I’m a little confused by the BFU bulletin.
They note that they found the Fenestron drive shaft broken.
Then they summarise that: “The on-site examination found no evidence of any technical defect in the helicopter, either in the Fenestron drivetrain or any impediment to the controls.”
They go on to report that manufacturer test pilots reconstructing the accident flight were able to arrest a right yaw without difficulty, both with left pedal input and by reducing power and increasing forward air speed to fly out of it. That only seems relevant if the Fenestron was working.
Then at the back they reference the RFM emergency procedures for tail rotor drive failure. That only seems relevant if the Fenestron was not working.
So did the drive shaft break in flight, or as a result of the crash? If it remained intact in flight, what could cause the right yaw seen in the UMS data – starting slowly and increasing in speed until it seemingly became unrecoverable?