What happened
The helicopter was engaged in controlled burning of timberland when it experienced a partial loss of engine power. At the time of the incident, the aircraft was being operated at an altitude of approximately 20 feet above windrows of organic material. The pilot reported that he was unable to maintain a hover due to the power deficiency. Consequently, he executed a landing on rough terrain.
Following touchdown, the helicopter rolled over. During this rollover event, the main rotor blades made contact with the ground. No further details regarding injuries or specific aircraft registration were provided in the source material.
The investigation
An examination of the engine was conducted to determine the cause of the power loss. The inspection revealed that all eight spark plugs showed evidence of lead fouling. Specifically, in the number four cylinder, the electrodes of the bottom spark plug were completely bridged with lead. The pilot also stated that the top spark plug in that same cylinder was dirty with carbon deposits.
Findings
The mechanical condition of the ignition system, specifically the severe lead fouling and carbon contamination on the spark plugs, contributed to the partial loss of engine power. This failure prevented the pilot from maintaining controlled flight at low altitude during critical burning operations.