What happened
On the evening of July 11, 1965, a Mucha 100-A glider, registration HB-642, was conducting a training flight near Kägiswil. The pilot was performing ridge soaring along the northwest slope of the Arnigrat ridge. At approximately 18:45, while flying at an altitude of roughly 1,000 meters, the aircraft struck a private hay transport rope that stretched between Dienenchlawen and Berggut Hell. The impact caused an object to be thrown from the aircraft, followed by a sudden left-hand roll and a crash into the rocky terrain of the Steckenmattgraben ravine. The pilot sustained fatal injuries, and the aircraft was destroyed.
The investigation
The investigation examined the flight conditions, the visibility of the obstacle, and the availability of aeronautical information. Investigators noted that the hay rope, which had a diameter of approximately 15 mm, spanned about 1,750 meters without intermediate supports. While the rope had been recently added to the official aeronautical obstacle map following a report from military authorities, the updated map had only been released on July 1, 1965. The investigation also looked into the pilot's flight experience and the operational procedures at the Kägiswil airfield regarding the dissemination of obstacle information.
Findings
- The pilot entered an obstacle-prone zone at a low altitude relative to the terrain due to diminishing updrafts near the ridge.
- The hay rope was extremely difficult to detect, even in good visibility, and the sun's position may have further obscured the obstacle.
- The aircraft's left wing sustained significant damage from the wire, which reduced lift and caused the subsequent roll and crash.
- Although the updated obstacle information was available at the airfield, it had not been specifically posted or highlighted to the pilots.
Safety action
While the pilot is responsible for reviewing the latest aeronautical information before flight, the investigation noted that it would be beneficial for airfield managers to officially post aeronautical obstacle maps or local notices to ensure all pilots are aware of newly identified hazards.