What happened
On Friday, June 17, 2011, a Cessna 150 L, registration F-BTFN, departed from Bourges aerodrome for a recreational flight accompanied by one passenger. Approximately 1 hour and 35 minutes into the flight, the engine stopped while the aircraft was in cruise. The pilot performed a forced landing in a field near Saint-Palais. Upon inspection on the ground, investigators found that the fuel tanks were empty and the fuel system was dry.
The investigation
The investigation focused on the pilot's pre-flight fuel planning and the actual fuel state of the aircraft. Before departure, the pilot had calculated an endurance of 3 hours and 15 minutes. This calculation was based on an assumed total endurance of 5 hours, from which the pilot subtracted 1 hour and 45 minutes of flight time recorded in the logbook since the last full refueling. Because the planned flight duration was under 2 hours, the pilot deemed the fuel levels sufficient.
During the flight, at a specific waypoint, the pilot noticed that the fuel gauges indicated a lower quantity than his calculations suggested. However, after re-calculating and arriving at the same result, the pilot concluded that the gauges were unreliable rather than questioning his initial math. The investigation also noted that the pilot did not perform a visual check of the fuel levels in the tanks prior to takeoff.
Findings
Several factors contributed to the engine failure:
- The pilot's incorrect estimation of the aircraft's actual endurance. While the flight manual suggests a theoretical endurance of 4.9 hours under specific conditions, the aircraft's actual average consumption was approximately 23 liters per hour. Based on the 85 liters of usable fuel available, the actual endurance was approximately 3 hours and 40 minutes, not 5 hours.
- The pilot's calculation of remaining flight time was significantly overestimated, leaving a much smaller margin than intended.
- The lack of a visual fuel quantity verification before departure prevented the pilot from identifying the discrepancy between his calculations and the actual fuel on board.
- A potential loss of approximately 10 liters of fuel may have occurred while the aircraft was parked on an incline, possibly due to fuel escaping through the tank vents. This loss would have reduced the actual endurance to approximately 1 hour and 35 minutes, matching the duration of the flight until the engine failure.