What happened
On 10 July 2022, a Cessna 152, registration F-GMOU, was conducting a circular cross-country flight departing from Poitiers-Biard airport. The flight path was intended to pass via La Roche-sur-Yon, Les Sables-d’Olonne, La Tranche-sur-Mer, and La Rochelle. After climbing to a cruise altitude of approximately 3,000 ft, the pilot began to experience engine misfires while passing abeam Tranche-sur-Mer.
Following the initial misfire, the pilot notified air traffic control at La Rochelle-île de Ré airport of the engine issues and his intention to land there. Shortly thereafter, the misfires became more frequent and louder. The pilot observed the tachometer oscillating rapidly between 2,200 and 2,400 rpm. Despite attempts to adjust the mixture and increase power, the engine failed to respond and eventually shut down completely.
After the engine failure, the pilot maintained a best glide speed of 60 kt. Realizing the aircraft could not reach the airport, the pilot attempted to head toward Île de Ré for a forced landing on land. However, determining that the island was out of reach, the pilot prepared for a ditching. The aircraft struck the water and tipped forward; the pilot successfully evacuated the aircraft before it sank. The pilot was subsequently rescued by a nearby sailing vessel.
The investigation
The investigation focused on the aircraft's mechanical condition and the sequence of the engine failure. The Cessna 152 had undergone a 100-hour inspection approximately 40 days prior to the accident, and maintenance records showed no anomalies regarding the engine.
Investigators recovered the wreckage from the sea on 21 July 2022. Examination of the fuel system showed the tanks contained a mixture of fuel and seawater, with approximately 65 liters of fuel remaining at the time of shutdown. The carburetor was inspected but could not be tested on a bench due to seawater contamination, though no pre-immersion damage was found.
Testing of the magnetos revealed that the left magneto was non-functional below 1,500 rpm and the right magneto was non-functional below 2,000 rpm. Further disassembly of the left magneto revealed an offset tooth in the rotor gear, which investigators determined must have occurred during a previous maintenance operation. However, this specific defect was not found to be the cause of the engine's total shutdown or the significant speed drops experienced during the flight.