Nick,
I just came across this thread thought I would add a slightly different, very dated, recollection of the events in posts 27 and 87. I was involved in both incidents, which happened a couple of months apart in 81.
The first incident was a collective to yaw lockup that resulted from the loss of the #2 hydraulic system. There I was at 6500 feet when the #2 system failed and the collective felt frozen. I was able to reduce the collective about 10-15% by forcing it down (maybe my feet moved but I wasn't feeling it at first.) Once I got lined up with the runway, I just drove it on with the cyclic. Interesting side note - once on the ground, I was so relieved, I reached up and yanked both throttles to idle. Big mistake. The nose jerked to the right and I instinctively stomped left pedal and I was flying again. The aircraft jumped about 5 feet into the air before plopping back down on the runway.
The second incident involved the transmission. Once again, there I was, when there was a big clunk that came from the #1 engine side. Split torque, with the #1 engine showing Hi temp and low N1 along with a pronounced grinding noise. We thought we had lost the #1 engine, so we shut it down and flew back to base. The next morning when I came to work, our lead mechanic told us, we were lucky - very lucky. The night before, they had done a run up using the #2 engine when the tail rotor failed.
I recall a phone conversation with you about the second incident and probably filled you in on the first one. I recall you issued a procedure that in the event of a partial #1 failure to beep it back but not shut it down.
Sikorsky sent a tech rep and a transmission guy to our base to look at the transmission. We all stood around as our mechanics opened the case. At first, nothing looked out of the ordinary, when your transmission guy pointed to the #1 side gears. As I recall what was later revealed, the bull gear at manufacture is pressed onto a spline that fits on a shaft that is then held in place with a nut. In this case the gear was not pressed all the way onto the spline, and after about 900 hours finally seated itself which allowed the retaining nut to work loose and the gear slid out on its shaft and started rubbing on the inside of the case but still had enough contact to drive the tail rotor.
Oh - and keeping with the theory that things happen in threes, I was the one that was flying the S92 that lost tail rotor control back in 2011.