What happened
While performing mustering duties, a Robinson R22 experienced significant vibrations, an increase in engine manifold pressure, and a simultaneous drop in both engine and main rotor RPM. In response to the power loss, the pilot attempted a forced landing. During the descent, the aircraft struck trees in a densely wooded area, resulting in the destruction of the aircraft.
The investigation
Investigators examined the engine's condition, noting that compression on the number 2 cylinder had been within acceptable limits during tests conducted on 23 June 2022 and 20 July 2022. Maintenance logs indicated no significant degradation in compression during that period. However, the investigation determined that during the 64 flight hours following the last periodic inspection, carbon deposits had accumulated on the valve stem and within the valve guide. This buildup likely caused the valve to stick, preventing it from closing fully, which led to the engine's rough running and power loss.
This failure mechanism, involving exhaust valve stem deposits, aligns with issues previously identified in the AHIA report. Because this progression can occur undetected by standard maintenance regimes, it remains a difficult failure to intercept.
Findings
- The primary cause of the engine performance degradation was the reduction of the number 2 cylinder exhaust valve clearance to well below the minimum limit due to carbon build-up.
- The pilot's initial instinctive reaction to raise the collective caused a drop in rotor RPM, but the pilot successfully corrected the error by rapidly lowering the collective to maintain rotor RPM and enter autorotation.
- While there has been a noted increase in cylinder failures in Robinson R22 and R44 helicopters used in Australian mustering operations, a lack of industry data has hindered the development of specific prevention strategies.
Safety message
The ATSB has issued a safety advisory notice to encourage operators, pilots, and maintainers to report engine cylinder issues via the Lycoming Cylinder Durability Investigation Group (LCDIG) defect report forms, as increased data is vital for identifying prevention strategies.