What happened
On July 19, 1965, an Agusta Bell 47-J3 helicopter, registration HB-XBX, was performing specialized aerial work near Saas-Almagell, Switzerland. The mission, operated by Heliswiss SA, involved stretching a 4,000-meter nylon rope across several high-voltage power line pylons to facilitate cable installation.
Prior to the operation, the pilot inspected the 8mm diameter rope, which was laid out in a serpentine pattern on the ground. The pilot noted a concern that the specific type of cabled rope being used was prone to twisting and looping, which could snag on obstacles, but the ground supervisor proceeded with the existing plan. After the pilot cleared the area of personnel and prepared for takeoff, the crew began the deployment.
As the helicopter took off, a light tailwind caused the rope to unspool much faster than anticipated. Without the pilot's knowledge, a ground supervisor had recently positioned two workers near the rope to monitor its deployment and untangle any twists. The sudden speed of the unspooling caused the workers to lose control of the line. One worker became entangled in a loop of the rope and was dragged across the ground and through a mountain torrent. The worker was eventually stopped by a low wall 40 meters away, which also caused the rope to snap. The injured worker was rescued by a colleague further downstream.
The investigation
The investigation examined the sequence of events, the equipment used, and the coordination between the flight crew and the ground team. Investigators reviewed the pilot's experience, noting significant flight hours, and evaluated the suitability of the 8mm cabled rope for the task. The inquiry also looked into the actions taken by the ground crew immediately prior to the helicopter's departure.
Findings
- The primary cause of the accident was a lack of foresight by the ground team.
- The ground supervisor's decision to place workers in the deployment zone at the last moment significantly increased the risk of injury.
- The supervisor failed to account for the possibility that the pilot might increase translation speed due to changing wind conditions.
- While the cabled rope was not ideal for this operation due to its tendency to form loops, the specific danger of the rope snagging a person was not a predictable risk for the flight crew.
- The flight crew was not aware of the workers' presence in the immediate path of the rope during the initial takeoff phase.