jimjim1 You are right regarding castellated nut holding duplex bearing. It is obvious that if failing bearing caused inner race to start rotating on shaft, this action would put UNSCREWING force to the nut if it has a normal, right tread - I believe it is. But nut stayed in place and only increased torque was reported. Will come back to that later.
Hovewer, torque transmitted to control shaft was strong enough to overcome the clamping force of castellated nut on the other side-at control lever and its pin carrier. Control shaft and nut (plus still intact but strained cotter pin) started rotating in pin carrier. I would speculate that nut immediatelly moved a little in UNSCREWING direction, thus slightly decreasing clamping force, (due to slack and/or initial deformation of cotter pin) . Enormous heat and clamping pressure then caused the nut to friction-weld to carrier pin, thus resisting rotation. Now cotter pin could not withstand the torque between control shaft and firmly welded nut and it snapped. Control shaft started unscrewing out of nut. Once shaft threads disengaged completelly, (it was very fast-just the length of the threads) control was lost. I suspect that while unthreading, rod moved (quickly) to the right for the length of the tread and changed pitch of blades, causing initial right yaw (?)
It remains unclear to me what exactly happened then , as in S2-2018 report there is not enough information how exactly hydraulic power cylinder engages with control rod and I do not have type knowledge-wether control shaft was hydraulically moved to the physical limit of actuator, or it was just free to move, thus leaving blades to rotate in pitch axis until equilibrium of all forces was established or pushed by runaway actuator.
Report says that T/R actuator continued changing the pitch of the blades until they reached physical limit. Sentence above says that rod was disconnected from lever.
Duplex bearing - if its castellated nut would unscrew or at least allow rotation of inner race on control shaft, the whole arrangement would still function until heating would weld inner race to the shaft. (with some lubricant, it might not happen very fast) Hovewer, evidence suggest that OUTER race rotated in its place. Wether inner race also rotated, report doesn`t say. I hope AAIB CT scanned also (duplex bearing side) castellated nut before they removed the cotter pin and found (abnormally high) nut torque. Any tell-tale deformation of the cotter pin would be altered-destroyed with removal.
I am not that surprised that nut torque on duplex bearing nut was found abnormally high, considering significant overheating, possible movement between to-be-clamped parts.. .
It often happens that correctly torqued assembly needs significantly higher unscrewing torque when disassembled again after a while, especially if exposed to elements, heat and dynamic loading.
Of course, if it is left-treaded, than torque increase is somewhat expected.
Bearing seizure obviously caused the accident, but what caused bearing failure, remains to be explained. Hovewer, I feel a bit uneasy that such failure dooms the helicopter.
I am trying to figure out if a feature, allowing rotation and thus integrity of flight controls, could (should?) be designed into the system, like alternative way of rotation between slider and control rod if bearing seizes-up and providing tell-tale sign (vibration) that would give time to pilot to recognize the problem and act before everything goes South. And I have a very personal interest in that.
hoistop
P.S. Could anyone provide a diagram of AW169 T/R ctrl sys that explains this description from S2/2018 report:
The lever pivots around the connection at the control shaft end and creates a demand on the hydraulic system vie the solenoid valve, which moves the hydraulic piston and control shaft of the actuator.