What happened
On the morning of July 24, 1966, a pilot was conducting unpaid passenger and material transport flights from Mollis to the Planura Hut. After completing an initial flight, the pilot prepared for a second leg, which involved taking two passengers in the rear seat of the two-seat Piper PA-11 HB-OLB.
During the flight, the pilot radioed a colleague to report his position over the Biferten Glacier, noting that the aircraft was struggling to climb. Shortly thereafter, the aircraft disappeared. The wreckage was discovered on July 26, 1966, partially buried by a snow avalanche triggered by the impact. The aircraft had struck a rock wall near the Biferten Gap (Bifertenlücke) at an altitude of approximately 3,000 meters, causing the destruction of the plane and the deaths of all three occupants.
The investigation
Investigators examined the wreckage and the topography of the Bifertenstock. Evidence at the impact site on the western ridge indicated that the aircraft had struck the rock face and slid down a steep glacier slope. The investigation also reviewed the pilot's flight history, noting a previous landing accident in the same aircraft type, and analyzed the aircraft's weight and balance.
While the aircraft was well-maintained and airworthy, the investigation found that the takeoff weight had initially exceeded the maximum allowable limit of 794 kg, though fuel consumption may have brought the weight back into the legal range by the time of the accident. Furthermore, the presence of two passengers in the single rear seat was found to be a violation of the operating instructions.
Findings
- The pilot attempted to cross the mountain pass from the west, which is a disadvantageous direction due to the required approach angle and terrain constraints.
- The pilot utilized an unsuitable tactic by attempting to fly over a mountain notch under unfavorable conditions.
- The pilot reported difficulty in climbing, likely due to a combination of the aircraft's heavy load and localized atmospheric turbulence caused by thermal activity between the sun-warmed and shaded slopes.
- The seating arrangement, with two passengers in the rear seat, was not permitted by the aircraft's operating manual.