What happened
On May 16, 2018, an AS 350 B3 helicopter, registration HB-ZKF, was conducting a test flight of a newly modified external fire suppression bucket at Steineji See, near Raron, Switzerland. The pilot was testing a self-constructed electric closing mechanism designed to fill and empty the bucket during flight.
After several successful cycles of filling and emptying the bucket using visual references from the shoreline, the pilot encountered difficulty submerging the bucket. To gain a better view, the pilot leaned into the cockpit's bubble window to look vertically downward at the bucket. During this period of vertical reference, the pilot used lateral and vertical movements to force the bucket into the water. As the pilot transitioned back to a normal seated position while lifting the filled bucket, the main rotor blades struck a high-voltage power line located approximately 30 meters to the right of the aircraft's initial position. The impact severed the line and caused the helicopter to become uncontrollable, leading to a crash into the lake. The pilot sustained one serious injury and managed to escape the wreckage.
The investigation
SUST examined the wreckage and the pilot's actions. The investigation confirmed that the aircraft's weight and center of gravity were within limits and that no technical malfunctions were present in the helicopter or the bucket. Investigators noted that the pilot was experienced and familiar with the local area. However, the investigation found that the pilot was not wearing a helmet at the time of the accident, as his helmet was being repaired. The investigation also looked into the operational procedures, noting that while company standards suggested having a ground observer for such operations, the pilot chose to perform the task alone.
Findings
- The primary cause of the accident was that the pilot did not notice the helicopter was drifting sideways while using vertical visual references on the water surface.
- The movement of the water caused by the rotor downwash created waves that likely gave the pilot the illusion that the helicopter was moving laterally, prompting an unconscious corrective input to the right.
- The decision to operate without a ground observer meant there was no one available to monitor the drift and warn the pilot via radio.
- The lack of a helmet likely contributed to the severity of the pilot's head injury sustained during the impact.