What happened
On April 28, 1997, an SA 313B Alouette II helicopter, registration F-BYCX, was conducting a local sightseeing flight near Toussus-le-Noble. The aircraft was carrying the pilot and three passengers when it entered the airspace near Villacoublay. During the flight, the aircraft encountered heavy turbulence and wind gusts.
While descending toward the airfield, a sudden gust caused the rotor to enter an overspeed condition. The aircraft's freewheel mechanism engaged to protect the engine, but it failed to re-engage the engine to the rotor. This caused the helicopter to enter an unplanned state of autorotation. As the pilot attempted to manage the descent, the aircraft struck the canopy of the Montéclain woods. The impact resulted in three fatalities and one serious injury.
The investigation
The investigation focused on the mechanical state of the transmission and the freewelle mechanism. Investigators examined the wreckage and found that while the engine and main gearbox were functional, the freewheel mechanism—the component connecting the turbine to the main gearbox—was contaminated with grease and metallic wear particles.
Technical analysis revealed that the freewheel's rollers were worn beyond allowable tolerances. Furthermore, the investigation found that the component had been installed 169 hours prior to the accident without the specific inspections recommended by the manufacturer's service bulletin. The investigation also noted that the emergency locator transmitter (ELT) failed to transmit because its power cable was severed during the impact, which delayed the rescue efforts.