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I've found a couple of sources that state the 727's APU cannot be started while flying, as it draws air from the wheel well.

It drew air from inside the wheel well, so it was only for use on the ground.

https://aviation.stackexchange.com/a/37715/34744

The APU on the Boeing 727 can be used for ground operation only.

https://www.boeing-727.com/Data/systems/infoapu.html

Additionally, the battery on the 727 doesn't power all of the systems on the "essential bus", and presumably cannot start the engines.

http://cyberpilot.tripod.com/electrical_system.html

So what happens if you lose all engine power in a 727? How can the engines be restarted?

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  • $\begingroup$ Ram air turbine? $\endgroup$ – David Richerby Jul 19 at 18:04
  • $\begingroup$ @DavidRicherby Using a RAT to start a jet engine? Not saying you're wrong, and there is the advantage that the engine is already windmilling, but still, wouldn't that require a bit more power than those can normally deliver, especially if you're already running some systems (hydraulics?) off it? $\endgroup$ – a CVn Jul 19 at 18:39
  • $\begingroup$ @aCVn Dunno -- that's why I put a question mark after it. $\endgroup$ – David Richerby Jul 19 at 18:41
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    $\begingroup$ Most engines can be windmill started above some speed. Some have to be going a lot faster than others depending on how strong the ram effect into the compressor inlet is. You just need power to the igniters and possibly boost pumps. $\endgroup$ – John K Jul 19 at 20:10
  • $\begingroup$ @JohnK that makes sense, I hadn't considered that you can rely on the ram air pressure since, well, you'd probably stal if you go much slower than the 150 knots mentioned by bclarkreston. $\endgroup$ – zymhan Jul 22 at 12:38
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According this rather detailed page on the 727's engines:

To restart in flight, an "air start" may be attempted. The start switch should be placed in the FLIGHT START position which will arm high energy ignition. The starter valve will not open. Ram air entering the engine is sufficient for start if the aircraft's speed is above 150 knots. When the start lever is moved to IDLE, fuel is introduced and the igniters are activated.

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