What happened
On May 6, 2013, a Centrair 101 glider, registration F-CGSF, was performing a local flight in the Pyrenees near Cierp-Gaud. While flying in ridge lift near the Hourmigué crest, the pilot observed an unidentified dark object in the sky. Shortly after, a Dassault Aviation Dornier Alphajet, registration F-TERB, appeared directly ahead at a similar altitude, closing the distance at high speed.
In an attempt to avoid a collision, the pilot of the F-CGSF attempted to fly beneath the jet and turn left. During this maneuver, the pilot experienced heavy vibrations and a significant drop in indicated airspeed. The glider subsequently lost control and struck trees, coming to rest vertically a few meters above the ground. The pilot of the F-TERB, a military crew consisting of a pilot and a passenger, did not observe the glider during their transit.
The investigation
The BEA investigation examined the flight paths of both aircraft using GPS and radar data. The F-TERB was flying at high speeds, between 340 and 430 knots, and was observed crossing several ridges at altitudes as low as 50 feet above the terrain. The F-CGSF was flying at a much lower speed, with its indicated airspeed dropping to approximately 83 km/h during the encounter.
Investigators analyzed the possibility of wake turbulence from the F-TERB affecting the glider, but concluded that at the distance maintained, the vertical air movement from the jet was negligible. Flight tests using a similar glider type were also conducted. While these tests did not replicate the specific vibrations reported by the pilot, they did demonstrate that an uncoordinated, aggressive left turn at low speeds could lead to a significant loss of airspeed and altitude, similar to the flight profile recorded during the accident.
Findings
- The accident resulted from a loss of control during a ridge soaring flight.
- The pilot of the F-CGSF was likely caught by surprise by the sudden appearance of the high-speed military aircraft, leading to an instinctive and aggressive avoidance maneuver.
- The glider's maneuver exceeded its available aerodynamic margins, particularly as the airspeed was already low and near the stall threshold.
- The high-speed, low-altitude flight profile of the F-TERB through mountainous terrain significantly reduced the time available for the glider pilot to detect the threat and execute a safe maneuver.
- Adverse meteorological conditions or local air turbulence may have further contributed to the loss of control during the maneuver.