Aircraft Accident/Incident Report

Pacific Ocean
Tuesady, June 16, 1998 14:00

Witnesses reported that the helicopter took off from the ship, and about 85 agl, it started spinning to the right. The pilot called 'No control, MAYDAY,' on the radio. The helicopter nosed down and impacted the water. The aircraft was recovered, except for the aft portion of the tail rotor driveshaft and tailboom, including the tail rotor gearbox and all tail rotor system rotating components. The wreckage was shipped to the United States for examination. The aircraft had undergone a major overhaul/rebuild and annual inspection following a previous accident on March 11, 1997. After completing 1.4 hours of test flights, the helicopter was then partially disassembled and transported to California, and subsequently shipped to American Samoa. Maintenance personnel in Hawaii reassembled the aircraft, after replacing several parts. The owner of the maintenance facility stated some of the life-limited components were replaced with parts with less accrued total time. He reported that they had to adjust the shim on the tail rotor driveshaft because it was 'way off, too tight.' No test run was performed after reassembly. Examination of the helicopter following recovery revealed the forward tail rotor driveshaft coupling evidenced a fracture and separation. The forward portion of the coupling remained attached at the main transmission output pinion, while the aft portion of the coupling remained attached to the forward end of the tail rotor driveshaft. The coupling bolts evidenced smearing of the bolt heads. Scanning electron microscopy revealed a total of eight fracture surfaces. Six fracture surfaces displayed features consistent with fatigue, although much of the mechanical damage prevented identification of origin locations, and the other two fracture surfaces exhibited features consistent with overstress separation. Further, the coupling is designed with a built-in fail-safe feature that will continue to provide torque transmission in the event that a flex frame leg or bolt fractured. When the fail-safe engages, the unbalance of the coupling increases with a resultant increase in the vibration, alerting the pilot to take the appropriate action and land.

The improper shimmying of the tail rotor driveshaft by maintenance personnel which induced fatigue in the tail rotor driveshaft coupling causing it to fail, which resulted in a loss of tail rotor control. A factor is the failure of the pilot to recognize the warning vibrations indicating impending failure of the coupling.