What happened
On 20 October 2018, a DJI Matrice 210 RTK was being operated over Manchester Piccadilly Gardens for emergency services duties. The flight, which involved maneuvering above rooftops, began at 1325 hrs under favorable weather conditions, including clear visibility and light winds. During the mission, the pilot replaced the existing batteries with a fresh pair of fully-charged TB55 units. Following a successful pre-flight check with no error messages, the aircraft reached altitudes of approximately 79 metres.
At 1340 hrs, while the drone was maintaining a static hover at an altitude of roughly 51 metres, it suddenly began a rapid, uncontrolled descent and drifted sideways. The pilot observed the battery indicator transition from green to flashing red, while an observer noted a battery system error on the remote controller. The aircraft struck the ground, resulting in extensive damage to the airframe, batteries, and cameras. Fortunately, no individuals or third-party property were struck during the incident.
The investigation
Investigators examined the aircraft's recorded data and the hardware provided by the operator. The manufacturer's analysis of the flight logs revealed that at the start of the flight, the two batteries displayed different voltages (22.5 VDC and 26.0 VDC), yet both reported a state of charge (SOC) near 100%.
As the flight progressed, the system drew current primarily from the second battery until the voltages equalized. Data showed that as the voltage dropped, the discrepancy in reported SOC became critical; while one battery showed 68% remaining, the other reported 0% despite having a recorded voltage of 17.6 VDC. Ultimately, when the voltage on the first battery reached 15.4 VDC, the aircraft triggered an automatic landing procedure. Less than three seconds later, the aircraft lost all power entirely, leading to the uncontrolled impact.