Incident Overview
The pilot of a helicopter had stabilized the aircraft in a 125-foot hover in preparation for picking up a load. At that point, the engine spooled down, and the helicopter entered tall trees with low rotor RPM, pitching nose-down as it descended through the trees. Ground witnesses reported hearing two loud bangs before the aircraft entered the trees.
Investigation Findings
Examination of the engine revealed that the fuel control and power governor schedules were set above the specified limits. These settings would increase fuel flow and engine power available while decreasing operating temperatures. However, they also reduced the engine surge margin and increased the possibility of an engine surge (compressor stall) and power drop. Additionally, the compression was found to be dirty, and the bleed air line was loose, which would have further reduced the surge margin.
Operational Context
The nature of the long line operation and the surrounding tall trees precluded the successful completion of the engine-out emergency procedure.
Conclusion
No probable cause was explicitly stated in the investigation.