There was a spate of accidents to quite experienced pilots in the late 60s and 70s (including a very public one at Farnborough). The scene went quiet for some years and now we are recycling with exactly the same type of accidents. Reading those accident reports should be de-rigeur. The two lethal problems appear to be mast bumping and unloading the rotor disc caused by a push-over or turbulence. Can't say much about the former except that it might be an inherent fault of the teetering hub rotor head. The latter is a gotchyer. You must maintain positive G in all manoeuvers. Any unloading of the rotor causes it to slow down and below a critical rpm it will not speed up again. The rotor flails and fails and the ROD becomes something akin to 32ft./sec./sec.
I've seen a film of it happening to Ken Wallis but he was only 20' high, yet amazingly, walked away from it.
[This message has been edited by Yogi-Bear (edited 20 June 2001).]