What happened
On 26 October 2018, a DJI Matrice 210 was being operated in Ledbury, Herefordshire, as part of an emergency services training session. The pilot was undergoing instruction on the Unmanned Aircraft System (UAS), which was equipped with two TB55 batteries and a Zenmuse Z30 camera. Following a brief test flight by an instructor, the pilot took control of the aircraft.
After taking off in an open field, the aircraft reached an altitude of approximately 4 m to perform control checks. During these maneuvers, the aircraft's tail indicator lights transitioned from green to red, and the controller displayed a 'Battery Error' message. Immediately following this, all four electric motors ceased operation simultaneously. The aircraft fell from its 4 m altitude and struck the ground inverted, resulting in damage to the propellers, camera, gimbal mount, and front motor arms.
The investigation
Investigators analyzed the aircraft's log files to determine the sequence of electrical failure. The data showed that at takeoff, both batteries displayed a State of Charge (SOC) of approximately 76%, with voltages around 20.1 V. However, only 16 seconds into the flight, the voltage on the two batteries had dropped significantly to 17.4 V and 17.7 V, even though the SOC indicator still erroneously reported 75% remaining charge.
Eight seconds later, the voltage had plummeted further to 14.4 V and 15.2 V. At this point, the system triggered 'Vol Not Safe/Dangerous' alerts and initiated a 'battery_force_landing' parameter. The log file concluded less than one second after the motors shut down.
Findings
- The primary cause of the crash was the sudden shutdown of the electric motors due to insufficient battery voltage.
- The battery State of Charge (SOC) was providing an erroneously high reading relative to the actual voltage levels.
- The specific cause of the inaccurate SOC reporting was previously documented in a separate investigation involving a similar incident on 4 September 2018.