Event Details
The pilot reported that during cruise, the engine began to lose power (RPM) as if experiencing a bad case of carburetor icing. The pilot applied carburetor heat after the engine had lost over 300 RPM. Despite this action, the engine continued to lose power, leading the pilot to elect a precautionary landing in an open field. The field was uneven, and the pilot found that he could not brake the airplane to a stop before colliding with a fence. According to the pilot, the total elapsed time from the initial power loss to landing was only 2 to 3 minutes.
Engine Examination
Examination of the engine revealed black, sooty deposits on the firing ends of the spark plugs. According to icing probability curves, conditions were favorable for the formation of moderate icing at cruise power. The airplane was equipped with a Supplemental Type Certificate (STC) to operate on autogas, and it was fueled with a mixture of 100LL avgas and auto gas.