I find that hard to comprehend? Given that the condition results from a low g condition that requires reasonable airspeed and height to achieve, it's hard to imagine what height above the ground is considered 'survivable' for the occupants of a freefalling Robinson fuselage that was probably doing around 100 kts at the point of seperation?
There's been other discussions on Robinsons and the teetering head in general, where someone posted the US military reports on the mast bumping phenomenon.
There was a training officer who survived one, but saw his copilot in the Cobra have his face severed by the departing blade.
A lot of those were occurring at terrain following, hence the low-g move as it crested hills.
Not saying this is the case with the noted Robinson accidents, but might be an argument used for this notice.
Also, not every "mast bump" causes separation before the cabin strike.
Example:
Robinson R66, ZK-IHU, Mast bump and in-flight break-up, Kaweka Range, 9 March 2013
Aviation Reports
4.2.1.
There was clear evidence of a severe mast bump and that a main rotor blade had struck the cabin.
The main rotor drive shaft separated under a combination of bending, torsional and inertial effects, resulting from its being driven by the engine while the blades were prevented from turning because they had struck the fuselage.
It seems there's a few instances like this, with cabin strikes precipitating the loss of the rotor.
http://www.aaiu.ie/sites/default/fil...20EI-MAC_0.pdf
The flight from Weston to Sligo, via the Newtownmountkennedy area, was uneventful and routine up to 10:10 hours. Very shortly after this time an in-flight catastrophic event occurred resulting in the disintegration of the Perspex windscreen, as it was struck by the main rotor blades.
...
This event would have caused a critical loss of rotor RPM and very severe airframe vibration and loss of control, leading to the final impact. Pilot aided recovery would have been impossible after such airframe strikes. Finally, just before or during ground impact the main rotor struck the tail boom.
http://www.pprune.org/rotorheads/292...om-strike.html