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Because, it might lead to reverse flow of air and could stall the plane. Is there some mechanism to prevent this?

There was a crash in Ireland a few years ago, when the pilot of a small charter plane accidentally put one of the turboprop's in reverse.

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    $\begingroup$ Are you referring to reverse-thrust as normally used on landing to slow the aircraft? $\endgroup$
    – RedGrittyBrick
    Apr 8, 2015 at 11:03
  • $\begingroup$ yes i am referring to that $\endgroup$
    – Firee
    Apr 8, 2015 at 11:19
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    $\begingroup$ The blades still run (i.e rotate) in the same direction, the blade angle is changed to produce reverse thrust. On the Pilatus PC-6 turboprop you can use beta-pitch (zero-thrust) when you want a near-vertical descent. $\endgroup$
    – RedGrittyBrick
    Apr 8, 2015 at 11:32

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Most manufacturers realise this is a serious risk so most are 'gated' in that you have to do something to get them into this range. This is like pulling up the reverse tabs on the throttle levers on the B737 or A320 that is only possible when they are in the idle position.

  • ATR 42/75: Has something called Idle Gate that prevents throttles from going back too far when engaged.
  • Dash 8: Has gated throttles that requires pulling up a pin to get them into reverse.

There is to the best of my knowledge nothing (in general at least) to stop to the thrust from being 'reversed', or more specifically to angle to blades to what is known as the 'beta' range, if you are intent on doing it. Unlike jets where doors open for instance and the process is a little more complex, this is just rotating the blade below a certain angle.

I found the accident report [PDF] for the Manx2 flight. There were multiple things going on here rather than a single cause:

  • Generally bad piloting for putting them in a late go-around situation to start with.
  • There were errors in the engine, causing a thrust mismatch. enter image description here enter image description here
  • This meant that one engine proceeded to continue into negative thrust. enter image description here

It does then not come as a complete surprise that you manage to flip the plane over from considerable unsymmetric thrust when you are close to stall speed.

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    $\begingroup$ Negative TORQUE isn't exactly the same thing as negative (or reverse THRUST). Negative torque means that instead of the engine driving the propeller, the propeller is turning the engine. That's a lot of drag, but it isn't reverse THRUST. When you use reverse thrust, the engine is driving the prop, and you'll have considerable positive torque - but the thrust itself is negative (due to blade angle). I'm not sure the report being quoted is the BEST example of the harm of going into the Beta range in flight (problems happened when they came OUT of Beta), although I'm sure those could be found. $\endgroup$
    – Ralph J ♦
    Apr 8, 2015 at 18:51
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As was pointed out in a comment, the "reverse thrust" of the Beta range, which is used to slow the aircraft on landing and which certainly can do disastrous things if deployed inadvertently in-flight, is a matter of propeller angle and not the direction of rotation of the engine. If it is running, the turbine engine is going to turn one way and that way only; all sorts of things wouldn't work right to try to run it turning it backwards.

The place where a prop and engine might try to go backwards is with a shut-down engine and a feathered prop. If the engine isn't running, then the lubricating oil that normally keeps moving parts lubricated and cool, isn't being supplied, and it's important that there NOT be any movement. With the prop in the "feather" position, it's generating a slight force to turn the engine the "wrong" way, and on the system I'm familiar with, we had a "prop brake" that would hold the prop stationary at that point. If you took the blade angle out of feather, then the brake would allow the propeller to turn the "right" way -- as in a restart attempt.

So to directly answer the question as asked, yes you can find turboprop engines with a system to prevent them from rotating the wrong way. But the reason for this has to do with preventing unwanted (unlubricated) rotation after an in-flight shutdown, rather than anything related to reverse thrust.

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