Look up the '[Dead Man's Curve][1]' for a helicopter.  It gives the zones for safe flying (in event of engine failure) as a function of speed and altitude.  Hover is the left-side of the curve (where V=0).

Very low, a helicopter can safely hover and have an engine failure -- from low altitude, it won't pick up enough speed to have terrible consequences when it reaches the ground.

Up high, a helicopter has enough altitude to use that potential energy to safely autorotate.

There is a region of altitudes in-between where the helicopter can not safely hover in the event of an engine failure.


  [1]: https://en.wikipedia.org/wiki/Helicopter_height%E2%80%93velocity_diagram