What happened
On May 4, 2016, a Dyn-Aéro MCR Sportster, registration HB-YKY, departed from Pontarlier aerodrome for a flight to Neuchâtel, Switzerland. The flight was intended to return the aircraft to its owner following maintenance performed on the oil system. Shortly after takeoff, while climbing through approximately 300 feet, the pilot experienced a sudden loss of power followed by a complete engine shutdown. The pilot attempted to follow emergency restart procedures without success. After extending the flaps to 25 degrees, the pilot performed an emergency landing in a steep field. The impact was severe, resulting in one pilot slightly injured and serious damage to the aircraft.
The investigation
The investigation focused on the engine failure and the condition of the aircraft's fuel system. While the engine itself showed no operational anomalies, investigators discovered a significant amount of water present in both carburettors and the electric fuel pump.
Examination of the fuel system revealed that the aircraft was equipped with a non-standard metal fuel tank, installed to replace the original composite tank. This replacement tank, manufactured by a German company, had not been approved by the aircraft manufacturer. The installation of this tank resulted in a discrepancy where the tank cap sat approximately three centimeters lower than the manufacturer's standard design. This configuration created a recessed bowl around the cap assembly.
Findings
- The primary cause of the engine shutdown was the presence of water in the fuel circuit.
- The specific design of the aftermarket metal tank allowed rainwater to accumulate in a recessed area around the tank cap.
- This accumulation allowed water to penetrate the tank at a rate of approximately 15 ml per minute.
- The pilot did not perform a fuel tank purge during the pre-flight inspection, which would have otherwise removed the accumulated water.
- The use of an unapproved fuel tank assembly contributed to the vulnerability of the system to water ingress.