CEN15LA414

The pilot reported that, while maneuvering the airplane at 3,000 ft mean sea level, the engine lost partial power and began running roughly. After unsuccessfully attempting to regain normal engine power, the pilot executed a forced landing into a field, during which the airplane nosed over. Postaccident examination of the carburetor revealed a gap between the throttle body and bowl assembly, and all four bolts connecting the throttle body to the bowl assembly were found loose. Both the engine and carburetor manufacturers had previously issued service bulletins (SB) regarding possible leakage between the bowl assembly and throttle body, likely due to loose bowl to throttle body screws. The SBs recommended inspecting the carburetor screws every 100 hours of engine operation. The last 100-hour inspection of the engine occurred 89 hours before the accident.

On September 16, 2015, about 1030 central daylight time, a Cessna 172P airplane, N65191, was substantially damaged during a forced landing near Nutwood, Illinois. The pilot sustained minor injuries. The airplane was registered to and operated by Sixninethreeone LLC under the provisions of 14 Code of Federal Regulations Part 91 as a personal flight. Day visual meteorological conditions prevailed for the local flight, which departed from the St. Charles County Smartt Airport (SET), St. Charles, Missouri, about 1000. While maneuvering at 3,000 feet msl, the pilot stated the engine lost partial power and ran rough. After unsuccessfully attempting to regain normal engine power, the pilot executed a forced landing into a field. The airplane nosed over during rollout and damaged the right wing. During post-accident examination of the carburetor, Federal Aviation Administration personnel noted a gap was present between the throttle body and bowl assembly. It was also noted that the four bolts which connect the throttle body to the bowl assembly were loose. Marvel Schebler Service Bulletin (SB)-17, dated August 12, 2010, with a subject of, "Body to Bowl Fuel Leaks", directs an inspection of the carburetor every 100 hours of engine operation. The SB indicates reports of loose body to bowl joints, with resulting leakage past the gasket between the bowl and throttle body. The SB further indicates that the leakage may be accompanied by loose bowl to throttle body screws. The compliance section of the SB indicates to visually inspect the carburetor for evidence of looseness of the bowl relative to the throttle body. The SB goes on to say that if looseness is detected, the carburetor must be removed from the engine and to inspect the threaded holes in the bowl and screws to assure the threads are serviceable. A new gasket and lock tab washers are to be installed, and the carburetor is to be tested in accordance with the appropriate service manual. Lycoming mandatory SB-366B, dated April 18, 2008, also directs an inspection of the carburetor every 100 hours of engine operation. The compliance section of the Lycoming SB is parallel to the Marvel Schebler SB. The last 100 hour inspection of the engine occurred 89 hours prior to the accident. The director of maintenance stated that maintenance personnel were aware of the Lycoming SB and were checking carburetor screws for looseness, but were not documenting accomplishment of the SB in engine logbooks.