What happened
On May 1, 2001, a Jubi Sportflugzeugbau ASK 13 glider departed from Unterwössen via winch launch for a thermal flight. The aircraft was occupied by a pilot in the rear seat and a passenger in the front seat. Approximately five minutes after takeoff, the glider was observed flying at a very low airspeed, estimated between 60 and 70 km/h, performing wide turns.
While flying on a northeastern course, the aircraft suddenly entered a near-vertical longitudinal pitch and disappeared from view. The glider struck a grassy roadside embankment at an 80-degree pitch. The impact resulted in two fatalities: the passenger died instantly, and the pilot passed away in a hospital two hours after the accident. The aircraft was destroyed, and damage was caused to the ground.
The investigation
The BFU investigation examined the aircraft's technical condition, the pilot's medical fitness, and the flight's weight and balance. Technical inspections of the flight controls revealed no mechanical defects. The aircraft had undergone its annual inspection recently, in April 2001.
Investigators determined that the aircraft's takeoff mass was 510 kg, which was 30 kg above the maximum allowable takeoff weight. While the center of gravity remained within limits, the weight of the passenger (including parachute) was 78 kg, which was slightly below the updated minimum required load for the front seat of 80 kg.
Regarding the pilot, although he possessed a valid license and significant flight experience, investigators found that he was not wearing his required corrective lenses at the time of the accident. Medical records indicated the pilot was farsighted, though it was noted that horizon references should have remained visible without glasses. Additionally, weather conditions at the time included Föhn winds and significant turbulence in the area of the crash.
Findings
- The glider was overloaded, which increased the aircraft's stall speed.
- The pilot failed to terminate his search for thermals in a timely manner.
- The pilot did not maintain the required stall speed during the low-altitude flight.
- Atmospheric turbulence may have contributed to the aircraft's loss of control.