What happened
During a mustering flight, approximately 25 minutes after departure, the VH-VHE experienced a significant reduction in engine power accompanied by induction backfires and a yawing motion. This sequence of events necessitated a forced landing. During the landing process, the tail rotor drive shaft fractured due to torsional overstress. While the pilot maintained pedal control until the skids made contact with the ground, the exact timing of the shaft failure remains unclear, though it appeared to occur concurrently with the aircraft touching down.
The investigation
Investigators examined the engine and found that the number-four cylinder's intake valve had sustained thermal damage. This damage compromised the seal between the valve face and the seat, allowing exhaust gases and the fuel-air mixture to enter the air induction system. This degradation was not detected by the pilot or the maintenance provider, even though a compression test had been performed only 17.6 flight hours before the incident.
Regarding the tail rotor, the ATSB investigated two potential causes for the drive shaft fracture: the impact of a foreign object on the blades or the combination of altered engine performance and pilot inputs during ground contact. Ultimately, there was insufficient evidence to confirm either theory, leaving the specific cause of the shaft fracture undetermined. Additionally, investigators discovered that the tail rotor blades had undergone unapproved modifications involving an epoxy addition to the leading edges.
Findings
- Thermal damage to the intake valve in the number-four cylinder caused the engine power loss.
- The tail rotor drive shaft failed due to torsional overstress, though the precise trigger remains unknown.
- The tail rotor blades featured unapproved modifications via an epoxy application on the leading edge.
- A compression test conducted shortly before the flight failed to reveal the progressing valve damage.