Hiding inside the tail of most every airliner is an extra engine called an Auxiliary Power Unit or APU. Time to find out what’s hidden in the tail of your aircraft!
The auxiliary power unit is an actual turbine engine, more appropriately called a turboshaft engine. Unlike the aircraft’s main engines, the APU outputs almost no thrust. Most of the power produced is used to run an electric generator and provide pneumatic (air) pressure.
the specific uses :
1- pneumatic power to start the engines
2-pneumatic power to run the air conditioning packs when the engines were not running.
3- electrical power on the ground and air, available regardless of whether the engines were running or not
Auxiliary Power Unit (APU) History:
The earliest APUs could be found on the B-29 Superfortress, looking essentially like a motorcycle engine installed inside the fuselage. The Convair XP5Y-1 also used an early APU, while America’s first jet airliner, the Boeing 707, was delivered without one. The 727 was the first Boeing to be APU-equipped.
Today, APUs can be found in medium-size and larger civil and military jets, some turboprop aircraft and a handful of military fighters. Smaller civilian jets like the Cessna Citation CJ or One Aviation’s Eclipse jet don’t carry an APU because the extra weight of even a small extra turbine engine can significantly impact the airplane’s useful load.