Originally Posted by
Bell_ringer
These days, how many unexplained in-flight breakups do we see on teetering aircraft other than Robinson?
The issue may not be the architecture but rather the Robbie implementation thereof, including their coning hinge.
Like aa777888, my search was finding all manufacturers of teetering systems showing up in the fatal accidents. My guess is that if you correct for the experience of the pilots, the Bells are experiencing a similar failure rate.
Interestingly
https://www.faa.gov/regulations_poli...l_Robinson.pdf
seems to support this. The teetering R22 and R44 saw accident rates lower than those of the Enstrom 280 and Schweizer 269 (but that's all accidents, not just fatal and not just mast bumping).
Comparing Bell and Hiller to Robinson, they show the Bell and Hiller substantially (25%) less accidents, but again this is not just fatals and not just mast bumping. Given that a large focus of SFAR 73 was mast bumping, I'm a little disappointed that they didn't break that specific form of accident out separately. Also interestingly, I saw at least one Bell mast bump / rotor separation where the cause was "inadvertent IMC" because iIMC caused the pilot to lose control and then bump the mast. You can't even depend on the summaries, you have to read the accident to be sure.
As for the Robinson Tri-Hinge rotor head... I'm not aware that it's ever been shown to have been a cause where a Bell style teetering head would have not. It was extensively studied during the SFAR 73 investigation, so in many ways it may be the most researched head in the industry. Still, I must admit that I worry that there may be some corner of the envelope, similar to the V-tail Bonanza, where that head design might do something unexpected. However, I have nothing to back up that fear.
In conclusion, the teetering head design has a large scary part of the envelope that some unlucky pilot enters on a semi-regular basis and pays the ultimate price for having done so. While all aircraft have restrictions on parts of their envelopes, this seems one that takes a large enough toll that we should question why we continue to allow it to be used?