APU - Auxiliary Power Unit

Auxiliary power unit – the APU

Tim Takeoff
1 picture
6 minutes

The operation of a large commercial aircraft requires an almost endless combination of different systems to ensure safe operation. One of the most basic components of an aircraft, besides the engines and the flight control, is undoubtedly the Auxiliary Power Unit (APU).

The aircraft’s ignition key

Putting a car into operation is easy. You just take the ignition key and start the engine. Complex electronics then take over the starting process in order to bring the engine to life via battery, glow plugs or spark plugs and an electric starter motor. Fuel is added via a pump.

With vintage cars – almost forgotten in modern times – you even still need idling mixture, a so-called choke, or you may have to pump fuel into the combustion chambers. If we go a little further back in time, we might even need a crank handle to get the crankshaft going.

APU - Auxiliary Power Unit
APU – Auxiliary Power Unit of an A380 / © wikimedia commons – David Monniaux

Cold and-dark

If you look at a modern commercial aircraft in this context, it is a rather helpless heap of metal in a “cold and dark” state – i.e. completely switched off and “cold”. There is no ignition key, no direct automatic start to wake up the electrics, hydraulics, air conditioning, and last but not least the engines. But how can such a highly complex system be put into motion at all?

Let there be light!

First, the aircraft can be connected to the “local” power grid on the ground or to a ground power unit. This is simply a diesel generator. If the position at which the aircraft was parked does not have such features, it is still not as helpless as you might think. Because even a jet has a permanently installed battery. This basically functions as a backup power supply unit, in order to supply the most important systems in the cockpit with power for a limited period of time, even in the event of a complete power failure. Once you have entered the “cold” aircraft and the cockpit via the door, you can activate the most necessary systems via this battery.

A small auxiliary turbine – APU

Electricity, water and air

After both battery and APU have been started, the aircraft is powered by that small generator (usually in the tail of the aircraft). The cockpit is brightly lit, all other systems can be started up. Light, water pumps for the toilets, infotainment and air conditioning, both in summer and winter, are provided. Now all that remains is to start the engines and lift off the aircraft.

Starting the engines

As we have already explained in our article on jet engines, an engine needs compressed air to start. This compressed air, also called bleed air, drives the turbine up to a certain speed, at which point it can ignite and continue to run by itself. The compressed air as well as the electricity can be supplied externally.

If external help is also missing at this point, the APU can help out. To start the engine, the air conditioning of the cabin is briefly suspended in order to divert the bleed air of the APU directly into the engine to be started. Once it has reached the required speed and has been successfully ignited, the system switches back. The compressed air goes back into the air conditioning until the second engine, or further engines are started up with the compressed air of the first engine.

APU - Auxiliary Power Unit
APU – Auxiliary power unit in the tail of the aircraft with open canopy covers

Logic of supply

Once the engines are running successfully, the power supply changes. The electronics now switches all supply lines to the engines. Compressed air and electricity can be taken from these. The APU is therefore “theoretically” superfluous and is switched off in most cases.

However, since the extraction of bleed air withdraws a certain percentage of the power from the engine, it may be necessary to demand full power from the engines on particularly hot days or at high altitudes. For this purpose, the APU is allowed to continue running until shortly after take-off. The packs can therefore continue to be supplied by the APU for take-off and do not draw any further energy from the engine. Only in the air does the pilot switch to power supply purely by the engines.

Always on the spot in an emergency

However, the APU’s job is not yet completed. Its task  always to provide the necessary redundancy in the system in the event of a major problem. If one or more engines fail in the air, the APU is immediately called “to the rescue”. In modern jets, it even starts up completely automatically in such cases and relieves the engine that is still running. If all engines fail, the APU takes care of what is probably the most important job: it supplies the hydraulics with power and compressed air to control the rudders, flaps and landing gear. In this condition, the aircraft can even be landed in gliding flight.

The “rat”

RAT - Ram Air Turbine
Ram Air Turbine (RAT) A350-900XWB – Can be seen in the lower left corner next to the letters XWB / © wikimedia commons – Laurent Errera

Should even the APU have a bad day, the on-board battery could supply the cockpit with power for at least 30 minutes. In this case, a small propeller will also pop out of the fuselage. This so-called ram air turbine, in short RAT, and affectionately called “rat” by pilots, functions as last instance. But more on this essential “creature” soon in another article.

The APU is thus a complex and incredibly important element of the system “aircraft”. Even after landing, the APU is switched on again immediately so that it is not “in the dark” after shutting down the engines.

If you soon have the opportunity to walk from the bus to the plane again, you can almost always hear the small turbine hissing loudly in the background.

Here “Captain” Joe explains to you once again on his YouTube channel what an APU is:

by Tim Takeoff

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