What happened
On May 3, 1970, a Pilatus Porter, registration HB-FDU, was conducting a non-scheduled air taxi flight for Air Glaciers S.A. The aircraft departed from Sion carrying seven passengers toward the Tsanfleuron Glacier.
As the pilot approached the glacier for a landing on a heading of 240°, the aircraft experienced intense vibrations. During this period, the pilot observed fragments of the propeller detaching from the unit. The aircraft's nose pitched up momentarily before dropping from a height of approximately 3 meters. Upon impact, the left landing gear leg fractured, causing the aircraft to slide to the left. The aircraft eventually came to a halt approximately 150 meters from the initial contact point. While the turbine was still running when the aircraft stopped, the propeller and its reduction gear were no longer attached.
There were no injuries among the pilot or the seven passengers, though the aircraft sustained severe damage.
The investigation
The investigation examined the mechanical condition of the aircraft and the environmental factors at the time of the accident. The pilot was found to be fully qualified for professional mountain operations, with extensive experience on the aircraft type. The aircraft's airworthiness certificates were valid, and no prior mechanical defects were identified in the flight logs.
Physical evidence from the wreckage indicated that the propeller blades had undergone significant deformation. Specifically, the three blades showed identical patterns of deformation and marks caused by the failure of the control pins within the blade roots. Furthermore, damage found at the engine mounting points and the firewall supported the conclusion that the propeller had made contact with the glacier's snow surface.
Weather conditions at the time of the accident included visibility exceeding 20 km, but the high sun position and snow-covered terrain resulted in low relief contrast. Winds were recorded at approximately 20 knots, with gusts up to 30 knots, causing irregular and occasionally strong turbulence.
Findings
- The primary cause of the accident was a pilot error in distance estimation during the landing phase.
- The error was exacerbated by diffuse lighting conditions, which reduced the visibility of the terrain's contours.
- The propeller strike with the snow surface caused the subsequent structural failure of the landing gear and the disintegration of the propeller assembly.