Accident Overview
Witnesses reported that the helicopter took off from the ship. At approximately 85 feet above ground level, it began spinning to the right. The pilot radioed 'No control, MAYDAY.' The helicopter then nosed down and impacted the water.
Aircraft Recovery and Examination
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.
Maintenance History
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 partially disassembled and transported to California, then shipped to American Samoa. Maintenance personnel in Hawaii reassembled the aircraft after replacing several parts. The owner of the maintenance facility stated that some life-limited components were replaced with parts having less accrued total time. He reported that the shim on the tail rotor driveshaft required adjustment because it was 'way off, too tight.' No test run was performed after reassembly.
Findings
Examination of the helicopter following recovery revealed that 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 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. The other two fracture surfaces exhibited features consistent with overstress separation. The coupling is designed with a built-in fail-safe feature that continues to provide torque transmission in the event of a flex frame leg or bolt fracture. When the fail-safe engages, the unbalance of the coupling increases, resulting in increased vibration, alerting the pilot to take appropriate action and land.