at any point in the flight to supply emergency power and pressure should the need arise. These jet engines have an exhaust port situated in or adjacent to the tail cone of the aircraft. Passengers who have embarked via the rear side doors, or in the case of Boeing 727s and DC 9s the rear tail doors, will certainly have heard the very loud jet noise at the rear of the aircraft that emanates from the APU exhaust.
Auxiliary power units are also commonly found on refrigerated trucks and trains that require the cooling cycle to continue even when the main engines are not running. These are generally diesel or electrically driven and typically supply power only to the vehicles refrigeration unit. Intensive care ambulances also have a small APU that can supply power to the emergency systems in the case of an engine failure. Another of the more common auxiliary power unit applications are those found in large boats and ships that require power during periods of main propulsion shut down while docked or anchored.
You see a lot about APUs when reading about aircraft. No one wants to be on board a stopped plane if the APU fails and the power (and air conditioning) goes out. That is so very not fun.
If you listen, you can hear the APU as you board the aircraft. You can usually hear the air conditioning, obviously, but the APU has a much higher pitch than the a/c motor. If you listen for the higher pitch, that's the APU. You can sometimes hear it cut off just before the pilot starts the main engines, too. It just has a different sound and if you're listening for it, you can hear it.