What happened
On 17 September 2022, an AS350 helicopter, registration D-HKMB, was performing a specialized geophysical survey near Auchmacoy, Aberdeenshire. The mission involved flying closely spaced parallel tracks at a low altitude to allow a large, 780 kg underslung antenna array to collect ground data. The antenna was suspended 50 m beneath the aircraft, with the helicopter intended to fly between 280 and 300 ft above ground level.
During a maneuver to transition between survey tracks, the pilot reduced airspeed to minimize the turning radius and avoid nearby buildings. As the aircraft slowed, the reduction in translational lift increased the power required to maintain altitude. During this turn, the helicopter descended, causing the underslam antenna to strike an 8 m tall electricity supply pylon. The impact severed the power cables, resulting in a loss of electricity to approximately 1,682 properties. The pilot was unaware that the collision had occurred and continued the flight until the mission was completed.
The investigation
The investigation examined the operational procedures, the risk assessment provided by the operator, and the regulatory permissions granted by the CAA. The operator had received specific permission to fly closer to structures than standard regulations usually allow, provided certain safety mitigations were in place.
Investigators reviewed the aircraft's performance capabilities, noting that at the low speeds and altitudes used during the survey, the helicopter was operating within the 'avoidance zone' of its Height/Velocity diagram. This meant that an engine failure during the maneuver would have left the crew with insufficient performance to achieve a safe landing. Furthermore, the investigation looked into the pre-flight planning and the availability of obstacle data to the pilot.