What happened
On December 5, 1999, a Robin DR 315, registration F-BROF, departed Saint-Inglevert for a planned 25-minute training flight to Saint-Omer. Approximately 15 minutes into the return leg, while cruising at 1,500 feet, the fuel pressure warning light illuminated. As engine RPM decreased, the instructor attempted to restore power but was unsuccessful. After notifying the Calais tower of the aircraft's position, the crew performed an emergency landing in a field near Alquines.
The investigation
The investigation focused on the fuel state of the aircraft and the decision-making process leading to the departure. Upon inspection, the 110-liter fuel tank was found to be empty. The electrical fuel gauge was found to be stuck, providing a false reading that suggested three hours of remaining endurance.
Because the Saint-Inglevert airfield lacks refueling services, the club's aircraft typically refuel at Calais. During the pre-flight inspection, the instructor noted that the gauge indicated a fuel level approximately one hour less than a full tank. Based on the aircraft logbook, which showed the previous flight from Calais had lasted one hour, the instructor incorrectly assumed that the aircraft had been refueited at Calais despite the lack of a corresponding logbook entry. Due to the design of the Robin DR 315, a visual verification of the fuel level was not possible.
Findings
- The primary cause of the incident was an erroneous mental representation of the situation regarding the fuel level.
- The instructor's decision to depart was influenced by three cognitive biases: a habit bias regarding refueling procedures, a confirmation bias linking the gauge reading to his assumptions, and a data selection bias where the gauge was treated as the sole source of truth.
- The electrical fuel gauge was malfunctioning due to worn insulation, which led to the incorrect reading.