What happened
On 19 November 2020, a DJI Matrice M210 Version 1 was being operated by emergency services for a police mission over Poole, Dorset. During the flight, a battery communication error occurred, which was caused by one of the two batteries losing physical connection to the terminal shortly after takeoff. This disconnection caused the aircraft's software to restrict its maximum pitch attitude to 15°, significantly reducing its top speed.
As the flight progressed, wind speeds at 400 ft reached up to 39 mph, exceeding both the initial forecast and the manufacturer's specified wind limits. Because of the reduced pitch capability, the pilot was unable to fly the aircraft back toward the takeoff point. The drone eventually drifted beyond visual line of 500 m, and communication with the controller was lost. The aircraft subsequently entered an auto-land mode due to low battery, but it struck the wall of a residential building, resulting in damage to the propeller blades and the aircraft's landing leg before coming to rest on a balcony.
The investigation
The AAIB examined data logs from both the aircraft and the master controller. The investigation established that while the pilot received a warning regarding the battery, the critical instruction to land immediately was likely missed because the message disappeared quickly and lacked an audible alarm. Furthermore, the investigation found that the manufacturer's wind warnings were ambiguous; the message displayed for both Level 1 and Level 2 wind warnings was simply 'Fly with caution,' which did not explicitly instruct the pilot to land when the manufacturer's limits were exceeded.
Additionally, the investigation revealed that the pilot and operator were unaware that the aircraft's speed and pitch were being restricted due to the battery issue, as this behavior was not documented in the user manual. The investigation also noted that the pilot's reliance on a 500 m visual line of sight (VLOS) distance made it difficult to maintain orientation and recover the aircraft once it began drifting.
Findings
- The primary cause of the accident was a battery disconnection that restricted the aircraft's pitch attitude and maximum airspeed.
- Wind speeds exceeded the aircraft's operational capabilities and the pilot's forecasts.
- The pilot did not notice the specific instruction to land following the battery error.
- The manufacturer's wind warnings were not sufficiently urgent or descriptive of the exceeded limits.
- The aircraft's software did not display the total battery capacity drop, masking the severity of the power issue.
- The user manual lacked information regarding speed and pitch restrictions during battery malfunctions.
Safety action
- The AAIB recommended that the manufacturer update its apps to provide clear instructions to land when wind limits are exceeded.
- A recommendation was made to implement audible alerts for safety-critical messages.
- The AAIB recommended that the manufacturer include information regarding pitch and speed restrictions in the user manual.
- A recommendation was made to the CAA to review and clarify Visual Line of Sight (VLOS) guidance in CAP 722.