What happened
On June 13, 1970, an experienced glider pilot was conducting a cross-country flight from Schänis to Gruyères as part of a national competition. During the flight, the pilot encountered declining thermal conditions near the Rossberg, which necessitated an outlanding. Seeking a suitable landing site, the pilot crossed Lake Zug and identified a field near Oberrisch.
While performing a straight-in approach to the elevated landing area at approximately 100 km/h, the pilot observed a dark shadow appearing just meters above the ground. The Elfe-Standard-S/2 P aircraft, registration HB-842, struck a steel wire cable. The impact caused the aircraft to lose forward momentum instantly; the tension of the cable then violently swung the glider sideways and backward, forcing it into a vertical crash. The pilot was able to extricate himself from the wreckage without serious injury.
The investigation
An investigation by the Swiss Transportation Safety Investigation Board (SUST) examined the aircraft, the pilot's credentials, and the site conditions. The investigation confirmed that the pilot was highly experienced, with over 700 hours of gliding experience and a history of successful outlandings. The aircraft, a single-seat glider, was found to be in good condition and properly registered, though it was noted to be slightly over its maximum weight limit due to the pilot's gear.
The investigation focused on the physical properties of the obstacle. The cable, an 8mm diameter steel wire used for a telephone line, was stretched between two steel masts approximately 500 to 600 meters in length. The investigation established that the cable was heavily oxidized and lacked any markers or supports to make it visible from the air. Furthermore, the cable was not recorded on any aeronautical charts or obstacle maps.
Findings
- The primary cause of the accident was the collision with a nearly invisible steel cable.
- The cable was extremely difficult to detect from the air because its oxidized surface provided no contrast against the landscape, and the background of wooded hills obscured the view.
- The cable was not marked or supported along its span, making it a significant hazard.
- The impact occurred at an altitude of only 5 to 7 meters, leaving the pilot with virtually no opportunity to perform evasive maneuvers.