Auxiliary Power Unit (APU)

Did you know that commercial planes have an engine which aren’t in wings? It is the auxiliary power unit and this post’s subject.

Location and functions

The auxiliary power unit is in rear end of airplane. The arrow indicates gas outlet.

Your functions are: Provide compressed air for start the turbines, CA electric power for airplane’s electric systems, air for air conditioning system and oil pressure for hydraulic systems. Before turn on main turbines, you must turn on APU. It is turned on when plane is on the ground with main turbines off or when it’s necessary during flight. Air flux for turbines is controlled by valves. When plane is grounded, can be used a ground start unit to provide compressed air.

In small planes, an electric motor is enough to turn on small engines. However, big turbines need a considerable quantity of compressed air to start.

How it works?

The auxiliary power unit is a jet turbine, uses the same fuel for wing turbines. Triggers an electric generator, pneumatic and hydraulic pumps.

APU is connected to AC generator through a gear box. A Flap is opened, allowing air inlet, which is compressed and provided by APU through bleed control valve. Generator supplies a Electronic control box (ECB), a digital controller which functions are:

Transmit data to panel when it’s on.
Control bleed control valve.
Monitor compressed air, called bleed air.
Start and monitor the generator.
Control fuel flux.
Control ignition unit.

In other means of transport

APUs also are present in helicopters, trains and trucks. This is auxiliary power unit for Mi-8, Mi-17, Mi-24 and Mi-28 helicopters.

In helicopters, APU is close to main rotor.

In trains, auxiliary power unit is also called static inverter. Provides energy to internal electric systems like lighting, air conditioner, battery, etc.