Magnetic interference caused by overhead railway cables led to the crash of an Aerialtronics Altura Zenith ATX8 in October 2019, according to a recently-released UK Air Accident Investigations (AAIB) Branch report. “Shortly after takeoff for a flight from a road bridge above two railway tracks, the UAS in GPS mode started to drift, accelerate and descend away from the pilot. The pilot was unable to regain control before it crashed into vegetation next to the tracks. A subsequent investigation by the operator found that magnetic deviations of up to 140° were observed over localised regions of the bridge below which the railway track’s overhead high-voltage wires were being ducted.”
“Following pre-flight checks for a flight from a road bridge above two railway tracks, the UAS, in GPS mode, took off normally, but as it reached approximately 5 m above the ground it started to drift to the west. As it reached about 10 m it suddenly accelerated to the west and began to lose altitude. The pilot attempted to correct the drift, but the UAS continued to fly away where, out of sight of the pilot and operating crew, it crashed into vegetation next to the tracks.”
When using a hand-held compass to check for any magnetic interference, the investigator reported deviations of up to 140° were observed over localised regions of the bridge below which the railway track’s overhead high-voltage wires were being ducted.
“As a result of the findings, the operator has made changes in its flying procedures to reduce the possibility of the event reoccurring. These include:
- a magnetic interference check using a manual compass of an area 10 m around the planned takeoff location;
- a visual check for objects and structures that might have a large magnetic field;
- takeoff and landings should take place as far away from any sources of magnetic interference;
- and the pilot should always be ready to switch out of GPS mode into atti(tude) or manual flight modes to retake control of the UAS if control is lost whilst in GPS mode”
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