What happened
On March 29, 2017, a LAK17A glider, registration F-CJJH, was performing a winch launch from the unpaved runway at Chambéry Challes-les-Eaux. The aircraft was being launched in a ballasted configuration. During the initial climb, the flight director (the person in charge of the session) observed that the cable tension was insufficient and requested the winch operator to accelerate the winch twice via radio. Approximately twelve seconds after the start of the launch, while the aircraft was at an altitude of roughly 25 meters, the glider tilted sharply to the right, inverted, and struck the ground within the airfield perimeter. The pilot was killed, and the aircraft was destroyed.
The investigation
The BEA examined the wreckage, which was found completely dislocated, and confirmed that the flight controls were intact with no mechanical anomalies detected. Investigators analyzed surveillance camera footage and audio recordings from the winch station, which captured the entire sequence of events. The investigation also reviewed the winch's technical performance, the club's adjustment charts, and the pilot's flight experience. The study revealed that the winch's speed control lever had a very narrow effective range, making precise adjustments difficult. Furthermore, the club's adjustment chart, which guided the winch operator, was based on empirical experience rather than technical calculations and did not account for the increased weight of ballasted gliders.
Findings
Several contributing factors led to the accident. The primary cause was the abrupt increase in cable tension caused by the winch operator accelerating the winch in response to the flight director's requests. This sudden surge in load, combined with the glider's low airspeed, induced a stall.
Additional contributing factors included:
- The use of an inaccurate adjustment chart that failed to provide sufficient winch speeds for ballasted aircraft.
- A deviation from standard emergency procedures, as the flight director requested acceleration instead of instructing an immediate launch termination.
- The pilot's failure to maintain the manufacturer-recommended minimum speed for a ballasted launch.
- Inadequate technical documentation from the winch manufacturer, which prevented the establishment of a verified relationship between winch settings and required aircraft speeds.
- The ergonomic design of the winch control lever, which made precise speed regulation challenging.