What happened
On 25 November 2021, a DJI Phantom 4 Pro was being operated for commercial purposes to document progress at a construction site in Rugby, Warwickshire. The flight was being conducted by a single pilot without the assistance of an observer. Shortly after takeoff, the pilot's attention was diverted by two warning messages appearing on the controller interface. One notification indicated that propulsion output was being limited due to battery health, while the second warned of magnetic interference and instructed the pilot to exit P-GPS mode.
Upon looking back up, the pilot found they had lost visual contact with the drone. An attempt to trigger the return-to-home function failed. A subsequent alert notified the pilot that the command link signal had become too weak, resulting in a total loss of connection. Without the ability to maintain a fixed position or follow a programmed return path, the aircraft drifted 630 metres over a congested area containing housing and a railway line before eventually coming to rest in a tree.
The investigation
Investigators examined the sequence of events leading to the loss of control. The investigation established that the magnetic interference prevented the aircraft from maintaining a stable heading or position. Because the aircraft could not rely on its heading reference, it was unable to execute the return-to-home command.
Additionally, the investigation looked into the impact of the battery health warning. This message indicated that the drone's ability to counter environmental forces was reduced. While the wind was measured at 13 knots, it was not definitively determined if this specific wind speed exceeded the reduced propulsion capabilities, though the risk of such a limitation in windy conditions was noted. The pilot also noted that the low position of the sun contributed to the difficulty in regaining visual contact once the controller messages had distracted them.
Findings
- The aircraft suffered from magnetic interference, which stripped it of its heading reference and prevented it from holding its position or returning to the launch point.
- The pilot's attention was diverted by controller notifications, leading to a loss of visual line-of-sight.
- The lack of an observer meant there was no secondary person to maintain visual contact during the period of distraction.
- The loss of the command link left the aircraft in an uncontrolled state, allowing it to drift over a populated area.