Originally Posted by
Super VC-10
How on earth does a propeller go into reverse in flight?
Pretty much the same way a turbojet/fan thrust reverser can deploy in flight (Lauda Air 004, TAM 402) - something
breaks, either in the primary operating system, or the safety interlocks, or both.
As evansb alludes to, a prop goes into reverse thrust simply by rotating the pitch of the prop blades from a position/angle that pushes air backwards (forward thrust) to a position/angle that pushes air forwards (reverse thrust).
As in, from <\ to </ (direction of aircraft travel <<<<). A matter of a relatively few degrees from one direction to the other.
This is a PT6/Hartzell prop hub (although schematic and not necessarily identical to that in the LET) - as you can see, the system operates on oil pressure, pistons, valves, springs, levers and a governor (rpm sensor/limiter).
https://i.ytimg.com/vi/tIAfmY42siI/maxresdefault.jpg
It shouldn't happen - it is not supposed to happen - but I expect one can see where a piece of metal snapping or an oil clog at the wrong moment can end with the prop blades blowing air in the wrong direction.