Thanks to all that answered my query.

Further info on AAIB website released today at 1400

AAIB Special Bulletin here ; https://assets.publishing.service.go...018_G-VSKP.pdf .

Already reported on the BBC .

If I read this right, the yaw went full travel to the right. Since the limit of right yaw is usually set for the ability to yaw right in autorotation, it would probably mean it was still yawing right even in auto with engines shutdown (normal recovery actions for a tail rotor drive fail in the high hover). Thoughts?

My reading of this is that the duplex bearing was overheating, binding and then seizing, causing rotation of the control shaft which undid the castellated nut and removed the pitch control of the TR allowing the servo to go to the 'full right pedal' position.

I don't care how many times you practice in the sim, that one is pretty unrecoverable from that height and speed.

The question is - is this an assembly issue (bearing end nut over torqued) or a maintenance issue? The aircraft was quite young.......

Looks like it should have been a left hand thread, although that might just have postponed disaster.

​​​​​​From the Special Update "The observed condition of the duplex bearing and the increased torque load on the castellated nut that remained on the spider end of the shaft is consistent with rotation of the tail rotor actuator control shaft."

I read that to mean that they think the increased torque on that nut was caused by the rotation of the control shaft - for the reasons you gave. Maintenance issue or defective bearing ??

What sober and horrible reading the S2 bulletin is - if only they had a few more minutes to transition into forward flight.... RIP!! Horrible sad loss for all involved

The report seems to be a little confusingly worded. I agree, the duplex bearing at the spider end seems to have failed and then spun the shaft. Result, the castleated nut at the input end of the shaft has spun, with the shaft, against the pin-carrier, until the nut friction-welded to the pin-carrier and then the split pin was sheared and the nut unscrewed and disconnected the input. That is the way I read it. There does not seem to be any prejudice against the assembly in that. The duplex bearing failure looks to be the culprit from my interpretation. :sad:

OAP

So, if my understanding is correct, it appears that the duplex bearing seized solid, causing what should have been a non-rotating component to become physically welded and locked to the rotating part of the shaft, so that it also rotated, unscrewed then threw off the locking nut, overwhelming its thread locking methods. Effectively, the pilots yaw pedals were no longer connected to the tail rotor pitch change mechanism and the servo went to full travel, pushing the tail rotor to a position where it would give maximum right yaw.

Unfortunately, JetScream32, transitioning to forward flight would not have made any difference. The aircraft would still have yawed hard right because the tail rotor made it do so as hard as it physically could. Shutting down both engines and entering autorotation would not have stopped it from yawing right. As I wrote earlier in this thread, the closest equivalent failure on a twin engined aeroplane would have been a full right rudder deflection with asymmetric thrust pushing it in the same direction, making it worse. If the "outer" engine of said aeroplane were shut down to remove the asymmetric thrust, the rudder was still at full deflection.

Irrecoverable and horrible to contemplate.

A balance-spring bias on the servo unit towards left pedal( power input) would have been useful...

Low Blade Angle
 

And that is why blade designers strive to get the overall c.g. of the blade forward of quarter chord or the center of lift, such that if one loses pitch link input on one or all of the blades they will drive themselves to zero (or neutral) pitch. If the c.g. is aft of quarter chord the blade drives itself to maximum blade angle which is a divergent condition and does not help a disastrous situation. If other control failures are present such that the blade is driven to some adverse pitch angle, the c.g. effect may be over-ridden. Otter

Otter - in this case the TR servo was allowed to drive the pitch change mechanism all the way to full travel which would easily oppose any aerodynamic backloads..

Sycamore - the Lynx had such a spring bias unit but it wouldn't have helped as it would, if designed in the same way, have gone past top dead centre and pushed to the right instead.