FTW04LA094 · Piper PA-28-151

Following a loss of engine power in the single engine airplane, the private pilot executed a forced landing while on approach to a private grass airstrip. While entering the downwind leg, the pilot reduced power and the engine began to run "rough." After carburetor heat was applied, the pilot advanced the mixture, and throttle with no effect. The engine began to develop power momentarily, and then, on base leg, the engine power became intermittent. Realizing the he may not make the landing area, the pilot selected an open field adjacent to the runway; however, he observed obstacles in the field and 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 from the initial impact point. A post-crash fire ensued. After the accident, the spark plugs were removed and dark, black carbon coating was observed on the tips of their respective electrodes and the sides of the electrode posts were "grayish" in color. The #4 cylinder exhaust flange was partially separated from the cylinder flange, and one nut was missing. There was visual evidence of heat concentration and thermal distress around the exhaust flange, and there was smoke residue observed on the lower fuselage. The engine was mounted on a test stand, started, and was operated for five minutes, 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 to be leaking from the mating surface between the carburetor bowl and carburetor upper plate. The engine was then shut down, and the 4 bolts that retain the carburetor bowl to the upper plate were tightened down approximately 1/4 turn each in an attempt to further compress the gasket and abate the fuel leak. While tightening the bolts, it was noted that the carburetor was sooted and it appeared that the gasket had 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 again 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.

Verbatim NTSB analysis and probable cause from the NTSB dataset