What happened
On March 29, 1988, at approximately 11:08 local time, a helicopter operated by Air Zermatt AG was conducting commercial external load operations in St. Niklaus, Valais. The pilot had previously dropped off a flight assistant in St-Nicolas and proceeded to a clearing known as "Ballacker" to retrieve a cargo package that had been left there a week earlier.
Two municipal workers attached a 34-meter-long sling to the load. As the helicopter lifted off and began to turn away from the slope, it collided with a 130 kV high-voltage line running parallel to the valley side, located approximately 70 meters above the loading site. The main rotor blades severed two of the middle conductors of the transmission line. The transport cable became entangled around the lower wires before snapping due to tension.
The collision caused a blade from the main rotor to cut through the drive shaft of the tail rotor gearbox. The helicopter began to spin uncontrollably around its vertical axis and descended rapidly into the village of Ballacker, crashing between houses on a grassy area next to the Mattertalstrasse. The aircraft was destroyed, and the pilot sustained injuries.
The investigation
The Swiss Transportation Safety Investigation Board (SUST) examined the flight path, weather conditions, and mechanical damage. The accident occurred in clear weather with visibility exceeding 20 km. The pilot had flown to the site from below, perpendicular to the slope, after previously visiting the location one week prior under poor visibility conditions where the power lines were obscured by fog.
The investigation confirmed that the helicopter passed between the transmission towers (masts 83 and 84). The main rotor cut the two middle conductors, which had a horizontal distance of 9 meters from the masts. The transport cable wrapped around the lower wires, creating enough friction to brake the helicopter momentarily before breaking. The tail section then contacted the rotor plane, severing the drive shaft.
Findings
The primary cause of the accident was the pilot's one-sided concentration on the external load, which resulted in the failure to notice the high-voltage line positioned above the loading area. Although the pilot was aware that power lines existed in the vicinity, he had underestimated their height relative to his flight path.
Contributing factors included:
- The absence of qualified personnel on the ground to monitor the loading operation or maintain radio contact between the ground crew and the cockpit.
- The pilot's prior experience with the same load under different weather conditions, which may have led to a misjudgment of the obstacle's position.
The helicopter HB-XDI, an SA 315B (Lama), was certified for flight and deemed airworthy prior to the incident. The power line sustained damage to two conductors in the middle tier and deformation in the lower tier. The pilot survived with injuries, but the aircraft was destroyed.
Safety action
The investigation highlighted the critical need for qualified ground personnel during external load operations to assist with situational awareness and communication. No specific regulatory changes were mandated by this report beyond standard operational procedures regarding obstacle clearance and ground crew qualifications.