What happened
On October 25, 2022, an Airbus Helicopters AS 350B3, registration LN-OGN, was performing specialized commercial operations near Bryne, Norway. Operated by Helitrans AS, the aircraft was engaged in stringing a new 132 kV power line. The mission involved using a heavy tool suspended from the helicopter's cargo hook to guide a pilot line through the crossbeams of newly erected masts.
During the process of hanging the tool on a mast, the helicopter suddenly entered a state of uncontrolled movement. The aircraft's power output fluctuated significantly, and a low hydraulic pressure warning light illuminated. In an attempt to stabilize the aircraft, the pilot released the underslung load. During this period of instability, the helicopter's right skid struck the top of a power line mast, causing damage to both the aircraft and the mast structure. The pilot managed to regain control and performed a safe landing nearby.
The investigation
The Norwegian Safety Investigation Authority (NSIA) examined flight data and video from an onboard recorder. The investigation focused on the sudden fluctuations in engine power and the subsequent loss of control. Investigators also reviewed the pilot's experience, the aircraft's configuration, and the specific operational procedures being used at the time of the incident.
Findings
- The primary cause of the instability was likely Rotorcraft Pilot Coupling (RPC), a complex phenomenon where unintended interactions between the pilot's inputs and the aircraft's movements create sustained oscillations.
- The pilot's experience with the phenomenon was consistent with Pilot-Induced Oscillations (PIO).
- Low friction on the collective control likely contributed to the occurrence of the oscillations, as higher friction can help mitigate such movements.
- The impact with the mast further exacerbated the loss of control.
- The low hydraulic pressure warning was a secondary effect caused by the rapid, large-scale movements of the flight controls during the event.
Safety action
- The NSIA recommends that flight safety forums for onshore helicopter operations provide further attention to the importance of adjusting collective friction.
- The NSIA urges the operator to implement specific procedures for the current method of stringing lines, as the existing manual only covered "needle flying" and did not account for the specific tool used in this incident.