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Auxiliary Power Units: A Look at the APU
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Auxiliary Power Units: A Look at the APU

Start Your Engines

Little did you know, if you have flown in a commercial airliner, you have experienced the benefits of the airplane’s auxiliary power unit or APU. Auxiliary power units are gas turbine engines which are used mainly in the operation of aircraft while they are on the ground.

APUs are located in the tailcone compartment of almost every large airplane. In fact it was the APU of a U.S. Airways A320 that landed in the Hudson River which provided the extra power necessary to keep the flight controls working as well as the displays so that the pilot was able to touch down safely at the lowest possible airspeed.

It’s a testament to the construction of these small turbine engines that the APU was able to start immediately when called upon to perform its duty. The two main manufacturers of APUs are United Technologies Corp. (via their subsidiaries Pratt and Whitney, Hamilton Sundstrand and Goodrich) and Honeywell.

What Does an APU Do?

The APU provides electricity, compressed air, and power to start the main engine. APUs also keep you cool by powering the air conditioning, give you light by providing electric power and help the pilot and flight crew by powering other systems in the airplane. Additionally, APUs can provide a source of backup electric power when the plane is in the air. They provide power for almost all functions of the airplane while it is on the ground except propulsion.

The main and most important purpose of the APU is to provide the power to start the main engines. An aircraft’s turbine engine needs to be accelerated to rotate at an extremely high speed for it to provide an adequate amount of air compression to keep it spinning. You can thank the APU for that task.

If you have a small airplane you do not need an APU as those engines are started using an electric motor. Big aircraft are started by an air turbine motor and thus the need for the APU to power that. Before the engine gets turned they need to start the APU (usually by battery or something called a “hydraulic accumulator”.

A “Power”ful Part Of The Airplane

Depending on the design, the APU can provide electric power, hydraulic power, pneumatic power or all three. Connecting to a hydraulic pump allows the crew to operate hydraulic equipment such as the flight controls or flaps. Having this function is also useful for a backup if there is an engine failure.

Aircraft APUs in general produce 115 volts of power at 400 hertz to run the electrical systems in the aircraft. Others can produce 28 volts DC. APUs can provide power through single or three-phase systems. The 400 hertz models are smaller and lighter than their 50 hertz counterparts but are more costly. Drawbacks of the higher frequency systems are voltage drops.

The aircraft APU is also able to get electrical or pneumatic power from ground equipment in case the APU failed or if the airport has restrictions on noise and pollution.

APUs are critical to aircraft safety as they supply the compressed air as well as backup electricity in place of a dead engine or failed main engine generator.

The APU In Military Aircraft

During World War II, APUs found their way into large military aircraft. They referred to them as “putt-putts” even when they mentioned them in training documents. The APU on the B-29 Superfortress bomber provided power to start the main engines and was used after take-off up to 10,000 feet. It took a break once airborne but when it was descending to land, the APU was restarted.

Instead of the tailcone where commercial aircraft now have the APU installed, some military aircraft had the APU located in the front of the airplane either inside the nose-wheel compartment like the B-24 Liberator or under the cockpit floor like the Douglas C-47 Skytrain transport aircraft.

The APU in Commercial Airlines

In 1963 the Boeing 727 was the first jetliner to utilize a gas turbine APU. That allowed it to operate at smaller airports that didn’t have ground facilities to power the airplane. You can locate the APU on modern airplanes by looking for the exhaust pipe in the tail of the airplane.

The typical APU on a commercial airliner is comprised of three main sections:


  • Power Section: This is the gas generator part of the engine and gives power to the shaft of the APU.
  • Load Compressor Section: This is a compressor mounted on the shaft. It provides pneumatic power to the aircraft. It has 2 actuated devices, the inlet guide vanes which regulate airflow to the load compressor and the surge control valve which allows the surge-free operation of the turbo machine.
  • Gearbox Section: This transfers power from the main shaft to an oil-cooled generator for electrical power. Inside the gearbox is where power is transferred to the engine accessories like the fuel control unit, the cooling fan and the lubrication module. There is also a starter motor connected through the gear train. It performs the starting function of the APU.

Boeing’s newest jet, the 787 Dreamliner, uses more electric power than the typical airliner so the APU is only used to deliver electricity to the airplane. Not having a pneumatic system simplifies the design but the tradeoff is the high demand for electricity means the requirement of heavier generators.

We hope you found our look at the auxiliary power unit interesting. ATC is one of the largest airplane spare parts providers and distributors in the world including APUs for the following aircraft:


  • Embraer E-Jets
  • Boeing 747-400
  • Boeing 747-8
  • Airbus A380
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