What happened
On September 5, 1996, an AS350 B2 helicopter, registration HB-XXV, was performing a series of livestock transport missions in the Bernese Oberland. The crew, consisting of a pilot, a doctor, and a paramedic, departed from Gsteigwoliwer to transport cattle from difficult terrain to accessible roads.
During the flight, the pilot communicated with another helicopter regarding significant solar glare caused by the rising sun behind the mountains. While maneuvering near the Sägistalsee area, the aircraft entered a left-hand turn and began a descent. Witnesses observed the helicopter descending rapidly toward the terrain. The aircraft's lower fuselage and tail rotor struck the rising slope, causing the helicopter to be catapulted across the landscape before the main structure impacted the roof of a barn. There were 3 fatalities among the crew.
The investigation
An investigation by the Swiss Transportation Safety Investigation Board (SUST) examined the aircraft's mechanical condition, the pilot's medical history, and the environmental conditions at the time of the accident. Technical inspections of the engine, hydraulic systems, and control linkages revealed no pre-existing mechanical failures. The engine was confirmed to be operational at the moment of impact.
Reconstruction flights were conducted under identical lighting conditions to assess visibility. These flights demonstrated that when flying on a course of 135 degrees, the pilot would be looking directly into the rising sun. The investigation found that while the terrain in the sunlit areas was visible, the areas in shadow appeared as a "black hole," making it nearly impossible to discern the ground's contours through the instrument panel.
Findings
- The pilot was flying a functional aircraft but was unable to detect the approaching terrain due to extreme solar glare and the high contrast between bright sunlight and deep shadows.
- The pilot was not wearing sunglasses, a cap, or any other form of eye protection to mitigate the sun's intensity.
- The crew's visibility was further compromised because the front left seat had been removed, forcing the doctor and paramedic to sit in the rear, which limited their ability to monitor the terrain.
- Although the pilot had flown in the area two days prior, the tactical decision to descend into the valley while facing the sun significantly increased the risk of controlled flight into terrain.