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What are two primary purpose of the APU?
The primary purpose of an aircraft APU is to provide power to start the main engines of an aircraft. APUs also generate electrical and pneumatic power to run the heating, cooling, and ventilation systems prior to starting the main engines.
Where does the APU receive fuel from?
One of the most important systems in the aircraft is the APU. It consists of a small turbine with a mostly purely electrical starter. The Auxiliary Power Unit taps fuel from the aircraft’s kerosene tanks and can in this way be started directly from the battery or external power supply.
Does APU have its own fuel tank?
The APU does not have its own fuel tank. The APU also has a fuel pump driven off the accessory gearbox that provides high pressure fuel required to produce atomized spray patterns from the fuel nozzels.
When should I turn off my APU?
In most cases, the APU is shut down before takeoff and reignited when the aircraft clears the runway after landing. While most of an APU’s active service life occurs as the aircraft sits on the ground, in some instances the APU is used as an emergency electrical power source while the aircraft is airborne.
Is APU always on?
Crews normally run the APU before flight for electrical power, air conditioning and engine start. After landing, the APU is started so engines can be shut down as soon as the aircraft arrives at the gate. If the APU is needed during flight, it can be started in about a minute.
What is the purpose of an APU on a plane?
The APU has several functions related to safety, convenience, and economy. The first and most straightforward use of the APU is to provide power when on the ground. It can be run when engines are shut down, and when boarding before the engines are started.
When does an APU need to be run?
It can be run when engines are shut down, and when boarding before the engines are started. The APU will run a generator that provides electrical power for the cockpit and cabin systems. It will also produce pneumatic pressure to operate the cabin air conditioning systems.
What happens when an APU is removed from an engine?
Pressure then builds up to further spin the engine and, once it reaches its idle power speed, the supply from the APU is removed. The other engines are then started, using either the APU or high-pressure air from the already started engine. This is known as ‘cross bleed’ and is also a technique used for re-starting a failed engine.
How is air supplied to the APU turbine?
Air supplied from the APU to the engines is at very high pressure, so traveling the distance to the engines has minimal effect. If you look carefully at the aircraft tail, you can also see an inlet valve for air for the APU turbine (this will be closed during flight).