Incident Overview
During a descent from 8,500 to 4,500 feet mean sea level (MSL), the pilot experienced engine roughness followed by a complete loss of engine power. Unable to maintain altitude, the pilot executed a forced landing in a farm field. Upon touchdown, the aircraft nosed over due to rough terrain.
Post-Accident Inspection
An examination of the engine was conducted. Manual rotation of the propeller found the engine rotated freely, and internal continuity was confirmed. The top spark plugs were removed and exhibited black, soft soot on the electrodes, indicating carbon fouling. When the starter was energized, the engine started successfully and ran for approximately ten minutes before being shut down.
Operational Context
The pilot/operator's handbook descent checklist specified the use of carburetor heat if carburetor icing conditions existed. Icing probability curves indicated that the aircraft was operating in conditions conducive to the formation of serious icing at glide power settings.
Conclusion
No specific probable cause was determined for the engine power loss. The observed spark plug fouling and the conducive icing conditions suggest potential carburetor icing, though the engine functioned normally during the post-accident test.