Incident Overview
A helicopter equipped with floats executed an autorotation following a loss of tail rotor effectiveness. During the landing on a river, the tail boom and tail rotor assembly made contact with the water surface.
Investigation Findings
Post-accident examination revealed that the tail rotor gearbox output shaft pinion retaining key had not been installed during the initial assembly of the component. As a result, the tail rotor hub and blade assembly were found to be free to rotate on the shaft at the time of the investigation.
Sequence of Events
The pilot initiated an autorotation after experiencing a loss of tail rotor effectiveness. The helicopter, fitted with floats, subsequently landed in the river. Upon touchdown, the tail boom and rotor struck the water.
Mechanical Condition
Inspection of the tail rotor drive system disclosed that the pinion retaining key, which normally secures the output shaft to the gearbox, was absent. This omission allowed the tail rotor hub and blades to turn independently on the shaft, rendering the tail rotor non-functional.