Bruce McPherson

25kV + lots of wiggly amps = big local magnetic field

A commercial drone fell from the sky after a flight across a railway line threw its internal compass into confusion.

The Aerialtronics Altura Zenith ATX8 craft crashed into bushes next to a railway line in October 2019, according to a recent Air Accidents Investigation Branch report.

The craft's operator started its flight from a road bridge over an electrified railway line. A subsequent investigation by the manufacturer discovered that the quadcopter's onboard magnetic compass reading "had varied through about 60° within a couple of seconds of takeoff". Its full flight lasted just 13 seconds.

Operators reported that when it reached a height of 16ft (5 metres), the drone "started to drift to the west" while still climbing. At about 30ft above ground level, it "suddenly accelerated to the west and began to lose altitude" before crashing out of sight.

Network Rail found the drone in trackside bushes, having been immediately alerted by the operators, who spotted that it came to rest away from the rails themselves.

After takeoff the drone's compass stabilised but the craft continued drifting to the west. The day after the crash, the operator went back and carried out an RF spectrum analysis of the takeoff site as well as using a handheld compass to check for magnetic interference. Deviations of up to 140° were immediately apparent "over localised regions of the bridge below which the railway track's overhead high-voltage wires were being ducted".

Overhead wires for trains carry AC current at 25kV. Current draw runs into the thousands of amps when a train is passing by.

Ian Hudson of drone blog UAVhive told The Register: "A drone shouldn't crash from a compass error. Good software should detect a compass error and switch to manual."

More modern designs, said Hudson, incorporate multiple levels of redundancy in key sensors, hopefully to mitigate the effects of malfunctions. It is unclear whether three similar magnetometers would have been immune to the strong electromagnetic field generated by a passing train.

Hudson also pointed out that Aerialtronics boasts on its website that its Altura Zenith product line is "TÜV tested to guarantee operations [fraught] with electromagnetic interference" and joked: "Clearly not well enough!"

Electromagnetic interference has been blamed for a number of unforeseen problems in electronic devices over the years, including smart meters delivering fictional utility readings to home Wi-Fi interference thanks to unshielded mains wiring. It seems drones flying near power lines is something to add to that list.