What happened
On August 24, 1988, at approximately 08:00 local time, an SA 315B (Lama) helicopter registered as HB-XDL was engaged in a commercial operation within the Kleines Melchtal valley in Switzerland. The pilot had been tasked with locating and removing the carcass of a cow from a remote alpine pasture inaccessible to ground vehicles. Prior to the flight, the pilot had plotted the animal's location on a 1:50,000 scale map, noting that a wire lay approximately 500 meters north of the target site.
Although the pilot was familiar with the terrain from previous flights, a technical issue with an earlier aircraft forced a switch to HB-XDL. Crucially, while this replacement helicopter carried standard obstacle charts, it did not include the specific map with the animal's marked location. The crew departed after dropping off a forest worker and proceeded into the valley. Flying at low altitude and reduced speed along the slope toward the Talalp basin, the pilot and flight assistant searched for the carcass. During a second pass in the southern sector of the valley, the helicopter struck an unseen wire. The impact severed two control rods, rendering the aircraft uncontrollable. It fell vertically approximately 20 meters before crashing onto its skids, resulting in total destruction.
The investigation
The Swiss Transportation Safety Investigation Board (SUST) examined the physical evidence and witness accounts to reconstruct the sequence of events. Investigators found a rusted, six-strand hay wire, roughly 13 mm thick, lying near the wreckage. This wire had been cut by the main rotor blades and had severed two control rods, confirming the loss of control mechanism. The wire was recorded on a 1:100,000 obstacle chart but was not explicitly marked.
The investigation highlighted several critical safety factors regarding the aircraft configuration and crew preparation. The helicopter was equipped with a "Bubble-window," which provided excellent downward visibility for reference but made it nearly impossible to wear shoulder harnesses. Consequently, neither the pilot nor the flight assistant wore helmets or shoulder harnesses; only pelvic belts were used. The weather in the immediate area was clear, but the specific search zone was in deep shadow, significantly reducing the visibility of the thin, rusted wire against the terrain.
Findings
The primary cause of the accident was the pilot's single-minded concentration on finding the animal during a low-altitude search flight, which led to a failure to maintain adequate situational awareness regarding obstacles. Contributing factors included:
- The inability to wear shoulder harnesses due to the Bubble-window installation, which allowed the pilot to be thrown forward into the instrument panel upon impact.
- The lack of the specific annotated map showing the wire's location during the final flight phase.
- The difficulty in detecting a thin, rusted wire from a moving helicopter, especially when flying in shadowed areas.
The vertical impact from 20 meters caused severe injuries to both occupants. SUST concluded that while the wire was difficult to see, the pilot's focus on the target rather than the immediate flight path was the decisive error.
Safety action
SUST emphasized that the use of Bubble-windows in utility helicopters creates a significant safety paradox: while they aid in precise hovering and ground reference, they physically prevent the use of shoulder restraints. This configuration poses a severe risk of head and chest trauma during hard landings or crashes. The report underscored the necessity of maintaining obstacle awareness even when familiar with an area, particularly when operating without specific annotated charts for the immediate search zone.