WPR19LA176

Shortly after takeoff during a practice go-around, the engine lost partial power. With limited time to troubleshoot, the flight instructor maneuvered the airplane for an off-airport landing, and the airplane sustained substantial damage after impacting a street. Postaccident examination revealed that the aft section of the carburetor heat butterfly valve seal had detached and partially blocked the air inlet of the carburetor venturi. The obstruction reduced the amount of air available for combustion, resulting in a rich fuel-to-air mixture, and partial loss of power. The valve seal was composed of a fiber-reinforced rubberized material that degraded and became brittle over the 45-year life of the airplane. The mechanic who performed the last inspection stated that he confirmed the operation and condition of the butterfly valve by observing it through both the airbox assembly forward inlet and the heated air inlet on the rear right side of the unit. He stated that the aft section of the seal is hard to view due to the way it folds behind the butterfly valve and that the only way to clearly see it is through the heated air inlet, which was small.

On June 20, 2019, at 1716 Pacific daylight time a Cessna 172M, N64460, was substantially damaged when it was involved in an accident near Fullerton Municipal Airport (FUL), Fullerton, California. The flight instructor and pilot receiving instruction were not injured. The airplane was operated under the provisions of Title 14 Code of Federal Regulations Part 91 as a local instructional flight. The flight instructor reported that he planned to perform a flight review of the pilot receiving instruction and remain within the traffic pattern. He confirmed that both fuel tanks were full prior to departure and that the engine contained over 5 quarts of oil. The preflight checks and the engine runup did not reveal any anomalies. After completing one circuit in the traffic pattern, they performed a practice go-around. During the initial takeoff climb of the maneuver, the engine began to lose partial power. The flight instructor took the airplane's controls and applied carburetor heat. The engine responded for a few seconds, and then power was lost again. With minimal climb performance available, and limited time to perform trouble shooting steps, the instructor decided to land the airplane on a road adjacent to the airport. The airplane sustained damage to the right wing and forward fuselage during the off-airport landing. The airplane 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. The system was designed such that when the pilot pulled the carburetor heat control (ON), the butterfly valve rotated about 90°, and redirected heated air from an exhaust shroud into the carburetor venturi through the back of the airbox. The butterfly valve was composed of two metal plates that sandwiched a flexible fiber-reinforced rubberized seal. 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 (See figures 1 and 2). The section was about 4 1/2 inches long and 3/8-inch-wide. It was brittle and exhibited four holes that matched the location of rivets on the lower side of the airbox (See figure 3). Black sooting was also observed on the inner surfaces of the exhaust pipe and the electrodes of all spark plugs (See figure 4). Figure 1 – Carburetor Heat Valve Seal Fragment Obstructing Carburetor Inlet Venturi Figure 2 – Carburetor Heat Valve in the ON Position (Area of Missing Seal Highlighted) Figure 3 – Carburetor Heat Valve Seal Fragment Figure 4 – Top Spark Plugs 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 that the induction airbox, air valves,doors and controls be inspected at 100-hour intervals. The mechanic who performed the last inspection stated that he confirmed the operation and condition of the butterfly valve by observing it through both the airbox assembly forward inlet and the heated air inlet on the rear right side of the unit. He stated that the aft section of the seal is hard to view due to the way it folds behind the butterfly valve and that the only way to clearly see it is through the heated air inlet, which was small. The only documented maintenance performed on the airbox assembly since the airplane’s manufacture in 1974 was the replacement of the airbox induction filter shroud on June 13, 2017. 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 172's 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.