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A comment to this answer discusses the possibility of evacuating passengers risking ingestion into the engines if evacuating over the leading edge of the wing. That particular question discusses the Boeing 737. For example:

"I always thought that the reason to go off the back of the wing was to avoid getting sucked into an engine."

and

"I'd imagine on a land landing, the leading edge could bring a passenger dangerously close to being ingested, should the engines be rotating.."

However, I really can't see any valid situation that calls for the following sequence of events:

  • Emergency landing (either on land or on water)
  • Keep engines running
  • Begin evacuation of aircraft
  • Shut off engines (during or after evacuation)

Instead, the sequence of events would seem to be:

  • Emergency landing (either on land or on water)
  • Shut off engines
  • Begin evacuation of aircraft

in which case some unlucky individual might possibly hit (come into contact with) some external or exposed part of the engine or engine pylon, but I have a really hard time seeing how anything could be ingested into the engine at that point.

For the purposes of this question, I suppose "engine running" can be interpreted as "sufficient intake airflow for large foreign object ingestion to be a concern".

Is there any valid situation where post-emergency-landing evacuation would begin while the engines are still running? If there is, then please describe at least one situation which would call for that and describe why such a course of action would be necessary in that situation as opposed to waiting for the engine front intake fans to spin down.

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    $\begingroup$ If the airplane landed in water, the engines would automatically stop running because water puts out the combustion going on inside the engine. $\endgroup$ – SMS von der Tann Jul 20 '16 at 12:54
  • $\begingroup$ Wild guess: if an engine on one side of the aircraft don't stop, you should evacuate by the other side. $\endgroup$ – Manu H Jul 20 '16 at 13:16
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    $\begingroup$ Pure speculation: for any landing in which the pilots are incapacitated or killed, the cabin crew will commence evacuation, but have no access to the cockpit to be able to shut down engines. $\endgroup$ – AndyT Jul 20 '16 at 15:20
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    $\begingroup$ I wonder if ingestion of humans is even a major concern, in terms of probability, at idle? $\endgroup$ – Ryan Mortensen Jul 20 '16 at 15:20
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    $\begingroup$ @RyanMortensen Ingestion of an adult human - probably not. Ingestion of items being carried by humans - more likely, and that scenario might also turn an "mostly harmless" idling engine into an uncontained fuel fire, bearing in mind that you are only in this situation because you can't shut down the engine normally! Note that the a common way for debris to be ingested is because of a vortex formed between the engine intake and the ground below it. At full engine power, those vortices are quite capable of lifting something the size of a 1-foot cube of concrete from the ground into the engine! $\endgroup$ – alephzero Jul 20 '16 at 18:05
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It is possible for the engine to not respond to shutdown sequence, so you'd have to get the passengers off before dealing with the rogue engine.

In the Qantas Flight 32 one engine had an un-contained turbine failure in engine No. 2 leaving them unable to shutdown engine No. 1. This was due to a piece of debris severing all communication to the engine, essentially leaving it out of control and running.

This in turn made it impossible for the crew to activate any safety mechanisms for the engine. It was eventually shutdown by dousing it until flameout.

Report by Australian Transport Safety Bureau

Wikipedia Article on Qantas Flight 32

It should be noted that in this specific case they didn't evacuate immediately due to a fuel leakage and the overheated brakes, though when they did evacuate engine No. 1 was still running.

Another situation where immediate evacuation may be preferable is if the aircraft had a cabin fire, meaning the highest priority would be cabin evacuation, which could potentially happen simultaneously to the crew attempting engine shutdown.

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    $\begingroup$ "It is possible for the engine to not respond to shutdown sequence" Good point, but why not pull the fire handle in that case which shuts off fuel to the engine? Sure, it's a somewhat unclean shutdown, but if the engine is not responding to a shutdown sequence in the first place, then surely it's going to have to be looked through thoroughly anyway. I doubt any pilot in their right mind will take off with an aircraft where the log just says "engine didn't shut down on command, nothing was done to investigate the problem or fix it, but trust us, it's fine now"... $\endgroup$ – a CVn Jul 20 '16 at 13:57
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    $\begingroup$ @MichaelKjörling in the Flight 32 case all contact to the engine was lost. All the cables were severed when No. 2 engine failed so even the safety systems couldn't be triggered. $\endgroup$ – Notts90 Jul 20 '16 at 14:05
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    $\begingroup$ @MichaelKjörling I've added that info. I also found the Australian Transport Safety Bureau report that states the evacuation took place with the engines running and the methods they attempted to use to shut the engine down. Have a look at page 8 onwards. It's a good read if you like the nerdy stuff like me :) $\endgroup$ – Notts90 Jul 20 '16 at 15:21
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    $\begingroup$ @called2voyage If the wind alone powered the engine (not saying that's possible) then the engine wouldn't actually be sucking air in. So the only thing that would cause you to go towards the engine would be the wind. So really, it's "Could the wind blow you into an engine?" $\endgroup$ – Notts90 Jul 20 '16 at 17:56
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    $\begingroup$ @called2voyage That would require hurricane force winds. Also, the engine would just be windmilling, so it wouldn't be spinning very fast and wouldn't be creating any suction at all. It wouldn't be significantly worse than being blown into any other large metallic object by the same wind. At that point, you should probably be more worried about this happening. $\endgroup$ – reirab Jul 21 '16 at 2:04

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