Seems like it was the feathering shaft that failed. He said it was a 5mm screw and the threads were gone but showed the feathering shaft still connected on one main blade holder. That's a pretty decent size screw to have fail. Maybe over tightening damage. Surprisingly he didn't notice it and tried to spin it up again after noticing the rough vibration. He only did a cursory couple turns of the main blades and never thought to check if there was play in the head. I've always wondered how the feathering shaft manages to hold those spinning blades together anyway. It's surprising we don't see more of this type of failure.
Saw in the comments that the feathering shaft bolts come with an anti corrosion lube on them. Loctite won't stick to oil and they unscrewed under vibration.
