Summary: APUs play a pivotal role in the aircraft startup process. This guide is designed to familiarize you with how they function.
Most large jet aircraft come built with an Auxiliary Power Unit (APU). If you aren’t familiar with an APU, it’s a turbine power source that is typically built in the back of the plane and can only be started by the onboard batteries. Now, once the APU starts, it provide a sufficient amount of electrical power, as well as bleed air, to fully start the engines.
Some Background Info on the APU
Not every plane has an APU equipped, but more commonly the modern ones will have an APU to bled air and provide a significant amount of air pressure to start up – most airports have mobile units on standby that are ready to handle this once the plane is on the ground.
Piston planes, which aren’t typically used for commercial purposes anymore, can be started purely by the internal batteries. If you’ve ever flown in a piston plane from the early 40s, you’ll encounter what’s known as a Coffman Starter, which essentially utilized a prop that’s similar to a blank shotgun shell to blast a significant amount of pressure into the cylinder to start the engine.
Turbo Props
There are various turbo props that have turbines that are small enough to be started by an electrical starter. Now, this extends out to small jets as well.
If a plane it on the ramp and the batteries are dead or they die whilst in the process of starting the APU, most airports have a Ground Power Unit (GPU) to help start the plane and provide a significant amount of power. This is an essential piece of maintenance equipment that is either portable or supplanted within the ground and connected to a central power point.
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