What happened
On May 3, 2009, a Robinson R-44 helicopter, registration G-KPAO, departed from Ampuria Brava (LEAP) airport in Gerona, bound for Castellon (LECN). The flight, operated by Avonair, carried a pilot and two passengers for a pleasure flight.
While en route and approximately 3 nautical miles past the Bagur VOR, the aircraft suffered a sudden loss of engine power. The pilot immediately declared an emergency and initiated an autorotation procedure. The helicopter landed in a nearby olive grove, during which the vertical stabilizer struck a tree. All three occupants escaped the aircraft without injury, though the helicopter sustained minor damage to its lower vertical stabilizer.
The investigation
The investigation focused on the mechanical failure of the Lycoming O-5-40-F1B5 engine. Post-accident inspections of the engine components revealed that the pushrod for the number 2 cylinder exhaust valve had broken and its external guide was bent. Additionally, damage was noted on the tappet.
Investigators measured the clearances between the rods and guides across all cylinders to check for compliance with Service Bulletin 338 C. While several cylinders were within the permitted limits, the measurements showed that the clearances were consistently near the maximum allowable thresholds. Specifically, the investigation looked into whether the clearance in the number 2 cylinder had exceeded the maximum specification.
Findings
- The primary cause of the engine power loss was that the number 2 cylinder exhaust valve became blocked in the closed position.
- This blockage caused the pushrod to break and the external guide to bend.
- It is believed that excessive clearance between the rod and the guide may have caused the guide to be misaligned, increasing friction and allowing oil to accumulate. The subsequent solidification of this oil likely contributed to the valve blockage.
- The investigation noted that the clearances in the remaining cylinders were all near the upper or lower operational limits, suggesting a pattern of wear or maintenance margins being reached.