Loss of control, 21 km north-east of Mount Gambier, South Australia, on 20 December 2006, Kawasaki KH4, VH-LFK

On 20 December 2006, a Kawasaki KH4 helicopter lost collective pitch control and impacted terrain while performing agricultural aerial spray operations approximately 21 km NE of Mount Gambier, SA. The helicopter was substantially damaged, but the pilot was uninjured. When the accident site was surveyed, the main rotor mast and main rotor blade assembly were found to have separated from the helicopter. They were located a short distance away.

Examination of the wreckage revealed that the helicopter's main rotor mast thrust bearing had failed catastrophically in flight. That bearing was a critical item for safe operation and continued airworthiness of the KH4. It supported the full weight of the helicopter and transferred thrust loads generated by the main rotor blades during flight.

The investigation was unable to conclusively establish the factors that led to failure of the mast bearing. No evidence was found of manufacturing or material defects. Nor was there any evidence of improper installation procedures or maintenance practice. Based on the inspection of aviation databases in Australia and North America, the main rotor mast thrust bearing failure appears to be an isolated event for the KH4-series helicopter.

Despite the low probability associated with a mast bearing failure of this type, the consequences of such an event could have been fatal for the pilot onboard. This report has been provided to Australian operators and maintainers of Kawasaki KH4 and Bell 47G3 series helicopters as a future alert for this type of occurrence.