Aircraft APUs Auxiliary Power Units

An aircraft is almost totally dependent on the engines to provide not only thrust for flight but also power to drive hydraulic and electrical systems. Most aircraft use powered flight controls, and if all supplies to these are lost the pilot may not even have the facility to make a safe forced landing.

The aircraft will have batteries, but the small amount of energy which they can contain, compared to the enormous power requirements of even a limited number of emergency systems, rules these out as a stand-by power source for flight controls. Batteries are limited to providing the initial power source for engine starting, and to supplying a small amount of emergency power.

It is possible to store energy to power hydraulic systems, as stored pressure in a hydraulic accumulator, but again this has very limited capacity and is normally used to lower flaps and undercarriage, and perhaps to power a very small number of control surface movements.

So what is needed is another engine, which can act as a standby power source if the main engines are shut down or fail.

Such devices are  small gasturbine engines, called the auxiliary power units, or APUs. The APU can drive electrical generators and hydraulic and pneumatic pumps, and is also frequently used to provide power, either electrical or air, to start the main engines. It can also supply pressurised air for cabin air conditioning if required.

On the ground, during passenger embarkation, the APUs are used to supply power for all of the systems used. Because the APU is a small engine, it can be easily started using battery power, therefore batteries can be smaller than if they were required to start a main engine. The output from many APUs is sufficient to allow simultaneous starting of all main engines, saving time on the ramp. It is not normally run in flight, unless a main engine fails, in which case it will provide a second power source for the aircraft systems.

Aircraft APUs may be located anywhere on the aircraft, but of course it needs a flow of air into it, and provision to disperse the exhaust gases. On many airliners, the APUs are located in the aftmost end of the fuselage, and on combat aircraft typically located above the main engines aft of the cockpit. Since they are fuelled by standard aviation turbine fuel, they can use fuel from the same tanks as the main engines.

Aircraft ram-air turbines

It is extremely rare for all main engines to fail during flight, but one possible cause for this may be a fuel-related problem, either with its supply or quality. In this event, the APU, fuelled from the same source as the main engines, will probably also be lost, and in any case it takes time to start.

Some aircraft, both civil and military, have ram-air turbines RATs, which are deployed into the air stream automatically if power is lost. Acting like a windmill, with the air stream turning blades, which in turn power electrical and/or hydraulic power generators. The RAT can provide control in the first few vital seconds, allowing time for the crew to take action to bring the aircraft down safely.