The hidden engine
The operation of a large commercial aircraft requires an almost endless collection of different systems to ensure safe operation. One of the most elementary components of an aircraft, besides the engines and flight control, is without doubt the Auxiliary Power Unit, or APU for short.
In order to understand the importance and functions of the APU, let us discuss a simple example from our daily life, which is driving and more specifically starting a car. This process is far from complex ; put the keys in the right place, turn the keys and magic ! The car will start with the help of electronical systems, that will take over the starting process and bring the engine to life via battery, sparks and fuel. However, if we look at a stationed aircraft waiting to be started, it will be in 100% of the cases in the "Cold-and-Dark" state, which means that it is nothing more than just a sitting pile of metal with no energy in it. With no ignition key, engines, hydraulics and no direct starting system the question remains ; how can such a complex system even be set in motion ?
The first solution would be to connect the aircraft to a "Ground Power Unit "(GPU), which is merely a diesel engine. However, if the position in which the plane was parked does not offer such facilities, the APU represents another alternative to start the aircraft.
An Auxiliary Power Unit or APU is the hidden engine installed in almost every modern aircraft providing electrical power to its systems as well as bleed air for air conditioning and engine start. It is the center piece of the starting process if external help is missing.
The unit consists of a conventional gas turbine engine with an electrical starter. Other main components of the unit are the load compressor, controlling the pressurized air supply, and a load gear box driving 1 or 2 generators depending on the aircraft's configuration. The APU first taps fuel from the tanks and can be started directly with the help of an electrical starter. The unit builds that way a complete power supply system, that controls various types of energies; electrical, pneumatic and even hydraulic. After the start, the generators, often located in the rear part of the plane, provide enough power to lighten up the cockpit and start the electrical systems and the AC-system in the cabin. If external help is still missing at this point, the air conditioning operations could be briefly suspended in order to divert enough compressed air to the engines' intakes. The compressed air, also known as bleed air, drives the turbine up to a certain speed, from which it can ignite and continue functionning by itself. Once the required engine speed is reached, the system switches back to ist initial configuration. The compressed air flows back into the AC system until all engines have started by using air from the first ignited engine. Once all engines are running successfully without external help, all supply lines are changed towards the engines. Compressed air and electricity will be taken from these lines. At this point, the APU is superfluous and switches off in most cases. However, the unit remains of a great importance during the flight, as it can provide pressurized air for the Environmental Control System (ECS or electronics) as well as for engine restarting in case of a failure.
Modern versions of the APU such as on the Boeing Dramliner 787 are developped as an all electric unit with no bleed air. Because the 787 uses more electricity than do other airplanes, it also generates more electricity, via six generators: two on each engine and two on the auxiliary power unit .On the ground, the 787 can be started without any ground power: The APU battery starts the APU generators, which start the APU to power the engine generators, which then start the engines without the use of bleed air in order to reduce fuel consumption and thus leading the way for eco-friendlier systems.
Photo credits : 787updates.newairplane.com - Institute of Aircraft Design _TU
