Incident Overview
A helicopter was on approach to an offshore platform, turning from base to final at 300 feet, when the pilot heard the low rotor rpm audio warning and noticed that the low rotor rpm warning light was illuminated. He initiated an autorotation and observed that the rotor rpm needle had dropped to zero.
Pilot Actions and Water Landing
As the helicopter descended, the pilot reported feedback in the flight controls and determined that a precautionary landing was necessary. He subsequently landed the helicopter in the water and shut down the engine.
Tail Rotor Damage During Rotor Coast Down
During the main rotor coast down, the tail rotor contacted a five-foot wave, resulting in separation of the tail rotor drive shaft.
Component Failure
Examination revealed that the internal splines of the shaft and sleeve assembly of the rotor tachometer generator (part number 206-076-373-001) were worn and not engaging. The failure of the rotor tachometer generator splines resulted in a loss of hydraulic pressure to the helicopter flight controls, since the hydraulic pump is driven by the rotor tachometer generator.