What happened
On 1 October 2019, an Aerialtronics Altura Zenith ATX8 unmanned aerial system (UAS) was being operated for aerial work near a road bridge in Stoke Gifford, Gloucestershire. The flight was intended to take place over two railway tracks. Following successful pre-flight checks, the aircraft took off in GPS mode.
As the UAS reached an altitude of approximately 5 metres, it began drifting westward. Upon reaching 10 metres, the aircraft suddenly accelerated in the same direction and began a rapid descent. Despite the pilot's attempts to provide corrective inputs, the aircraft moved out of visual range and crashed into vegetation adjacent to the railway tracks. There were no injuries or fatalities during the incident, though the airframe and gimbal were destroyed beyond repair.
The investigation
The investigation involved a review of the flight log data by the manufacturer and a site inspection by the operator. Analysis of the flight logs confirmed that the aircraft was operating in GPS mode and that the magnetic compass values experienced fluctuations of approximately 60° within seconds of takeoff. The logs showed that the compass stabilized to the correct heading at the exact moment the aircraft accelerated and lost altitude.
During the site inspection, the operator performed a radio frequency spectrum analysis, which revealed no issues. However, testing with a handheld compass identified significant magnetic interference. Specifically, magnetic deviations as high as 140° were detected in certain areas of the bridge, which was situated above high-voltage overhead wires for the railway tracks.
Findings
- The primary cause of the loss of control was magnetic interference from high-voltage cables ducted beneath the bridge.
- The sudden magnetic fluctuations caused the GPS-stabilized flight controller to react erratically, leading to the westward acceleration and descent.
- The flight lasted only 13 seconds and covered a distance of roughly 100 metres.