What happened
On 14 January 2019, a DJI Matron 210RTK unmanned aircraft system (UAS) was conducting an aerial survey mission in Colwyn Bay, Conwy. The aircraft was equipped with a third-party lidar pod for the mission. During the execution of a pre-programmed flight route, the drone climbed to an altitude of approximately 70 m, exceeding its programmed 30 m limit.
Upon noticing the altitude deviation and receiving a critical battery/current error notification, the remote pilots attempted to land the aircraft by initiating a return-to-home command. However, the aircraft became unresponsive to these inputs. Following a period of instability, the drone entered an uncontrolled descent, rapidly increasing in speed before striking the ground. The impact resulted in extensive damage to the aircraft and payload, though no injuries were reported.
The investigation
The AAIB examined the flight data available from the autonomous flight app, as the primary onboard flight data was unavailable. The investigation focused on the aircraft's weight, the compatibility of the third-party software, and the physical configuration of the payload.
Investigators determined that the total takeoff weight of the aircraft was approximately 6.37 kg, which exceeded the manufacturer's maximum specified takeoff weight of 6.14 kg. Analysis of the flight data showed that 14 seconds into the flight, the motors experienced a heavy load and high current draw, causing the battery voltage to drop significantly. Additionally, the investigation looked into the positioning of cables from the lidar pod's battery pack, noting that an unsecured cable might have interfered with the aircraft's downward-looking ultrasonic sensors.
Findings
- The aircraft was being operated above its maximum specified takeoff weight.
- The high current draw required to lift the overweight aircraft likely caused the low battery voltage and subsequent warnings.
- The use of the 'Litchi for DJI Drones' app was noted, despite the specific aircraft model not being listed as compatible by the app developer.
- An unsecured cable from the lidar pod's battery pack may have obstructed the ultrasonic sensors' field of view, potentially affecting the aircraft's ability to maintain altitude.