What happened
On 30 October 2020, a Cessna 401B, registration N517HC, was performing a local test flight near Toussus-le-Noble aerodrome. The aircraft, which had recently undergone work to return it to airworthy condition after being stored for over twelve years, was carrying a pilot, a passenger, and a mechanic.
Shortly after takeoff, the engine computer indicated that the temperature of cylinder No. 4 on the left engine had exceeded the manufacturer's limit. Although the crew initially believed this might be a false alert and managed to stabilize the temperature by reducing power, the warning light reactivated during the return leg of the flight. At this point, a fire broke out in the left engine. The pilot attempted to cut the fuel supply and feather the propeller, but the fire continued. Fearing the aircraft would not reach the runway, the pilot performed an emergency landing in a ploughed field. The aircraft sustained substantial damage, but the three occupants evacuated safely without injury.
The investigation
The BEA examined the aircraft's engine computer (CGR-30) and the wreckage of the left engine. The investigation revealed that the engine had been stored for approximately 14 years without specific preservation measures, exceeding the manufacturer's recommended overhaul interval by eight years.
Analysis of the engine components showed significant corrosion and friction marks on the cylinder liners. Data from the engine computer showed that during ground tests, temperatures in cylinders No. 2 and No. 4 had been trending high, and during the flight, the temperature of cylinder No. 4 reached 536°F, well above the 460°F limit. The investigators also noted that the propeller failed to feather despite the pilot's input.