Accident
During cruise flight, the main rotor mast fractured and the main rotor separated from the helicopter. Witnesses to the accident reported seeing the helicopter falling to the ground in multiple sections. The aft 7.5 feet of tailboom, including the tail rotor gearbox, was located 220 feet from the main wreckage. The main rotor hub and blades were found 146 feet from the main wreckage. The main rotor blades remained attached to the rotor hub assembly.
Investigation
Examination of the main rotor mast revealed a fracture approximately 6 inches below the rotor hub assembly. The fracture was transverse to the longitudinal direction of the mast. The interior diameter of the main rotor mast was corroded and had several areas of surface pitting. According to the NTSB Materials Laboratory Factual Report, the main rotor mast contained a "flat" crack that contained crack arrest features indicative of fatigue cracking. The report stated, "The crack arrest features in the flat fracture region emanated from corrosion pits on the inside diameter surface of the mast. The entire inside diameter surface had been affected by this corrosion in the area of the flat fracture region."
Findings
The fracture of the main rotor mast resulted from fatigue cracking that initiated at corrosion pits on the interior surface. The presence of widespread corrosion in the fracture area contributed to the crack propagation.