What happened
On June 30, 1976, an SA 315 B Lama helicopter, registration HB-XEY, was performing commercial transport duties in the Verzasca Valley, Switzerland. The aircraft, operated by Eliticino SA, was tasked with moving construction materials between Lavertezzo and the Agro hamlet. After successfully completing an initial flight with timber, the pilot attempted a second transport involving a heavy load of cement, gypsum, and sand, weighing approximately 800 kg.
Due to the increased weight, the pilot adjusted the flight path to utilize thermal currents from a sunlit slope to maintain sufficient altitude for the approach. During the final descent, the aircraft suddenly plummeted. Witnesses reported hearing machine-gun-like noises and observing the main rotor blades bending upward before the helicopter struck the ground. The impact resulted in 1 fatality (the pilot) and the total destruction of the aircraft.
The investigation
Investigators examined the wreckage distribution and the mechanical condition of the engine. While the engine showed signs of overheating and micro-cracks on the turbine wheels, experts ruled out lightning strikes or static discharge as causes. Forensic analysis of the wreckage identified distinct abrasion marks on the mirror bracket assembly. These marks were consistent with contact by a cable, a finding supported by experimental testing using a sample of the transport cable found at the site.
Findings
- The primary cause of the accident was the late detection of a transport cable crossing the valley.
- The cable, which spanned from Arai to a nearby peak, was not registered with the Federal Office of Civil Aviation and was not marked on official aeronautical charts.
- Although the pilot was aware of the presence of cables in the region, the specific cable was likely invisible against the dark background of the valley during the approach.
- The aircraft was flying significantly heavier than the previous load, which necessitated a wider flight path to gain altitude.
- It is suspected that a sudden reaction to seeing the cable may have caused an engine compressor stall, explaining the unusual noises heard by witnesses.
Safety action
- It is essential that during transport operations in areas known for overhead obstructions, flight paths are thoroughly pre-scouted and all known cables are identified.