What happened
Following departure from the Geraldton storage depot, a Robinson R22 experienced a loss of engine power. This malfunction necessitated that the pilot perform a forced landing. During the process of attempting to reposition the aircraft after the initial ground contact, the pilot lost directional control of the helicopter. This loss of control was caused by the complete failure of the anti-torque thrust from the tail rotor system.
The investigation
Investigators examined the engine and found that the power loss originated from thermal damage to an inlet valve within the number-four cylinder. This damage created a clearance that compromised the valve's ability to seal, allowing combustion gases to bypass the valve seat. This process likely caused the reported backfire, power loss, and airframe yaw. Evidence suggested that previous instances of yawing and reduced performance in the Geraldton region were indicators of this developing damage.
Regarding the tail rotor, the investigation determined that the drive shaft fractured due to torsional overstress. This fracture led to the shaft separating from its connection to the rear flex plate and the tail rotor gearbox. While the ground contact marks from the forced landing were straight, suggesting the shaft was intact during the initial landing, the fracture likely occurred during the subsequent running landing. The ATSB found no evidence of pre-existing damage, mechanical rubbing, or engine resonance as causes for the shaft failure.
Findings
- Thermal damage to the number-four cylinder inlet valve reduced engine performance and necessitated the forced landing.
- The tail rotor drive shaft fractured under torsional loading during the repositioning attempt.
- Thermal damage to the inlet valve was the primary cause of the initial engine power loss.