Incident Overview
A helicopter engaged in an external load operation experienced a vibration or shudder shortly after lifting off. The pilot, attempting to maneuver the helicopter away from ground personnel, allowed the right skid to contact the ground, resulting in a rollover.
Post-Inspection Findings
Subsequent inspection of the helicopter revealed that the main rotor drive shaft had separated approximately three inches from the splined end of the shaft. The separation point was identified as a failure location.
Metallurgical Examination
A metallurgical examination of the failed shaft determined that the fracture occurred due to unidirectional fatigue. The fatigue cracks initiated from surface pits present on the shaft. These pits served as stress concentrators, leading to progressive crack growth and eventual separation.
Sequence of Events
The helicopter had lifted off during the external load operation when the pilot noted a vibration or shudder. In response, the pilot attempted to move the helicopter away from nearby ground personnel. During this maneuver, the right skid made contact with the ground, causing the helicopter to roll over. The rollover was a direct consequence of the abnormal dynamics induced by the drive shaft failure and the pilot's evasive action.