Engine Power Loss
Shortly after departing from a storage depot in Geraldton, a Robinson R22 helicopter sustained an engine power loss, forcing the pilot to conduct a forced landing. Investigation revealed that the power loss resulted from thermal damage to an inlet valve in the number-four engine cylinder. The in-service thermal damage created a clearance on the valve head, reducing its sealing capacity. This likely allowed combustion gases to bypass the valve seat and extend back through the engine intake, producing a backfire, power loss, and associated airframe yaw. Previous instances of yawing and reduced engine performance while the helicopter was operated in the Geraldton region had indicated developing inlet valve damage. The damage and its effects on engine performance were consistent with industry advice from the Civil Aviation Safety Authority.
Tail Rotor Drive Shaft
Examination by the ATSB found that the tail rotor drive shaft had fractured in overstress from torsional loading, separating from its connection to the rear flex plate and tail rotor gearbox. The pilot lost directional control when attempting to reposition the helicopter after the forced landing due to a complete loss of anti-torque thrust from the tail rotor system. Ground contact marks from the helicopter skids during the forced landing were relatively straight, indicating the drive shaft was likely intact at ground contact. The ATSB could not conclusively determine the reasons for the driveshaft fracture, which probably occurred during the running landing. Surfaces around the fracture showed no evidence of pre-existing damage, mechanical rubbing, or rotational contact. There was no evidence of the engine operating at a resonant speed or damage to the tail rotor blades from foreign object contact that could have overstressed the shaft. Previous damage to one rotor blade was considered unlikely to have contributed, as the shaft had been inspected after that occurrence and found serviceable. A search of databases revealed limited previous instances of similar failures in R22 helicopters, and the manufacturer reported no instances of tail rotor drive shaft fracture due to a rough running engine.