What happened
On June 2, 1966, a Pilatus-Porter PC-6, registration HB-FAO, was conducting an official inspection flight of mountain and glacier airstrips. The flight, operated by the Federal Air Office, included the pilot and two passengers. After successfully completing two landings on the Zanfleuron Glacier at the Sanetsch Pass earlier that morning, the aircraft returned to the glacier for a third approach at approximately 12:07 PM.
During the final approach, the pilot attempted to use tracks from previous landings as visual references. However, due to hazy atmospheric conditions and a lack of contrast on the fresh snow, the pilot lost sight of these landmarks. As the aircraft approached the ground, the pilot initiated a pull-up maneuver and increased engine power to avoid impact. This maneuver was performed too high and too early, causing the aircraft to lose lift and drop several meters. The aircraft struck the glacier surface with significant sink rate, causing the left wing and landing gear to deform and the propeller to be torn from the engine.
The investigation
The investigation examined the aircraft's mechanical condition, the environmental conditions, and the pilot's flight path. The Pilatus-Porter PC-6 was found to be in good mechanical standing, with no relevant defects identified. The investigation also analyzed the terrain, noting that while the Zanfleuron Glacier is a recognized landing site, the specific approach path involved significant changes in terrain gradient.
Findings
- The primary cause of the accident was that the pilot pulled the aircraft up too high during a glacier landing under unfavorable visibility and lighting conditions, leading to a sudden drop in altitude.
- Atmospheric haze and a high sun angle contributed to a loss of depth perception and visual references.
- The lack of shadows and the uniform white surface of the new snow created a near-total loss of contrast, making it difficult to judge altitude.
- The specific approach path used was particularly difficult because the terrain gradient obscured the previously established landing tracks.