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Why do the jet turbines spin while the engines are turned off? Is this as a result of the wind? Or are the turbines deliberately spun using the APU? If so what is the reason behind this?

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    $\begingroup$ I could swear this is a duplicate but I can't find it. $\endgroup$ – GdD Jan 16 '18 at 9:46
  • $\begingroup$ Perhaps a vague resemblance to aviation.stackexchange.com/questions/8840/… That question asks about the rattling noise heard due to the windmilling of turbofans. $\endgroup$ – Sanchises Jan 16 '18 at 16:15
  • $\begingroup$ They have "turning gear" ; The purpose is to keep the shaft/s turning as the engine cools down to avoid any "sag" . The shafts must be kept perfectly straight. $\endgroup$ – blacksmith37 Jan 16 '18 at 20:42
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After shutting down they still have momentum. Or like you said, because of Windmilling - freely turning when the wind impacts the compressor blades.

The APU can, but more often than not doesn't, rotate the engines (overly simplified, not to go into unrelated details).

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    $\begingroup$ An aircraft parked in a tailwind can even cause the engines to rotate backwards. Can you please expand your comments about the APU? You are implying that the APU is rarely used to rotate the engines when in fact it is used every time when starting the engines. $\endgroup$ – Mike Sowsun Jan 16 '18 at 12:29
  • $\begingroup$ Yes you are correct of course, but what I meant is that an APU is used for longer periods when powering the aircraft on the ramp - before starting and after shutdown, while when starting the engines it is used only for a few seconds. Emergencies situations during flameouts are another example $\endgroup$ – PIXP Jan 17 '18 at 13:24

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