What happened
On July 8, 1991, a pilot operating an SA 315B Lama helicopter (HB-XXZ) was conducting supply flights in the upper Val Bavona, Ticino. After completing several trips between Sabbione and the Capanna Laghi della Crosa hut at 2,155 meters, the pilot prepared for a return flight to the valley. One passenger, who had accompanied the crew up the mountain, requested to fly down with the pilot.
At approximately 11:00 local time, the helicopter lifted off vertically from the landing pad near the hut. The pilot initiated a left turn onto a heading of roughly 130 degrees while simultaneously pushing the cyclic forward to accelerate toward the valley. This maneuver placed the aircraft in a steep nose-down attitude over Lago della Crosa, a deep alpine lake situated in a depression below the hut.
As the helicopter approached the water surface at an estimated speed of 40–60 knots, the pilot attempted to pull out of the descent by applying aft cyclic and collective pitch. However, he reported that both controls felt blocked or unresponsive. The aircraft struck the water surface with a significant nose-down angle and rolled over almost immediately, sinking within seconds. Both occupants escaped into the 4°C water. While the pilot managed to reach the shore after struggling with hypothermia and exhaustion, he was unable to save the passenger, who could barely swim and succumbed to drowning approximately 50 meters from safety.
The investigation
SUST examined the flight path, wreckage condition, and technical systems of HB-XXZ. The helicopter was recovered days later from a depth of 37 meters. Technical inspections revealed no pre-existing mechanical failures in the control or hydraulic systems; damage to the rotor blades and engine mount was consistent with the impact forces. The pilot held valid licenses and had extensive experience on the type, with no medical issues reported. Weather conditions were ideal, with clear visibility and light winds.
Findings
The accident was caused by an unsuitable departure tactic in mountainous terrain and an unnecessary flight maneuver. The pilot chose a descent path over the lake that required excessive nose-down attitude to clear the terrain on the opposite side, rather than maintaining altitude. This resulted in a loss of control near the water surface. Contributing factors included the extremely cold water temperature (4°C), which rapidly incapacitated both occupants, and the passenger's lack of swimming ability.
Safety action
The report highlights the dangers of unnecessary low-level descents over alpine lakes during mountain operations. No specific regulatory changes were mandated in this summary, but the findings underscore the need for careful flight planning to avoid terrain-induced control limitations.