Accident Summary
On 16 May 2018, at about 10:08 local time, an Airbus Helicopters AS 350 B3, registered as HB-ZKF, operated by Air Zermatt AG, sustained a collision with a high-voltage power line during a hover operation to fill a water bucket on Steineji See, approximately 600 meters northeast of Heliport Raron (LSER) in the canton of Valais, Switzerland. The pilot, the sole occupant, was seriously injured and the helicopter was destroyed.
Aircraft and Pilot Information
The helicopter was a single-engine AS 350 B3 owned and operated by Air Zermatt AG. The pilot, a Swiss national born in 1982, held a commercial pilot license (CPL(H)) issued by the Federal Office of Civil Aviation (FOCA). He had accumulated approximately 2,860 total flight hours, of which about 2,240 hours were on the AS 350 B3. His flight time in the last 90 days was 103 hours total and 94 hours on the accident type.
Accident Narrative
The pilot had conducted two transport flights earlier that morning totaling about 1.5 hours. After a half-hour break, he was tasked with testing a modified firefighting bucket (a custom design by the company) that had been previously tested in November 2017. The bucket was attached to the helicopter's cargo hook via an 8-meter long line.
The pilot departed alone from Heliport Raron at approximately 10:00 for the Steineji See, a gravel pit lake, to fill the bucket with water. He chose a location in the northwestern part of the lake, which he had used before. The wind was from the west, and he used two trees on the western shore as visual references. He was aware of a high-voltage power line running along the northern shore but considered it sufficiently distant.
The pilot performed two or three successful fill-and-empty cycles, monitoring the bucket via a mirror under the helicopter. On the next attempt, the bucket did not submerge properly. The pilot leaned to the right to look vertically through the bubble window and attempted to submerge the bucket by lateral and vertical movements. After the bucket filled, he closed the mechanism and increased power to lift it. As he returned to his normal position, he perceived a bright flash and a bang. The main rotor blades had struck the lowest wire of the power line, severing it. The helicopter vibrated violently, yawed 90 degrees to the right, and tilted. The pilot, who was lifted from his seat and struck his head on the cabin roof, let go of the controls and braced himself. Shortly after, the helicopter impacted the lake surface and came to rest on its left side, partially submerged.
The pilot freed himself, reached the shore, and alerted rescue services. He was hospitalized with serious injuries. The helicopter was destroyed. The severed power line ignited some bushes on the shore, but the fire was quickly extinguished by the fire department.
Cause
The accident was caused by the pilot not noticing that the helicopter was drifting sideways during a hover conducted using visual references on the water surface, which led to a collision with a high-voltage power line during a bucket filling mission. The decision to forego a person who could monitor the maneuver on the ground prevented the pilot from being warned of the drift.