Correspondents appear to believe that I am saying you can't pull of a successful landing following an engine failure from any where - go back to my original post - I have carried out many thousands of engine off landings from any of those scenarios successfully. However, as an example, I can assure you that a loss of power from an IGE hover or an OGE hover almost anywhere inside the HV curve the rotor system will NOT be in fully developed autorotation before you need to pull pitch to cushion the landing. Let's not get too pedantic the height loss maybe say 800 feet for fully developed aerodynamically stable autorotation but stable aerodynamic autorotation is not instantaneous. As AOW stated when carrying out an auto rev check test flight the auto was commenced at least 1000 feet above test height to ensure fully developed stable autorotation was achieved by that test height.
A word on underslung teetering heads - years ago the service I was attached to suffered two fatal accidents, four pilots lost, as a result of main rotor separation - one flight was simulated IF which was to include UA recovery. The other was a two ship low level tactical formation. Somehow these pilots pulled the rotor heads off - there was no sign of a pre-existing failure. A senior Bell test pilot was invited to discuss the aerodynamics of this system. He emphasised that extreme care should be taken with cyclic movement with anything less than optimum RRPM particularly at less than 1G - exactly where you will be should you drop the lever after an unalerted power loss.
When constructing the HV curve manufacturers take into account many factors including pilot reaction time - low inertia/high inertia rotor system to name a couple. The result is the avoid curve - in other words avoid flying in this zone as we don't guarantee a successful landing following a sudden power loss.
GAGS
E86