What happened
Immediately following lift-off and the transition to a hover, the helicopter began rolling to the right. The instructor pilot attempted to correct the movement by applying left cyclic; however, the instructor realized that the student pilot was also applying left cyclic at the same time.
As the aircraft continued its rightward roll, the instructor lowered the collective pitch. Despite this corrective action, the roll continued until the main rotor blades struck the ground, causing the main rotor head to separate from the aircraft.
The investigation
The investigation examined the flight controls and the condition of the rotor components. The instructor reported that the flight controls maintained proper feel and that no unusual vibrations were present prior to the incident.
Upon inspection of the fractured main rotor mast, investigators found excessive porosity, though it was determined that the component failed due to overload. Additionally, a gouge discovered in the ground indicated that the rear tip of the right skid dragged during the lift-off phase, which precipitated the dynamic rollover.