What happened
On July 5, 1974, a Hughes 269 C helicopter, registration HB-XDU, was engaged in a series of transport flights near Mollis, Switzerland. The pilot was conducting these flights as part of advanced training for a commercial pilot license. After completing several cargo deliveries to a high-altitude site, the pilot transported a passenger to an alpine hut located near Sienen ob Schwändi.
To reach the destination, the pilot landed on a mound of stones used for avalanche protection, which extended toward the roof of the hut. Following the initial landing, the pilot returned to Schwänd and then flew back to the same location to pick up a second passenger. During the second landing attempt on the rocky surface, the pilot performed maneuvers to verify the stability of the ground. After reducing engine power, the helicopter suddenly tilted sharply to the left. The rotor blades struck the roof of the hut, causing the fuselage to rotate and tip onto the structure. The incident resulted in one injury to the pilot and significant damage to the aircraft, while the hut's roof sustained minor damage.
The investigation
The investigation examined the pilot's qualifications, the aircraft's airworthiness, and the environmental conditions at the time of the accident. The pilot was found to be fully licensed and medically fit, with no evidence of impairment. The Hughes 269 C was confirmed to be in an airworthy condition, with the weight and center of gravity within permissible limits. Weather conditions were favorable, with high visibility and calm winds.
Investigators also analyzed the damage to the landing gear. While there was initial speculation that a structural failure of the left fiberglass skid caused the instability, the examination revealed that the breakage was caused by downward bending forces that occurred after the helicopter had already overturned and rotated.
Findings
- The primary cause of the accident was the use of an objectively unsuitable landing site.
- It is highly probable that the movement or sliding of one or more stones under the landing gear disrupted the helicopter's equilibrium.
- The damage to the landing gear was a consequence of the impact and rotation during the capsize, rather than a contributing factor to the initial loss of stability.