Accident Overview
On June 20, 2019, at 1716 Pacific daylight time, a Cessna 172M, registration N64460, was substantially damaged during an off-airport landing near Fullerton Municipal Airport (FUL) in Fullerton, California. The flight instructor and pilot receiving instruction were not injured. The aircraft was operating under 14 CFR Part 91 as a local instructional flight.
Flight Sequence
The flight instructor planned to conduct a flight review of the pilot receiving instruction within the traffic pattern. Both fuel tanks were confirmed full before departure, and the engine contained over 5 quarts of oil. Preflight checks and engine runup revealed no anomalies.
After completing one circuit in the traffic pattern, the pilots performed a practice go-around. During the initial takeoff climb of that maneuver, the engine began to lose partial power. The instructor took the controls and applied carburetor heat. The engine responded briefly, then lost power again. With minimal climb performance and limited time for troubleshooting, the instructor decided to land on a road adjacent to the airport. The right wing and forward fuselage were damaged during the landing.
Aircraft and System Details
The Cessna 172M was equipped with an airbox assembly mounted to the carburetor venturi inlet on the underside of the engine. Inlet air passed through a filter on the front of the airbox and was directed up into the carburetor venturi by a butterfly valve. When carburetor heat was selected ON, the butterfly valve rotated about 90°, redirecting heated air from an exhaust shroud into the carburetor venturi through the back of the airbox. The butterfly valve consisted of two metal plates sandwiching a flexible fiber-reinforced rubberized seal.
Investigation Findings
Examination of the airbox assembly revealed that the aft section of the carburetor heat butterfly valve seal had broken away from the valve and partially obstructed the air inlet of the carburetor venturi. The fragment was about 4½ inches long and 3/8-inch wide, brittle, and exhibited four holes matching the location of rivets on the lower side of the airbox. Black sooting was observed on the inner surfaces of the exhaust pipe and the electrodes of all spark plugs.
Maintenance History
According to maintenance logbooks, the last inspection was an annual inspection completed 73.5 flight hours before the accident on May 16, 2019, at a total airframe time of 3,052.5 hours. The Cessna 172 service manual recommended inspecting the induction airbox, air valves, doors, and controls at 100-hour intervals. The mechanic who performed the last annual inspection stated that he confirmed the operation and condition of the butterfly valve by observing it through both the forward inlet and the heated air inlet on the rear right side. He noted that the aft section of the seal was hard to view due to its position behind the butterfly valve, and that the only clear view was through the small heated air inlet.
The only documented maintenance on the airbox assembly since the airplane’s manufacture in 1974 was replacement of the airbox induction filter shroud on June 13, 2017.
Similar Occurrences
Review of the Federal Aviation Administration's Service Difficulty Report (SDR) database revealed three similar occurrences involving carburetor-equipped Cessna airplanes. Two occurred in 1977/78 N-model 172s and one in a 1974 M-model. The total time in service of the affected components was 3,706, 4,925, and 7,993 hours, respectively.