What happened
On 2 September 2021, a survey flight involving a DJI M600 Pro drone, registered ZT-WIV, ended in a crash in the Ghaamsberg region of the Northern Cape. While performing a mission to survey terrain, the pilot was executing a return flight to the launch site when the aircraft began transmitting critical error messages. These alerts indicated that the motors were reaching their maximum allowable speed, followed by failures in the compass and the inertial measurement unit (IMU).
As the technical issues progressed, the drone disconnected from the controller, causing the pilot to lose all telemetry data. During this period of lost communication, the pilot lost visual contact with the aircraft. The drone subsequently impacted rocks at the base of the terrain it had been surveying, resulting in the total destruction of the aircraft and its underslung payload.
The investigation
The investigation focused on the navigation methods used during the mission. The drone's flight path relied on a digital terrain model to maintain safe altitudes. While the operator had been provided with a high-quality Lidar terrain model, analysis revealed that certain steep cliffs and gorges within the survey area were not accurately captured. Because the navigation software uses waypoints that only sample the terrain directly beneath them, certain vertical features were effectively invisible to the system.
It was determined that the terrain model was inaccurate by approximately 10 meters in specific critical areas. These discrepancies meant the drone's programmed flight path conflicted with the actual physical height of the terrain, leading to the collision.
Findings
- The primary cause of the accident was the pilot losing visual line-of-sight with the drone, which led to the aircraft striking the mountain base.
- A significant contributing factor was the use of inaccurate terrain data provided by a client, which the operator did not independently verify.
- The inaccuracies in the digital terrain model prevented the navigation software from correctly accounting for vertical cliff faces and sudden drop-offs.