What happened
On August 11, 2001, a student pilot was conducting his 16th solo flight in an ASK 23B glider, registration HB-1880. After completing a scheduled two-hour flight, the pilot was returning to Buttwil airfield. At an altitude of approximately 500 to 600 meters above ground level, the aircraft's flight attitude changed abruptly. Witnesses observed the glider enter a leftward slip, followed by a rapid nose-down pitch and a forward roll over the left wing. This maneuver transitioned into an inverted spin, during which the aircraft maintained a constant angular velocity and sink rate. The aircraft eventually disappeared from view and impacted a field, resulting in the fatal injury of the pilot and the destruction of the aircraft.
The investigation
An investigation conducted by the Swiss Transportation Safety Investigation Board (SUST) examined the aircraft, the pilot's medical history, and the flight circumstances. Technical inspections of the ASK 23B revealed no mechanical failures; the control linkages, including the ailerons, rudder, and spoilers, were intact and properly secured. The investigation also found no evidence of medical impairment, as toxicology reports were negative for alcohol or drugs, and the pilot's recent eye surgery was not a contributing factor. Witnesses noted that the aircraft's sink rate increased just before the loss of control, which may have been due to the deployment of air brakes or a localized downdraft.
Findings
The accident was caused by a loss of control of the glider shortly before the final approach. A significant contributing factor may have been the phenomenon of "sub-gravity sensations" (low-g). In such instances, a reduction in vertical acceleration can lead an inexperienced pilot to instinctively push the control stick forward to counteract the sensation of falling. In this specific aircraft type, such an input can trigger a significant nose-down moment, potentially leading to the unrecoverable inverted spin observed during the accident.