What happened
On September 7, 2004, an Eurocopter AS 350B3, registration HB-ZCB, was performing external load operations at a mountain construction site near the Cabane de Brunet in the Valais region of Switzerland. The aircraft was equipped with an 18-meter long lifting sling. Following a landing at the site, the pilot initiated a takeoff, flying eastward across the plateau.
During the initial climb, the load hook struck the terrain. This impact caused the lifting sling to be thrown into the tail rotor, leading to significant damage to the rotor blades. The impact of the sling on the tail rotor caused the helicopter to lose control and begin spinning around its vertical axis. The aircraft subsequently crashed into the Dyure de Sery gorge, resulting in the fatal death of the pilot.
The investigation
Investigators examined the wreckage in the riverbed of the Dyure de Sery and analyzed the components of the tail rotor and the lifting sling. Physical evidence, including paint transfers from the tail rotor blades found on the sling and marks on the tail rotor gearbox, confirmed that the hook had struck the ground and the sling had subsequently struck the tail rotor.
Technical inspections of the Eurocopter AS 350B3 revealed no pre-existing mechanical failures that contributed to the accident. The investigation also focused on the pilot's operational status, noting that the pilot had completed 192 rotations that day and had been on duty for approximately 11 hours. Furthermore, toxicological analysis of the pilot's blood and urine revealed the presence of Citalopram, an antidepressant, which the pilot had been taking for two years without informing the aviation medical examiner.
Findings
- The primary cause of the accident was the load hook colliding with the ground immediately after takeoff, which sent the sling into the tail rotor.
- The pilot was experiencing significant fatigue following an exceptionally long workday, having exceeded the daily limit of 160 rotations.
- The pilot's decision-making and perception may have been impaired by the regular use of a psychotropic medication (Cital_opram), which, according to medical experts, renders a pilot unfit for flight.
- The pilot was likely under time pressure to complete the remaining scheduled tasks for the day.