What happened
On March 11, 2019, an Airbus AS 350 B3 helicopter, registration HB-ZCZ, was performing a commercial mission to transport three workers to the Tseuzier Dam. The flight originated from Sion Airport under Visual Flight Rules (VFR), but deteriorating weather conditions, including light snow and low cloud bases, forced the pilot to alter the planned approach.
Instead of the usual approach from the right bank, the pilot attempted an approach via the left bank. Relying on prior knowledge of the area, the pilot intended to fly over a known power line before final approach along the dam wall. However, during the approach, the main rotor blade struck an electrical line. The pilot noted no immediate engine vibrations or parameter exceedances and proceeded to land the aircraft at the intended site. Upon returning to base after a brief period of idling, a physical inspection revealed damage to the leading edge of a main rotor blade.
The investigation
SUST examined the accuracy of aeronautical obstacle databases and the effectiveness of onboard warning systems. The investigation established that the power line's recorded position in the Swiss obstacle database was offset by approximately 20 to 50 meters from its actual location. Furthermore, the line was nearly invisible to the naked eye due to a lack of marking or lighting.
The investigation also reviewed the aircraft's equipment. While the HB-ZCZ was equipped with a Flarm obstacle alert system, the acoustic warning was only audible within the cabin and could not be heard through the pilot's headset due to high ambient noise levels. Additionally, the pilot had not performed a reconnaissance flight to visually confirm the obstacle's exact position and height before committing to the approach.
Findings
- The pilot relied too heavily on inaccurate obstacle database information, leading to a misjudgment of the power line's precise location and altitude.
- The Flarm obstacle warning system failed to serve as an effective safety net because the alert was not audible to the pilot through the headset.
- Reduced visibility and the lack of a preliminary reconnaissance flight increased the risk of the collision.
- The electrical line was difficult to detect visually because it lacked any specific markings or lighting.