What happened
While operating in cruise flight, the helicopter experienced a loss of engine power. The pilot attempted to restore power but was unsuccessful. Due to the darkness at the time of the incident, the pilot could not clearly see the terrain during the subsequent forced landing. This resulted in the aircraft hitting the ground with significant force.
The investigation
An examination of the engine identified several mechanical discrepancies. Two lower screws from the No. 3 valve cover were missing. Additionally, the attachment bolts for the intake pipes on both the No. 1 and No. 3 cylinder assemblies were found to be loose. Specifically, the No. 3 intake pipe had rotated approximately 180 degrees from its proper installation position. The gaskets that are typically installed under the intake pipe flanges for both the No. 1 and No. 3 pipes were also missing.
Regarding the ignition system, the spark plugs for cylinders No. 1, No. 2, and No. 4 were found to be wet with fuel, whereas the No. 3 spark plug was dry and showed tan-colored combustion deposits. A review of maintenance documentation indicated that the engine had been in operation for 38.2 hours since its last inspection.