What happened
Between May 21 and May 22, 1963, a Sikorsky S-58 helicopter, registration F-OBON, conducted three separate attempts to recover two aircraft stranded at the Clariden Pass. The first two aircraft involved were a Champion 7 GCB HB-UAM and a Piper Super Cub PA-18 HB-ORP, both of which had previously been involved in separate accidents at the pass.
During the first attempt on May 21, the crew attempted to lift the HB-ORP. As the load was being taken up, the aircraft became snagged, causing the pilot to lose control of the load system. To stabilize the helicopter, the pilot was forced to jettison the load, which slid down a slope.
On May 22, the second attempt focused on the HB-UAM. This operation was initially successful, and the aircraft was transported to Mollis airfield. However, during the release of the load, the pilot misjudged the altitude, causing the aircraft to drop from a short height and sustain additional damage.
In a third attempt later that day, the crew returned to the site of the HB-ORP. After the load was secured, the pilot accelerated the helicopter's horizontal speed too aggressively immediately after takeoff. This caused the heavy load to swing violently, forcing the pilot to release the aircraft. The HB-ORP fell several hundred meters into a rocky crevice, resulting in its total destruction.
The investigation
An investigation by the Swiss Transportation Safety Investigation Board examined the technical aspects of the Sikorsky S-58, the meteorological conditions, and the pilot's maneuvers. The inquiry noted that while the helicopter was technically capable of the task and the weather was favorable, the complexity of high-altitude sling load operations requires significant experience.
Findings
- The primary cause of the failed recovery attempts was the insufficient experience of the pilot in performing complex mountain rescue operations involving large external loads.
- During the first attempt, a "snagging effect" occurred when the load caught on the ground, creating a tipping moment that led to a loss of control.
- During the second attempt, the pilot's misjudgment of altitude during the release phase caused unnecessary damage to the HB-UAM.
- During the third attempt, the excessive horizontal acceleration applied by the pilot immediately after lifting the load induced unstable oscillations in the cargo.