What happened
On July 19, 1963, a Cessna 150-B, registration HB-CRW, departed from Zurich-Kloten with the intention of transporting a passenger to a meeting in Bex. Despite receiving weather briefings that indicated poor visibility and low cloud bases in the Mittelland and Prealps regions, the pilot proceeded with the flight.
After flying toward Bern, the pilot attempted to navigate along the edge of the Prealps. During the flight, the aircraft descended to an altitude between 4,500 and 5,000 feet. While attempting to maneuver through the terrain, the aircraft encountered sudden fog or obscured terrain. The pilot attempted to pull up or execute an emergency landing, resulting in the aircraft striking a slope near Rüschegg. While the aircraft was destroyed, both occupants escaped without injury.
The investigation
An investigation by the Swiss Transportation Safety Investigation Board (SUST) examined the weather conditions, the pilot's flight path, and the aircraft's performance. Investigators reviewed the weather briefings obtained by the pilot, which had explicitly warned of conditions below VFR (Visual Flight Rules) minimums in certain areas. The investigation also analyzed the pilot's decision-making regarding altitude and navigation through mountainous terrain under deteriorating visibility.
Findings
- The pilot flew into an area where the terrain and weather conditions made the safe continuation of the flight impossible.
- The pilot's altitude of approximately 1,400 to 1,500 meters was too low to maintain the required safety margin of 150 meters above ground in such complex terrain.
- The pilot lost visual orientation while navigating through heavy fog and mountainous ridges.
- The decision to fly through the Prealps under such conditions carried an extreme risk of becoming lost or encountering terrain.
- While the pilot claimed a sudden weather change, investigators noted that the stationary fog and high-pressure system made a sudden, unexpected deterioration unlikely; however, the pilot was still responsible for anticipating such changes in the mountains.