What happened
On September 14, 2016, a Binder EB29D touring motor glider, registration D-KVEB, was conducting a VFR flight near the Jura mountains. The aircraft, which had been in service for only about 24 hours, departed from Courtelary for a flight over the region. During the return leg of the flight, the crew directed the aircraft toward the Côte de Châtel area, a location known for its rising air currents.
While maneuvering near the terrain, the aircraft entered a stall and subsequently lost control. Flight data recorders indicated that the aircraft performed a series of tight spirals, with the rate of descent accelerating significantly to approximately 7,000 feet per minute. Witnesses on the ground observed the aircraft performing tight rotations before it plunged into the trees. The high-energy impact with the terrain resulted in two fatalities.
The investigation
Investigators from the SUST examined the wreckage and analyzed flight data from the onboard Logger LX 9000 and the Open Glider Network. The physical inspection of the D-KVEB revealed no mechanical failures; the flight controls were functional, and the engine and landing gear were in the retracted position. The aircraft's weight and center of gravity were within prescribed limits.
The investigation also reviewed meteorological data, which showed a strong Bise wind at the time, creating mechanical turbulence and thermally active conditions. The investigators also evaluated the flight profiles, noting that the aircraft was flying at a relatively low altitude of approximately 1,500 meters AMSL during the maneuvers.
Findings
- The aircraft encountered turbulent atmosphere and potential wind shear while flying near the terrain.
- The aircraft entered a stall during a left-hand turn, which then transitioned into a spiral dive.
- The crew attempted recovery maneuvers, as evidenced by the reversal of the rotation direction just before impact, but the remaining altitude was insufficient to recover level flight.
- The pilots had limited experience specifically with this aircraft type, having flown it for less than ten days.
- The combination of mechanical turbulence from the terrain and thermal activity created difficult, fragmented updrafts that made the flight highly demanding.