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This question already has an answer here:

A fixed wing aircraft is able to glide in case of an engine failure and potentially land safely.

What would happen if a helicopter engine fails at high altitude?

Is it possibly to land it safely?

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marked as duplicate by Jay Carr, ymb1, kevin, mins, Pondlife Oct 12 '16 at 13:22

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

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    $\begingroup$ Look up "Auto rotation" or "autorotate" or other forms of those words. Helicopters don't just fall out of the sky (they also don't typically fly at 20,000 feet either). $\endgroup$ – Ron Beyer Jul 25 '16 at 19:51
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    $\begingroup$ As a helicopter pilot, I would say this is not a duplicate. It asks specifically about high altitude. because of density altitude, a high altitude auto is much more dangerous. I edited the question to focus more on high altitude autos. $\endgroup$ – rbp Jul 25 '16 at 22:22
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    $\begingroup$ Hi. Welcome to Aviation.SE. I hope that your question gets re-opened if it is about high altitude autorotations. An important point, which will make a big difference to the answers, is to edit your question to indicate whether you mean starting the auto at high altitude and finishing the auto at high altitude, e.g. 20,000 feet to 10,000 feet or 20,000 feet to 0 feet. $\endgroup$ – Simon Jul 26 '16 at 6:55
  • $\begingroup$ Does the helicopter you are asking about have one or two engines? $\endgroup$ – KorvinStarmast Oct 5 '16 at 16:26
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    $\begingroup$ @ymb1 Agreed, voting to close. Chopperman: Altitude really doesn't make a difference, provided you have a enough space to actually perform an autorotate. If your rotors can get enough grip on the air to climb that high, the air can get enough grip on your rotors to auto rotate them for you. $\endgroup$ – Jay Carr Oct 12 '16 at 3:15
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Yes. See the Wikipedia's article on autorotation!, which mentions a real-world example of the case you are describing:

The longest autorotation in history was performed by Jean Boulet in 1972 when he reached a record altitude of 12,440 m (40,814 ft) in an Aérospatiale Lama. Because of a −63 °C (−81.4 °F) temperature at that altitude, as soon as he reduced power the engine flamed out and could not be restarted. By using autorotation he was able to land the aircraft safely.

The biggest concern here is the disk loading of the helicopter's rotor - the greater the disk loading (the mass of the aircraft related to the rotor's area) the worse the autorotation capabilities. See this Google Books link for an in-depth discussion of the autorotation capabilities of different aircraft based on their characteristics.

The weight of the very rotor also plays a significant role here - the heavier the rotor, the more kinetic energy it stores, and the greater the probability of a successful entry into the autorotation state. See the Osprey's design, for example - it has light propellers on each wing which are practically unable to autorotate because of their small mass.

As you can see, the aforementioned helicopter - Aérospatiale Lama - is a light aircraft which is geared towards high altitudes - it has three propeller blades, which translates to low disk loading, and sufficiently heavy blades to allow easy entry and execution of the autorotation stage.

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  • $\begingroup$ Welcome to av.se! $\endgroup$ – ymb1 Oct 12 '16 at 1:37
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When a helicopters engine fails, it would start falling until Auto rotation kicks in.

Whether this happens and the pilot shuts off the engine or in the case of actual engine failure, once the engine drops below a certain number of revolutions per minute, relative to the rotor RPM rate, a special clutch mechanism, called a freewheeling unit, disengages the engine from the main rotor automatically. This allows the main rotor to spin without resistance from the engine.

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    $\begingroup$ When you quote a source, please include a link to it if at all possible $\endgroup$ – Pondlife Oct 4 '16 at 16:50
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    $\begingroup$ 'When you quote a source, please include a link to it.' There, fixed it for ya', @Pondlife. $\endgroup$ – FreeMan Oct 4 '16 at 18:32
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    $\begingroup$ This is a poor answer. It starts falling (descending) when it loses the engine (unless there is some airspeed cushion to trade kinetic for potential energy), and while autorotating it continues to fall (autorotate). This does not look like an expert answer. $\endgroup$ – KorvinStarmast Oct 5 '16 at 16:25
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    $\begingroup$ Auto-rotation doesn't "kick in", its not something that happens automatically. Auto-rotation is a technique you perform to arrest your descent. $\endgroup$ – Ron Beyer Oct 12 '16 at 1:40

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