Incident Overview
During cruise flight, the helicopter's engine lost power, prompting the pilot to initiate an autorotative descent. The aircraft subsequently collided with high vegetation. Moments before touchdown, engine power was restored, but the pilot decided to focus on completing the forced landing successfully. The helicopter touched down in high vegetation and sustained substantial damage.
Engine Examination and Testing
Following recovery from the remote site, the engine underwent visual examination and was placed in a test cell for runs. During the first run, the engine started and warmed up normally. However, it exhibited lean operation when accelerating between 2,250 and 2,300 rpm with the fuel boost pump off. When the test cell's fuel boost pump was turned on, the engine accelerated smoothly between 2,700 and 2,800 rpm without hesitation.
A second engine run was conducted using a carburetor from another Lycoming O-540 engine. With this carburetor, the accident engine operated normally during run-up to full power without the test cell's fuel boost pump.
The accident carburetor was subsequently examined, including a flow bench test. No abnormalities were found that would have prevented it from functioning within prescribed design specifications.
Conclusion
The reason for the excessively lean mixture or restricted fuel flow that reduced the engine's available power was not ascertained.