UK investigators have determined that a failure of the connection linking the yaw control pedals and the tail rotor was behind the high-profile fatal crash of a Leonardo Helicopters AW169 (G-VSKP) in late October.
A control rod that should have remained stationary was somehow caused to rotate, the Air Accidents Investigation Branch says. Sufficient force was applied to the rod to unscrew a lock-nut and shear off a split pin holding the mechanism together.
This lead to the helicopter developing an "uncontrollable right yaw," says the AAIB in its latest special bulletin on the 27 October incident. The AW169 came down shortly after take-off from about 400ft, killing all five passengers and crew on board.
Inspections at the crash site in Leicester revealed that the input lever mechanism for the tail rotor servo control had become disconnected from its associated control shaft.
This "prevented the feedback mechanism for the tail rotor actuator from operating and the tail rotor actuator from responding to yaw control inputs", says the AAIB. "Loss of the feedback mechanism rendered the yaw stops ineffective, allowing the tail rotor actuator to continue changing the pitch of the tail rotor blades until they reached the physical limit of their travel."
Components that should have held the assembly together were found loose in the tail rotor fairing, the report says, and "were bonded together"; later analysis revealed they had become joined by friction welding.
The AAIB says a split pin, which should also contribute to the integrity of the system, was missing and was later found to have been sheared off "in rotation".
Subsequent examination of parts at the other end of the control shaft revealed that the inner races of a set of duplex bearings could only be "rotated a few degrees in either direction by hand".
The control shaft sits within these bearings and remains stationary, while the tail rotor's spider/slider assembly rotates with the outer race of the bearing.
A build-up of black grease was also detected around the inboard face of the bearing, as well as evidence of "burnt-on grease" along the adjacent portion of the control shaft.
CT scans revealed fractures to the bearing cases and "significant damage to the surface of the inner bearing races", as well as evidence of debris accumulating in bearing raceways.
Disassembly of the unit showed "evidence of relative rotation" between the sliding unit and bearing outer ring.
"No grease in its original form remained in the bearing," the AAIB adds.
Investigations into the root cause of the incident are continuing "as a priority", the AAIB says.
The European Aviation Safety Agency has already issued a number of emergency airworthiness directives for the AW169 covering mandatory inspections of the tail rotor assembly, castellated nut and duplex bearings.