What happened
On September 5, 2005, at approximately 11:10 UTC, an SA 315 B Lama helicopter was performing transport flights near the Rettenbachferner in the Sölden ski area, Austria. The pilot was tasked with transporting several cubic meters of concrete from the Pitztaler-Jöchl parking area to a construction site at a higher altitude.
During a routine flight at an altitude of approximately 2,800 meters, the helicopter was climbing at 1,000 feet per minute with a speed of 50 to 60 knots. Approximately 30 seconds after picking up the external load, the sling assembly detached from the cargo hook, causing the concrete bucket to fall toward the ground. The sudden loss of weight caused the helicopter to surge upward.
The falling load struck the cables of the "Schwarze-Schneid-Bahn I" cable car system. The impact caused the carrier cable to derail from its pulleys, leading to the immediate stoppage of the system. The impact also struck a descending cabin, tearing it from the cable and dropping it approximately 10 meters onto a debris field. This incident resulted in 3 fatalities and 2 injuries.
Furthermore, the violent vibrations caused by the impact threw six children from another cabin. They fell from a height of approximately 40 meters, and all 6 children died at the scene. Two other passengers in the same cabin sustained minor injuries, while one person was thrown from a third cabin, sustaining serious injuries. In total, the accident caused 9 fatalities and 5 serious injuries.
The investigation
The investigation examined the helicopter's external load hook system and the electrical release mechanism. Investigators analyzed the microswitch located within the cyclic stick's trigger. Technical experts from the TU Wien and other specialized institutes performed forensic analyses on the switch components. The investigation also reviewed the maintenance history of the SA 315 B Lama and the mechanical integrity of the cable car system.
Findings
- The investigation established that the external load was released unintentionally.
- Analysis of the microswitch in the cyclic stick revealed the presence of metallic shavings and molten droplets.
- The microswitch design featured a high risk of creating conductive silver debris due to its specific contact arrangement and internal play.
- Conductive silver shavings were able to bridge the electrical gap between contacts, creating a path for the 200 mA current required to trigger the release relay.
- The pilot was unable to manually reach the release switch during the incident because the cyclic stick was held in a position that blocked access to the trigger.