What happened
On 18 July 2018, a DJI Matrice 210 RTK was conducting a commercial survey flight over a farmer's field in Keith, Aberdeenshire. This was the fourth flight of the day for the unmanned aircraft system (UAS), with the previous three missions having been completed successfully using fresh batteries.
Approximately five minutes into the fourth mission, while the aircraft was at an altitude of roughly 60 m, the pilot received a low battery voltage alert on the controller. Shortly after this alert, the four electric motors ceased operation simultaneously while the aircraft was 25 m above the ground. This sudden loss of power caused the aircraft to fall, resulting in the destroyed airframe.
The investigation
The investigation involved an analysis of the manufacturer's report, as the onboard recorded data was unavailable at the time of the request. The manufacturer's analysis confirmed that an automatic landing sequence had been triggered by an abnormally low battery voltage at the 5.5-minute mark of the flight. Seventeen seconds after this trigger, the aircraft's data recording ceased as the motors stopped.
Investigators also noted that the batteries' State of Charge (SOC) had been displaying an erroneously high level of remaining energy, which masked the actual depletion of the battery cells.
Findings
- The primary cause of the loss of power was an abnormally low battery voltage that triggered an automatic landing mode.
- The aircraft's monitoring system provided an inaccurate State of Charge (SOC) reading, which suggested much more power remained than was actually available.
- This incident shared significant similarities with three other reported accidents involving the DJI Matrice 210 model during 2018, all involving landings initiated by critical battery voltage drops.