What happened
On 16 November 2020, an Airbus Helicopters AS 350 B3, registration LN-OAX, was engaged in a utility mission in Songesand, Sandnes municipality. Operated by Heli-Team AS, the aircraft was performing a sling load operation to deliver concrete for a new power transmission line pylon foundation.
During the delivery of the concrete bucket, the pilot attempted an altitude correction. This maneuver may have been prompted by a wind gust or contact between the bucket and the mast foundation. Immediately following this correction, the aircraft was struck by intense vertical oscillations. The pilot attempted to disconnect the suspended load and maneuver the aircraft away from the site, but the onset of the instability was too rapid and occurred at too low an altitude to prevent the crash. The pilot was able to evacuate the aircraft unassisted following the impact. There were no injuries.
The investigation
The Norwegian Safety Investigation Authority (NSIA) examined the wreckage and the flight history of the aircraft. The investigation included technical reviews of the helicopter's systems in collaboration with the manufacturer, Airbus Helicopters, and the French investigation authority, BEA. Investigators focused on the flight dynamics and the specific mechanical configuration of the aircraft's hydraulic systems during the period of the accident.
Findings
- The primary cause of the accident was a phenomenon known as rotorcraft-pilot coupling (RPC), which triggered severe vertical oscillations.
- The aircraft's servo configuration, characteristic of AS 300 series models manufactured before 2006, contributed to an increased sensitivity to these oscillations.
- The pilot had insufficient time and altitude to recover control once the oscillations began.
- Environmental factors, such as wind gusts or the physical interaction of the load with the foundation, likely initiated the sequence of events leading to the instability.