why do aircraft engines continue to spin even when they are on the ground?

Question by Jona: why do aircraft engines continue to spin even when they are on the ground?
Jet engine turbines spin slowly even when the a/c is at the gate. Is this just the wind blowing them or is there some reason to keep them spinning by powering them.

Best answer:

Answer by zhornback
The aircraft draws electrical power from the turbines’ rotation…like a wind farm generator. So if the plane is disconnected from the airport (meaning not drawing power from the airport terminal, regardless of if it is physically connected or not) and the pilots need power, at least one engine will continue to idle while the plane is on the ground, to power the plane’s systems. Not sure how many engines are needed to power the plane, but thats what they are for.

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  1. This is called “windmilling” in the aircraft industry. Lets imagine you were holding an un-powered fan into the wind outside. The wind would “push” the fan, or make the blades spin on their own. Now lets put this theory into an aircraft engine. Most aircraft engines today on larger planes are what’s called “high-bypass” engines. Without getting to involved in the operation of an engine, most of the air passing through the engine itself (fan air) is passed around the inner core of the engine giving the thrust needed to propel the aircraft forward. Because of this, the front “fan” you see at the gate is shaped just like a fan in your house only with more blades and of course the blades are elongated. As a result, even a very slight breeze from either the rear or the front of the engine can cause the engine to appear like it is in operation. For short periods of time this isn’t a problem but when aircraft are not going to be flying for an extended period of time, covers are placed over the intakes and exhausts of the engines to keep them from windmilling and to keep foreign objects out. Long term windmilling causes damage to the blades and undue wear to the internal parts of the engine.

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