Primary finding
Probable cause
A partial loss of engine power due to the rear cylinder's spark plug coming out of the cylinder head.
Investigator assessment
Analysis narrative
According to the sport pilot, during cruise flight in his experimental light sport airplane he noted that the two-cylinder engine's cylinder head and exhaust gas temperature gauges decreased in temperature. Thereafter, the number 1 cylinder (rear) stopped producing power, and engine power was partially lost. The pilot made a forced landing on upsloping, rough terrain. During landing, the airplane's forward fuselage buckled. The pilot said that examination of the engine revealed that the spark plug had come out of the engine's rear cylinder head. The pilot had changed the spark plugs about 2.5 hours prior to the mishap.
Source record
Factual narrative
According to the sport pilot, during cruise flight in his experimental light sport airplane, he noted that the two-cylinder Hirth 3203 engine's cylinder head and exhaust gas temperature gauges decreased in temperature. Thereafter, the number 1 cylinder (rear) stopped producing power, and engine power was partially lost. The pilot made a forced landing on upsloping, rough terrain. During landing, the airplane's forward fuselage buckled. The pilot said that examination of the engine revealed that the spark plug had come out of the engine's rear cylinder head. The pilot had changed the spark plugs about 2.5 hours prior to the mishap.