Accident Overview
On 6 December 1992, at approximately 0950 local time, an AS 350 B2 helicopter (registration HB-XXL) operated by Heli Bernina AG was involved in an accident at the valley station of the Lagalb cable car in Pontresina, Switzerland. The helicopter was on a commercial transport flight to carry blasting specialists for avalanche control in the Lagalb ski area. During a landing maneuver, the tail rotor collided with two 220 kV power cables, resulting in damage to the helicopter and the power lines. The pilot, the sole occupant, was not injured.
Flight Details
Following a mission to transport specialists for avalanche blasting, the pilot initially selected a landing zone on a snow track of a piste machine approximately 100 m southeast of the valley station. While approaching from the northwest, the pilot noted the slope of the chosen area and decided that a 180° turn would be needed for optimal tail rotor clearance. However, this would have placed the tail rotor toward the buildings and required the specialists to approach from behind, which the pilot considered dangerous. Consequently, the pilot chose a different landing site: a cleared area in front of the garage of the valley station, where two blasting specialists were waiting with explosives. The pilot maintained visual contact with a 220 kV power line running to the left of the helicopter and judged the distance sufficient for landing.
To facilitate boarding through the left-side sliding door, the pilot intended to hover at about 10 m above ground and turn the helicopter 180° to the right. During this turn, a loud bang was heard, immediately followed by an uncommanded left yaw. The pilot recognized a tail rotor failure, lowered the collective pitch lever, and the helicopter made a hard landing after yawing 180°. The engine was shut down using the fire shutoff valve.
Damage and Findings
The tail rotor, including the fin, was torn off and hung loosely on the tail rotor support. The two tail rotor blades had cut through two cables of the 220 kV power line, which were approximately 10 cm apart. The cables, made of an aluminum alloy with 37 strands, had a diameter of 22.54 mm and a breaking strength of 90 kN. The power line, crossing the Bernina road between masts 36 and 37, was marked on the aeronautical obstacle map. The distance from the accident site to the power line was about 16 m. The parking lot and pass road were not cleared to black pavement; fresh snow covered the area. Weather conditions included diffuse lighting due to fresh snow, with visibility over 15 km in cloud-free areas, wind from 300° at 3 knots, and temperature around -8°C. The sun was at an azimuth of 134° and altitude of 6°.
Investigation revealed no pre-existing technical defects with the helicopter. The pilot held the required licenses and had no known health issues. There was no prior coordination between the pilot and the blasting personnel regarding the landing site or post-landing procedures.
Causes
According to the official investigation, the accident was attributed to:
- Lack of prior agreement on the landing site.
- Pilot's misjudgment of the safety distance to the high-voltage line under diffuse visibility conditions.