What happened
On April 20, 1974, during a national glider team training camp in Schänis, a Standard Libelle glider, registration HB-950, was participating in a long-distance triangular flight. The flight path covered approximately 248 km, passing through Schänis, Sonthofen, and Klosters. While the initial stages of the flight proceeded normally, the pilot encountered significant descending air currents over the western portion of Lake Walen, with sink rates reaching up to 6 m/s.
Due to this rapid loss of altitude, the pilot was unable to reach the intended destination at Schänis and decided to perform an outlanding near Süstli. During the final approach, the pilot was blinded by the low position of the sun and failed to detect a 16 kV high-voltage line. The glider's left wing struck the grounding cable of the power line, causing the aircraft to rotate 46 degrees to the left. The aircraft then descended further, with the right wing striking a parallel 380-volt line. The impact caused the left wing to break near the aileron and the aircraft to come to rest at a 70-degree bank angle. There were no injuries to the pilot, though the aircraft sustained heavy damage.
The investigation
The investigation focused on the pilot's flight conditions, the aircraft's airworthiness, and the environmental hazards at the landing site. Investigators confirmed that the pilot was highly experienced, with 3,000 total flight hours, 1,100 of which were in the Standard Libelle model. The aircraft was found to be in good technical condition, with its weight and balance within legal limits.
Investigators also examined the landing site, noting that the area is intersected by several electrical lines. Crucially, the 16 kV high-voltage line involved in the collision was neither marked nor included in aeronautical charts. The investigation also reviewed meteorological data, confirming the presence of strong downdrafts in the area and low sun elevation at the time of the accident.
Findings
- The pilot had planned the approach with an altitude reserve that would have been sufficient under normal conditions.
- Strong descending air currents were present, as confirmed by other pilots flying in the same area.
- The 16 kV power line was an unmarked hazard and was not documented in obstacle charts.
- Solar glare prevented the pilot from identifying the power lines during the approach.
- The chosen landing field was suitable for an experienced pilot, provided the electrical hazards had been visible.