About 20 minutes into the cross-country flight, the pilot of the experimental amateur-built airplane detected a vibration of the airplane’s engine that “didn’t sound good,” and noticed a drop in the rpm of the automotive-conversion engine. He immediately applied carburetor heat, but the engine continued to lose power until the airplane could no longer maintain altitude and began to descend from its cruise altitude of 1,500 feet agl. The pilot subsequently performed a forced landing to the trees below, resulting in substantial damage to the airplane’s fuselage, wings, and empennage. A Federal Aviation Administration (FAA) inspector examined the wreckage at the accident scene, found no anomalies associated with the airplane’s fuel system, and noted that the fuel remaining was absent of any contamination. A subsequent examination of the engine was also unable to note any obvious mechanical deficiencies of the engine. The temperature and dew point reported at an airport located about 6 nautical miles from the accident site were conducive to “serious icing at cruise power,” according to a carburetor icing probability chart published by the FAA. Following the accident, the pilot stated he believed that the airplane’s custom carburetor heat system may not have possessed an adequate heating capacity to recover from an encounter with carburetor icing.