What happened
On 20 December 2017, a DJI Matrice 210 was being operated for a commercial training flight near Albert Bartlett Farm, St Helier, Jersey. The aircraft, which was configured with a takeoff mass of 4.57 kg, began the flight with an estimated 31 minutes of battery life remaining. During the operation, the pilot encountered intermittent satellite signal loss and a compass error message. While hovering, the aircraft experienced an uncommanded rotation around its yaw axis, which the pilot addressed by switching flight modes.
As the pilot initiated a descent to land, the battery indicator showed 12 minutes of remaining flight time. Upon reaching an altitude of 84 m, a 'low voltage battery warning' appeared. Simultaneously, all four electric motors ceased operation, causing the aircraft to enter a rapid descent. Although the warning cleared and the motors restarted approximately one second later, the aircraft could not be recovered in time and struck the ground at a considerable speed. The impact resulted in damage to the landing gear, arms, propellers, gimbal, and battery case.
The investigation
The manufacturer of the UAS conducted an analysis of the onboard flight data following the accident. The investigation focused on the electrical performance of the dual battery system. Data logs showed that the main controller measured a voltage of 23.4 V at the time of the incident. While a full battery sits at 26.3 V, the recorded 23.4 V was still a sufficient level to maintain flight.
However, the investigation identified significant discrepancies in the readings for the second battery. While the voltage of the first battery decreased gradually from 25.5 V to 23.3 V, the second battery maintained a constant reading of 22.6 V despite showing a steady, high current output. The manufacturer could not provide a specific reason for this measurement anomaly.
Findings
- The primary cause of the sudden motor shutdown was a battery firmware issue.
- This firmware error caused the system to ignore actual battery levels and incorrectly trigger a power cut to the motors because it perceived the voltage as being too low.
- The aircraft's descent was unrecoverable due to the insufficient time between the motor failure and the ground impact.