Accident Narrative
A private pilot was flying a single-engine airplane and experienced a loss of engine power while approaching a private grass airstrip. During the downwind leg, the pilot reduced power and the engine began to run "rough." Despite applying carburetor heat and advancing the mixture and throttle, the engine did not respond. The engine briefly developed power but became intermittent on the base leg. Realizing he might not reach the landing area, the pilot selected an open field adjacent to the runway. However, after observing obstacles in that field, he turned back toward the grass strip. Subsequently, the airplane impacted small trees and brush approximately 300 feet short of the intended landing strip and came to rest within 10 feet of the initial impact point. A post-crash fire ensued.
Engine Examination
After the accident, the spark plugs were removed and examined. A dark, black carbon coating was observed on the tips of the electrodes and on the sides of the electrode posts, which appeared "grayish" in color. The #4 cylinder exhaust flange was partially separated from the cylinder flange, and one nut was missing. Visual evidence indicated heat concentration and thermal distress around the exhaust flange, and smoke residue was observed on the lower fuselage.
Test Run Findings
The engine was mounted on a test stand, started, and operated for five minutes, reaching up to 1,700 RPM. With full throttle applied, the engine RPM did not exceed 1,700 RPM. During the test run, fuel was observed leaking from the mating surface between the carburetor bowl and the carburetor upper plate. The engine was then shut down, and the four bolts retaining the carburetor bowl to the upper plate were tightened approximately 1/4 turn each to further compress the gasket and abate the fuel leak. While tightening the bolts, it was noted that the carburetor was sooted and the gasket appeared to have some degree of thermal distress. The engine was then started again. Magneto checks were performed at 1,800 RPM with 75–100 RPM drops, and then at 2,100 RPM with 75 RPM drops. The engine operated for approximately 10 minutes at 2,100 RPM. According to carburetor icing probability charts and the available temperature and dew point at the time of the accident, the airplane could have been susceptible to moderate or severe carburetor icing at glide power settings.