What happened
On the morning of May 11, 1975, a Robin DR 400/180, registration F-BSQO, departed from Annemasse, France, bound for Geneva-Cointrin airport. The flight, which included the pilot and three passengers, was intended to be a technical stop before continuing to Calvi, Corsica.
While performing holding patterns in the approach circuit for runway 23, the aircraft descended rapidly into the Foretaille forest at an altitude of approximately 100 meters. The impact was followed immediately by a fire that consumed the aircraft. All four occupants perished in the accident.
The investigation
Investigators examined the wreckage and engine components to determine the sequence of events. Witnesses near the airfield reported seeing the aircraft flying at a very low altitude and noted that the engine sound became irregular before disappearing into the trees. One observer specifically noted that the engine noise had diminished, leaving only the sound of airflow and the propeller.
Technical analysis of the engine showed that the propeller blades were bent backward, and the engine was found embedded in the ground, suggesting a near-vertical descent. While the carburetor could not be identified due to fire damage, no other mechanical defects were found in the engine components. The investigation also noted that the aircraft was operating near its maximum allowable weight.
Findings
- The primary cause of the accident was a loss of airspeed at a low altitude, likely resulting from a reduction in engine power due to carburetor icing.
- High humidity and low temperatures (10°C) created significant risks for ice formation in the carburetor.
- Poor visibility and low cloud ceilings contributed to the difficulty of the flight conditions.
- The pilot's modest flight experience and the decision to fly at low altitudes over rugged terrain made managing the engine power loss extremely difficult.
- The aircraft's heavy weight near its maximum limit further reduced the margin for error during the power loss.