Auxiliary Power Unit – (APU) of Plane


What is APU, what are its main functions? Why does an airplane have an APU? Let’s answer these questions.

I think at least some of the people who travel by plane get the attention. When you get off the plane or get on the plane, you will hear a thin turbine engine sound even though you see the engines of the plane not working. That voice belongs to APU Auxiliary Power Unit.

apu plane
Auxiliary Power Unit – APU

The APU is a turbo shaft engine generally in the tail section of aircraft. It is used to meet the electric and bleed air needs of the aircraft. Its main purpose is to meet the electricity needs while the aircraft is on the ground, to ensure the operation of the main engines by supplying bleed air and to provide air conditioning (air conditioning effect).

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.

Aircraft APUs generally produce 115 V alternating current (AC) at 400 Hz (rather than 50/60 Hz in mains supply), to run the electrical systems of the aircraft;


There are different APU engines for different aircraft models. The APU is a gas turbine engine. As we mentioned that the turbo shaft engine is a gas turbine jet engine variant. However, APUs are not used for purposes other than what we have mentioned since they do not have a response output that can be considered as thrust. Depending on the different APU types, there are certain service bases. They are generally inactive when the aircraft is flying, they are operated below a certain altitude/ flight level.

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