Collective control rigging
In the case of the 412 , that lost collective control because of collective lever severance , I have seen this many times in the past , and it has been attributed mostly to improper rigging of the collective flight control , which allows the collective levers to bottom out on top of the swash plate mounting bolts when the collective is in the full down position . This clearance is critical especially during the auto-rotative rpm check during the test flight , and after rigging has been accomplished . if this clearance is not there , and the levers bottom out on the mounting bolts , coupled with the 1500 psi hydraulic boost , the collective levers will be stressed to the point of fracture , and eventual complete severance , and failure .