On May 6, 2021, about 1045 eastern daylight time, a Dà-Jiang Innovations (DJI) Matrice 300 RTK small unmanned aircraft system (sUAS, commonly known as a drone), FA3HW43WTF, was involved in an accident near Young Harris, Georgia. The remote pilot in command (RPIC) sustained serious injuries. The flight was operated as a Title 14 Code of Federal Regulations Part 107 demonstration flight.
The purpose of the flight was to demonstrate for the local sheriff’s office the drone capabilities with a payload of a Zenmuse H20T camera and a Wingsland Z15 spotlight. The flight location was very close to a DJI GEO Zone no-fly zone (NFZ) designated as a “restricted zone.”
Figure 1. Restricted Zone in red and approximate accident site denoted with a yellow circle.
The RPIC stated that he performed a normal takeoff and flew the drone to the west over a wooded area about 393 ft above ground level (agl). He stated that, during the spotlight demonstration, the drone was unresponsive to control inputs, so he utilized the return-to-home (RTH) function. During the RTH descent toward the home point, the drone hovered about 7 ft agl over a vehicle in the parking lot and was still unresponsive to any control inputs. When the drone would not land, the RPIC grabbed onto the landing gear and attempted to physically move the drone away from the vehicles. The drone resisted the physical displacement and maintained its position over the vehicle. The RPIC requested assistance from a demonstration attendee and handed him the remote controller. With guidance from the RPIC, he attempted several times to shut down the motors while RPIC held onto the landing gear with both hands. Finally, the RPIC attempted to remove the drone batteries when a propeller blade struck his right hand several times, which resulted in tendon and nerve damage. The RPIC continued to hold onto the drone for several minutes until the batteries were exhausted and the motors stopped.
Figure 2. Accident flight path in yellow
According to DJI, in restricted zones, which appear red on the DJI application (app), users will be prompted with a warning and flight is prevented. GEO Zones that prohibit flight are implemented around locations such as airports, power plants, and prisons. NFZ’s feature a “buffer zone” defined as an area about 66 ft wide surrounding the NFZ. The purpose of a buffer zone is to account for estimation and control errors in order to avoid breaching the NFZ when the drone has forward speed. When approaching the buffer zone, the drone will reduce speed and stop at the buffer zone border. After the accident, the RPIC stated that he was initially unaware of the restricted zone and never saw a notification on the remote controller during the flight. He added that he only discovered the Restricted Zone after a discussion with the National Transportation Safety Board (NTSB) investigator.
The accident flight logs and data were extracted and revealed that the home point was set within the buffer zone of the restricted zone. About 7 minutes after takeoff at 393 ft agl, the RTH function was initiated. The drone returned to 56 ft from the home point and stopped at the boundary of the buffer zone, when the DJI app prompted a restricted zone warning and the drone remained in RTH mode. The drone remained about 7 ft agl and would not respond to the RPIC’s multiple control inputs since it was still in RTH mode. The logs showed multiple altitude fluctuations and increased motor thrust, without corresponding control inputs, which indicated an external force was exerted on the drone. On four occasions, the RPIC exited RTH mode then reactivated RTH a few seconds later, and the drone would not respond to manual control inputs with RTH mode active. The battery level reached 9% and a battery installation error was prompted. The battery level reached 7%, and a critically low battery auto landing was initiated. The drone did not maintain altitude despite increased motor thrust, which indicated an external force was again exerted on the drone.