Cessna 172M Engine Incident at Alijó Aerodrome (CS-AVT)

Incident Overview and Flight History On 19 August 2010, a Cessna 172M, registration CS-AVT, operated by Aero Clube de Chaves, departed Chaves Municipal Aerodrome at 07:30 UTC for Portimão Municipal Aerodrome. The aircraft landed at Portimão at 10:30 UTC. After refueling and completing standard procedures, the pilot and two passengers departed Portimão at 11:40 UTC for the return flight to Chaves. The flight was authorized for Flight Level 080. The pilot maintained two-way radio contact with Faro Approach Control and Lisbon Air Traffic Control. ## In-Flight Anomaly and Precautionary Landing During the return leg, while flying north of the Douro River and after passing Vila Real Aerodrome, the pilot observed abnormal vibration followed by a partial loss of engine power. The pilot attempted corrective actions, including engaging the carburetor heat, cycling the magnetos, and adjusting the power lever, but these measures were unsuccessful. Unable to assess the severity of the situation, the pilot contacted Lisbon Air Traffic Control (LISACC) and declared an intention to make a precautionary landing at Alijó Aerodrome, which was approximately 10 nautical miles to the right. The aircraft landed at Alijó at 13:40 UTC without incident. No injuries or damage to the aircraft were reported. ## Aircraft and Maintenance Details The aircraft was a single-engine, high-wing, metal monoplane with a fixed tricycle landing gear, seating four persons, and a maximum takeoff mass of 1,043 kg. It was powered by a Lycoming O-320-E2D engine and equipped with a McCauley L-28649-27A two-blade fixed-pitch propeller. The engine had undergone an overhaul and had accumulated 198 hours of flight time since the last inspection. The aircraft's total flight time was 7,218 hours. ## Investigation Findings Following the landing, a maintenance technician inspected the engine. The initial examination revealed a bent pushrod tube for the exhaust valve of cylinder number 2. Further disassembly confirmed that the exhaust valve pushrod was bent and the valve was stuck in the open position. Carbon deposits were found on the exhaust valve seat, preventing the valve from closing. The maintenance of the valve in this open position caused the pushrod to bend and resulted in irregular engine operation and vibration. ## Analysis of Contributing Factors The aircraft's logbook indicated approximately 190 hours of flight time over the previous six years, averaging just over 30 hours per year. In the month preceding the incident, the aircraft logged 19 hours and 10 minutes, with intervals of inactivity ranging from one to ten days. The report notes that during winter months, the aircraft likely experienced long periods of inactivity without specific engine preservation treatment. The investigation highlights that valve sticking, particularly of the exhaust valve, is a known phenomenon attributed to high temperatures, lack of cylinder cleanliness, and prolonged engine inactivity. During inactivity, oil accumulates combustion residues such as moisture, acids, and lead sludge, which can form deposits on valve walls, causing corrosion and preventing normal operation.