What happened
On July 30, 2000, at approximately 14:50 local time, a Ventus C glider, registration HB-1975, was participating in a private soaring training flight near St. Stephan (Wistätthorn), Switzerland. The aircraft had departed from the Zweisimmen airfield earlier that afternoon.
While flying in the vicinity of the Wistätthorn ridge, the aircraft was observed by witnesses performing a standard turn. During the maneuver, the glider's nose began to drop rapidly. Witnesses noted that as the aircraft continued through a 270-degree turn, the descent rate increased significantly. The aircraft eventually entered a vertical dive, accelerating to an estimated double its previous speed, before impacting the ground at a high velocity. The impact was so severe that the forward section of the fuselage became embedded in the earth, leaving the aircraft stuck in a vertical position.
The investigation
Investigators from the Swiss Transportation Safety Investigation Board (SUST) examined the wreckage and the circumstances surrounding the flight. The aircraft was found to be airworthy and properly maintained, with the center of gravity within permissible limits. No trim weights were present, and the wing and tail tanks were empty.
Technical examination of the airbrake system revealed damage, including a broken linkage on the right airbrake and bent metal on the left. However, forensic analysis by EMPA determined that this damage was caused solely by the force of the ground impact and not by any pre-existing mechanical failure. Furthermore, consultation with the manufacturer, Schempp-Hirth, yielded no technical explanation for the sudden change in flight path.
Medical examinations were also conducted. An autopsy performed by the Institute of Forensic Medicine in Bern revealed that the pilot had a 60% narrowing of the coronary arteries. Toxicological tests were negative for alcohol, drugs, or medication.
Findings
- The aircraft was in a functional, airworthy condition prior to the accident.
- There were no mechanical or technical defects found that could have caused the sudden dive.
- The pilot was experienced, holding a valid glider license and significant hours in both gliders and motor aircraft.
- Medical reasons are the most likely cause of the accident, as the pilot's underlying coronary artery disease could have led to a loss of consciousness, particularly in high-altitude environments where oxygen levels are lower.
Safety action
No specific safety recommendations were issued, as the investigation could not definitively prove a medical event occurred, though it remains the most probable cause.