Gordy,
To quote Nick Lappos from post #81 & #87 in this thread 2nd Nov 2001 !!!
It has happened -
Steve76 asks:
Hello Nick and fellow 76 drivers,
With regard to a internal gearbox failure of the #1 engine, N2 input driveshaft and the subsequent channelling of 100% of the turbines power into the tail rotor. Is it not unreasonable to expect that the tail rotor will overspeed to the point that it suffers a catastrophic failure and disintegrates?.
Nick sez:
You are postulating an internal main rotor gearbox failure, I think, where the engine is still connected to the tail rotor, but disconnected from the main rotor bull gear.
Such a failure would leave the engine driving a much smaller load than the big main rotor, so an upspeed might occur, depending on how big the power reduction is (the reduction from normal drive to just the tail rotor.)
In all cases, the engine overspeed protection will catch the drive up speed if it should get out of hand, since the overspeed protection system is designed to shutdown the engine if the engine shaft breaks, which is a very big power reduction, the biggest possible. The tail rotor is quite healthy at speeds up to somewhere over 130% (I have been to 128 in flight during tests - this is done by professionals, do not try this at home!)
Most likely, the failure you describe would create a situation where the engine would speed up for a second or two, then settle down to about 109% or so, and it would be at about 2% torque, spinning happily with almost no load (just driving the tail rotor). The 109% is because unloaded, but with the cruise collective pitch setting, the engine would be trimmed up a bit by the collective bias. The rotor and the #2 engine would be driving the helicopter, and you would be in single engined flight.
The only problem would be if you decided to then shut down the #1 engine, because that would cause the loss of tail thrust.
The procedure is based on an actual failure that occurred about 15 years ago, where the input gear attachment bolts lost torque and the separation that you describe actually occurred. The gear was redesigned, and no repeat failure occurred. The flight crew noted the problem as noise and rumbling, a momentary upspeed of #1 engine, a swing to the left (extra tail thrust) and then back to normal, with very low #1 torque and high #2 torque.
After a bit of discussion, the crew left well enough alone, and flew home without shutting down #1 (what a good pair of guys! If it works, leave it alone!).
When they landed, they noted that the failure, in that the tail rotor was not connected to the main rotor.
We id'd the problem and fixed it asap, of course, and no repeat has occurred. We inspect all boxes on overhaul for signs of lost torque on that gear to see if any recurrence is creeping back, and everything is fine now.