Accident Sequence
Shortly after takeoff and transition to forward flight, the helicopter's engine stopped. During the ensuing autorotative landing glide, a main rotor blade struck a tree about 30 feet above ground level. The helicopter then fell to the ground.
Pre-flight Conditions
This was the first flight of the day. The helicopter had been stored outside overnight in conditions of cold temperatures and light snowfall without engine air inlet covers installed. Prior to the flight, the engine was difficult to start. Five start attempts were aborted before the engine was successfully started. During the first three start attempts, fuel was introduced, ignition occurred, the engine acceleration hung at 30 percent, and the starts were aborted.
After the third start attempt, a mechanic noted that the engine inlets were approximately half blocked by snow and cleared the snow from the inlets using a towel. He did not check the plenum chamber aft of the particle separator and forward of the engine inlet for snow.
Post-accident Examination
In the wreckage, the plenum chamber inspection window was found to be opaque. After the accident, another company mechanic who had dealt with the same starting difficulty on another helicopter of the same type parked under the same circumstances without inlet covers reported finding the plenum chamber of that helicopter contained several cups of snow.
The engine manufacturer reported that tests of the engine had shown that ingestion of as little as 6 ounces of snow or slush could flame out the engine.
Operational Context
The operator's procedure requiring that the last flight crew at the end of the day install the inlet covers on aircraft stored outside had not been followed.