"Auxiliary power unit", i.e. APU in aviation jargon and hereinafter, is an auxiliary propulsion unit and serves as an additional source of energy for the aircraft. In most commercial aircrafts, the APU is installed at the very end of the aircraft, i.e. at the tail of the aircraft. So an APU is a turbine engine that like any jet engine takes air, compresses it, adds a mixture of fuel and ignites it.
Although you haven’t paid so much attention to it, we’re sure you’ve had a chance to see the APU so far, or at least hear it while it works. For example, in Croatian airports, passengers are often transported by buses to aircraft parked at the parking positions that do not have the ability to connect to passenger bridges. If you find yourself in such a situation, the deafening noise you hear when disembarking from the bus and boarding the aircraft is the sound produced by the APU. This deafening sound is also one of the main reasons why ground personnel wear noise protection while working around aircraft.
As a reminder, listen to the sound and see how the APU works on the link below.
Why do planes have APUs?
As we said in the introduction, the APU is an auxiliary propulsion unit whose main purpose is to make the aircraft self-sustaining while parked on the ground. The APU serves to provide the aircraft with electricity on the ground and to enable the supply of compressed air to the cabin and the use of the air conditioning system. Also, the APU is used to start the aircraft engines themselves. In most cases, the APU turns off before the takeoff and turns on again when the aircraft approaches the parking position. The APU is mostly used while the aircraft is on the ground. There are also cases when the APU is used in the air, primarily for safety reasons. For example, an APU can be used as an additional power source if one or more of the aircraft’s main generators have a malfunction during flight. The APU can also provide compressed air and prevent the fuselage from freezing in the event of a pneumatic failure on the aircraft's main engines. Probably the APU is also the main reason why twin-engine jets are allowed to fly long overseas flights.
The reason why APU is often necessary lies in the fact that there are a number of airports that do not have enough equipment to provide each aircraft with the energy needed for the necessary functions of the aircraft while standing on the ground. Also, some airlines have their own procedures that prescribe the use of an APU in case of a short stay on the ground.
In the end, it should be noted that everything has its price, including the operation of the APU unit. Expensive aviation fuel is used for propulsion. Depending on the aircraft model and the operating conditions themselves, the APU may require exceptional amounts of aviation fuel to operate normally. For example, an APU on a Boeing B777 can consume about 230 liters of jet fuel per hour while the B747 can consume even about 400 liters of jet fuel per hour. Therefore, most airlines have their own procedures when the APU is used, and when airport equipment is used as an energy source for the aircraft on the ground.