Accident Sequence
On 20 July 2024 at 1347, an Airbus Helicopters AS 350 BA helicopter, registered ZK-HJM, departed from Queenstown Aerodrome for a non-revenue repositioning flight to Franz Josef. The helicopter was operated by Amuri Helicopters Limited and had recently undergone maintenance by Salus Aviation (AW) Limited. On board were two people: a pilot and a passenger, who was also the operator's chief pilot.
After approximately 52 minutes of flight, the pilot felt a jolt and noticed the main rotor blade tips were no longer following the same path. About seven minutes later, during a descent for a precautionary landing near the Paringa River mouth, the cyclic control became significantly harder to push in one direction. Despite this, the helicopter remained controllable.
As the helicopter descended to land at about 10 to 20 feet above the ground, it experienced an unintentional roll to the left and collided with terrain. The helicopter came to rest on its left side, facing about 180 degrees from its original direction of travel.
The helicopter's emergency locator transmitter activated automatically, alerting the Rescue Coordination Centre, which dispatched a rescue helicopter. Both occupants were transported to hospital with minor injuries. The helicopter suffered significant damage.
Maintenance Issues
Prior to the accident flight, during a maintenance check that included a 150-hour airframe and 300-hour engine check, the left and right main rotor hydraulic servo actuators were swapped over at the operator's request to balance wear. The duplicate safety inspection, required to verify critical maintenance tasks, was certified as complete by trained personnel.
However, it is virtually certain that the bolt securing the lower end of the left-hand servo became detached during flight, leading to a change in control characteristics. It is very likely that the engineers performing the duplicate safety inspection were distracted and did not adequately verify the swap. Additionally, it is likely that the engineer who certified the second part of the inspection was experiencing cumulative fatigue, which contributed to the error.
After the accident, the missing bolt was found on the helicopter's transmission deck, but the nut was not located.
Aftermath
The pilot and passenger inspected the helicopter post-accident and discovered the missing bolt at the lower attachment point of the left servo. The rescue helicopter arrived about 1600 and transported the occupants to Grey Hospital in Greymouth with minor injuries.