What happened
On the afternoon of January 31, 1996, an AS 350 B helicopter, registration HB-XOQ, was performing a flight from Sommeri toward Zurich Airport. The aircraft was carrying four occupants, including the pilot and three passengers. While flying over the Holzweidsattel ridge in the municipality of Wattwil, the helicopter struck the treetops of a forested area.
The impact caused the aircraft to lose critical components, including parts of the main and tail rotors, the horizontal stabilizer, and the landing skids. Following the collision, the helicopter plummeted into a valley below and caught fire, resulting in the total destruction of the airframe. All four occupants sustained fatal injuries.
The investigation
Investigators reconstructed the flight path using eyewitness accounts and physical evidence from the crash site. Witnesses near the ridge observed the helicopter approaching from the direction of Wil, noting a significant reduction in forward speed before it turned southwest. The investigation established that while the weather at the departure and arrival points was clear, the accident site was shrouded in dense fog with horizontal visibility of less than 50 meters.
Technical examinations of the wreckage, though limited by the intensity of the post-crash fire, revealed no evidence of mechanical failure or engine malfunction. The investigation also confirmed that the aircraft's weight and balance were within permissible limits and that the pilot's flight and rest hours had been properly maintained.
Findings
- The pilot continued to operate under Visual Flight Rules (VFR) despite encountering Instrument Meteorological Conditions (IMC).
- The flight involved an inadequate flight tactic, as the pilot navigated through dense fog with extremely low visibility.
- The aircraft struck the tops of fir trees at an altitude of only 3 to 5 meters above the ridge crest.
- While the aircraft was well-equipped with advanced instrumentation, including an autopilot and radar altimeter, the pilot chose to fly at a low altitude through terrain with minimal visibility.