Incident Overview
During cruise flight, the pilot reported a slight vibration that became more pronounced over several minutes, accompanied by an unusual noise. The pilot initiated a precautionary landing to an offshore platform. During the approach, the vibration and noise level increased again, and total engine power was lost. The pilot performed an autorotation to the water, deployed the skid floats, and landed safely. While the helicopter was being towed in the water, it rolled inverted.
Post-Recovery Examination
After recovery, examination revealed that the KAflex engine-to-transmission driveshaft was fractured, and the forward tail rotor driveshaft segment had separated. The KAflex driveshaft showed fatigue fractures originating from a bolt hole in the first flex frame of the coupling at the transmission end, as well as fatigue fractures in the end fitting at the transmission end. The primary fracture was a fatigue crack that occurred in a bolt hole where a bolt joined the first flex frame to the center flex frame. The driveshaft was found to have been manufactured within engineering specifications. The separated tail rotor driveshaft exhibited evidence of torsional overstress.
Driveshaft History and Manufacturer Action
The KAflex driveshaft coupling had accumulated a total time of 2,114.2 hours, against an airworthiness life limit of 5,000 hours. Following the accident, the manufacturer issued a service bulletin introducing a new KAflex driveshaft with a 1,250-hour overhaul interval.