What happened
On February 14, 1973, a Hughes 300 C helicopter, registration HB-XDW, was conducting training maneuvers at the Sézegnin heliport in Geneva. The flight, involving two pilots preparing for professional examinations, included various exercises such as precision autorotative landings and hover maneuvers.
During a specific exercise involving an autorotation with a subsequent power application, the engine failed. The pilot attempted to manage the descent, but as the aircraft approached the target at a speed between 30 and 50 mph, the engine failed to restart when power was applied. At an altitude of approximately 3 to 4 meters, the aircraft suddenly dropped and struck the ground with significant force. Despite the heavy damage to the aircraft, both occupants emerged uninjured.
The investigation
Technical examinations of the engine revealed that the Fuel Control Unit was delivering an excessively rich mixture, particularly at low engine speeds. Furthermore, analysis of the fuel from the fuel strainer showed heavy contamination; the residue left after evaporation was significantly higher than the permitted limits.
The investigation also noted that the refueling process at the Geneva airfield involved using plastic canisters without any specific filtration, which was contrary to the manufacturer's flight manual instructions. While the pilot's decision not to attempt a restart due to low altitude was deemed correct, the investigation noted that the pilot's specific technique for the autorotation did not fully align with the flight manual's procedures regarding the 'flare' maneuver.