What happened
On June 5, 1983, an experienced pilot departed Buttwil airfield at 14:27 local time in a Standard-Libelle 201 B glider, registration HB-1542, for a training flight under favorable soaring conditions. After releasing from the aerotow at approximately 1,300 meters above mean sea level (MSL), the aircraft operated in the airspace surrounding Mount Rigi.
Around 15:50, while flying horizontally at an altitude of 1,725 m MSL, HB-1542 collided with the 18 mm steel support cable of a freight cable car. The impact occurred roughly 250 meters southeast of the Rigi-Kulm summit and 29.80 meters below the mountain station. The collision caused the glider to tumble vertically down a steep, rocky cliff face, traversing approximately 110 meters in altitude before coming to rest upside down on a snowfield.
The investigation
The Swiss Transportation Safety Investigation Board (SUST) determined that the pilot held a valid glider license and was medically fit for flight. The aircraft was airworthy, with weight and balance within limits, and no mechanical failures were found in the wreckage. Weather conditions included good visibility and thermal activity, but the north-eastern flank of Rigi was in shadow while the glider remained in sunlight.
Investigative findings highlighted that the cable car wires were unmarked and difficult to see against the dark rock face due to unfavorable lighting conditions. Additionally, the glider’s downward blind spot likely hindered visibility of obstacles below the horizon. Witnesses reported other air traffic in the vicinity, including another glider and hang gliders, suggesting potential distraction.
Findings
The primary cause of the accident was the pilot's failure to maintain the required minimum altitude during ridge soaring, leading to a collision with the unmarked cable car wire. Contributing factors included:
- Poor visibility: The contrast between the dark cables and the shadowed rock face made detection difficult.
- Visual obstruction: The glider’s design created a significant blind spot directly below the aircraft.
- Airspace congestion: Potential distraction from monitoring other local air traffic may have diverted attention from terrain awareness.
Safety action
The investigation confirmed that the cable car had been declared safe for aviation in 1974, though it remained unmarked on civil obstacle charts. The accident underscored the critical importance of maintaining minimum safe altitudes above obstacles in complex terrain, regardless of weather conditions or pilot experience.