What happened
On June 6, 1984, an SA 315B helicopter registered as HB-XLT, operated by Air Glaciers SA, was engaged in a commercial agricultural mission near Fully, Valais. The pilot departed Sion Airport early that morning to treat vineyards in the area. After completing initial low-altitude sulfating work, the pilot decided to make another pass at a higher altitude to utilize remaining chemical product.
While approaching the end of the vineyard row and preparing to turn back toward Martigny, the helicopter collided with a telephone line. The impact shattered the cockpit's acrylic canopy. The pilot immediately initiated an autorotation while attempting to steer toward suitable emergency landing terrain. During this maneuver, he noticed a power line intersecting his flight path but chose to maintain turbine power to clear it. Shortly after passing over the power line, as he began braking maneuvers, he lost all control of the aircraft. He pulled the collective pitch lever fully back at the last moment to cushion the impact. The helicopter struck the ground on its left side and was destroyed. The pilot sustained fatal injuries.
The investigation
The Swiss Transportation Safety Investigation Board (SUST) examined the flight record, the wreckage, and the pilot's qualifications. The pilot held a valid professional license and had logged 3,226 hours of total flight time, including 2,140 hours in the relevant helicopter type within the preceding 90 days. No evidence suggested the pilot was unfit for duty due to health issues.
The aircraft possessed valid airworthiness certificates. Examination of the wreckage revealed no mechanical defects that contributed to the accident. However, forensic analysis showed that one end of the severed telephone cable had wrapped around the main rotor hub. As the rotor continued to spin, the cable tightened, eventually snapping one of the two cyclic control levers just before impact. This mechanical failure explained the total loss of control.
The pilot was wearing a protective helmet but lacked shoulder harnesses. The telephone line in question was marked on Swiss aeronautical obstacle charts (1:300,000 edition January 1984 and 1:100,000 national maps) and had been highlighted in Class C NOTAMs (Nos. 1018-1022/74) issued by the Federal Office of Air to warn pilots of the obstacle.
Findings
The primary cause of the accident was the pilot's loss of control following the collision with the telephone line, which mechanically jammed the cyclic controls.
Contributing factors included:
- Insufficient pre-flight preparation: The pilot admitted to not consulting aeronautical obstacle charts or NOTAMs before beginning the spraying mission. Although he had conducted a ground reconnaissance days earlier and noted a cable carrier and telephone line, he failed to account for the telephone line during the high-altitude pass.
- Fatigue and attention lapse: Agricultural spraying requires intense concentration. The report notes that as such demanding work concludes, fatigue can lead to a relaxation of attention, resulting in critical oversights like forgetting known obstacles.
Safety action
The investigation highlighted the necessity for pilots conducting low-altitude agricultural operations to rigorously consult all available obstacle charts and NOTAMs prior to flight. Proper preparation significantly reduces the risk of overlooking hazards that may not be visible from the ground or during initial reconnaissance passes.