Incident Summary
During level flight at an altitude between 500 and 700 feet, the pilot perceived a high-frequency vibration in the tail rotor anti-torque pedals. Subsequently, a low-pitched hum and a shuddering vibration were felt, followed by a loud bang from the rear of the aircraft. The helicopter then began to rotate to the right, and full application of left pedal failed to arrest the rotation.
After several complete 360-degree turns, the pilot attempted unsuccessfully to streamline the helicopter. An autorotation was initiated, during which the aircraft's rotation diminished to a slow right-motion spiral or flat spin. As the autorotation was completed, the left float contacted the water first, causing the helicopter to roll inverted.
Examination Findings
A post-accident examination of the helicopter revealed that a bolt had separated from the tail rotor aft short shaft forward flex coupling, and the drive shaft was fractured.
Conclusion
The investigation did not provide an explicit determination of probable cause.