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The APU and its benefits

Picture of Alberto Herranz

Alberto Herranz

Ibérica del Espacio, S.A.


Though the general public are familiar with many of an aircraft’s pieces of equipment and systems, the APU (Auxiliary Power Unit) is a relatively unknown system despite the many benefits it provides. Along with the ECB (Electronic Control Box), the electronic system which controls it, it makes up the APS (Auxiliary Power System). This article, however, solely focuses on the APU.

The APU and e-Taxi will soon allow a 4% saving in aircraft fuel, as the engines will not need to be started up until just before takeoff.

The APU is a completely independent system which operates on the ground and in flight. Its main function consists of providing energy should it be necessary. It does not provide propulsion to the aircraft. Rather it provides electrical and pneumatic energy, either in cases where support is needed or in the event of an emergency. With this supply of energy, the APU allows the main engines to be started up, electrical energy to be supplied and the air conditioning to be turned on when the engines are shut down. It also provides energy during the flight.

The APU is usually located at the tail section of the aeroplane (Section 19.1), where exhaust fumes can be seen when it is turned on.

Who hasn’t wondered why the lights go out very briefly in some older aircraft when we have already boarded and are waiting for the aircraft to depart? The answer is that the APU is working. That is why the lights go out briefly (transition) when the power supply is transferred from the APU to the main engines, which have just been started up.

On a technical level, an APU is a gas turbine which is divided into three sections:

  • Power section: turbine engine which provides power to the APU.
  • Load compressor: which supplies pneumatic pressure to the aircraft.
  • Gearbox: which transmits the APU’s power to an electric generator that in turn supplies electricity to the aeroplane.

The most interesting thing about the APU is the benefits it provides to airlines:

  • Cost savings: When the aeroplane is on the ground, it does not have to keep its engines running to supply air conditioning or lighting to the cabin. This reduces fuel consumption and maintenance costs, since it can obtain this energy without having to keep its main engines running.
  • The aircraft’s energy independence: When the aircraft is on the ground or when maintenance work is being done, the APU allows the aircraft to operate independently, without the need for an external source of energy or having to start up its main engines. It can thus supply energy to the aircraft’s electrical and pneumatic systems. Hence, the airline will not be obliged to pay the airport for this additional service, since it can obtain energy from the APU and has the possibility to choose the cheapest option. Moreover, it should be taken into account that the option of external power supplies is not available at some small airports.
  • Supply of emergency energy: To a great extent, the APU makes it possible for the aircraft to be granted ETOPS (Extended Twin Operations) certification. In the event of an engine failure, the APU will be essential, since it has the capability of supplying the electricity and the air conditioning the engine should provide. Aircraft granted ETOPS certification can service long-distance routes, especially routes crossing deserts, oceans and polar areas, which twin-engined aircraft were formerly not allowed to service. The APU therefore provides an additional benefit to airlines, as they can operate much longer routes with the same aircraft.
  • Essential for some in-flight phases/situations: The APU can be essential to start up the engines in flight. In the improbable case that both engines shut down, the APU can provide the power needed to start them up in flight and land at the nearest destination. It also provides pneumatic energy until the aircraft reaches a certain height.

Like any other system, the APU is a system undergoing continuous evolution. Its future is therefore linked to its continuous improvement. The new equipment under development seeks:

  • An increase in efficiency through a considerable reduction in fuel consumption, which also reduces polluting emissions.
  • A reduction in the equipment’s weight. All the equipment – in other words the APU + ECB – currently weights between 140 and 170 kg in an A320.
  • An increase in the system’s reliability.

Future applications will include e-Taxi, an application which allows the aircraft to taxi with a partial or total supply of energy from the APU, without the need to use the main engines. Fuel savings of 4% for each 500 nautical mile flight in an A320 are forecast, because it won’t have to start up its engines until it is time to take off. Polluting emissions are also reduced while the aircraft is taxiing.

In the longer term, the trend will be to progressively install fuel cells, which provide significant efficiency enhancements, while reducing polluting emissions and fuel consumption.



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