What happened
On April 7, 1960, a Bell 47-G helicopter, registration HB-XAK, was deployed to assist in a rescue operation following an avalanche near the Wissigstock, above Engelberg. The mission was initiated after a skier was buried in an avalanche at approximately 2,800 meters.
Upon arriving at the area, the pilot encountered extremely difficult flying conditions. The sun had already set behind the mountains, leaving the terrain in very diffuse and milky lighting, which made depth perception and terrain assessment difficult. Due to the high altitude, the aircraft lacked the performance required for a hover, forcing the pilot to attempt a landing via a gliding approach.
While performing a low-altitude pass parallel to the slope to identify a suitable landing spot, the right skid made contact with the snow. Because the aircraft had no remaining power reserves to compensate for the sudden deceleration, the helicopter settled onto the moderately sloped ground. The momentum caused the aircraft to tip downhill; the rotating rotor blades struck the ground and broke, and the helicopter eventually came to rest on its side with the cockpit canopy shattered.
The investigation
The investigation established that the mission was prompted by an initial report that suggested the accident site was at 1,500 meters, whereas the actual elevation was approximately 2,800 meters. The pilot proceeded with the mission despite the altitude discrepancy because no other suitable aircraft were available for the evening.
Technical analysis of the aircraft's performance at the time of the accident—at an altitude of approximately 2,700 meters—revealed that the helicopter's ceiling for hovering with ground effect was only about 2,300 meters, and even lower without ground effect. This meant the aircraft was operating well beyond its functional limits for stable hovering.
Findings
- The primary cause of the accident was the unintentional contact of the right skid with the terrain during a landing attempt under poor lighting conditions.
- The pilot was operating the aircraft at an altitude significantly exceeding the maximum ceiling for stable hovering.
- The lack of power reserves prevented the pilot from recovering the aircraft once the snow contact caused deceleration.
- The steepness of the slope caused the aircraft to capsize downhill once the landing was interrupted.
- Poor visibility due to the lack of sunlight and diffuse lighting contributed to the difficulty in assessing the landing site.