What happened
On 5 March 2020, a DJI Phantom 4 was being operated for aerial survey purposes at the Bristol sewage treatment works. The mission involved surveying an area containing four wind turbines. During the flight, the unmanned aircraft collided with one of the turbines, resulting in the destruction of the aircraft.
Prior to the flight, the pilot used the NATS Drone Assist app for planning and risk assessment. Because the app did not identify the wind turbines as hazards, the pilot performed an internet search for the height of the turbines, which suggested a height of 328 ft. Based on this information, the pilot planned a flight at 400 ft above the ground, believing this would provide a 72 ft clearance from the turbine blades. However, the turbine in question actually stood 413 ft above the ground, leading to the collision.
The investigation
The AAIB examined the pilot's flight planning process and the tools used to identify ground hazards. The investigation established that while the pilot was aware of the turbines' presence, they could not find accurate height data through their primary planning app or initial internet searches.
Investigators noted that CAA aeronautical charts display all known land-based obstacles exceeding 300 ft above ground level. These charts, and the databases used by many flight planning applications, provide a reliable method for identifying tall structures. The investigation also found that the pilot had received training regarding aeronautical charts but did not utilize them during the risk assessment for this specific operation.
Findings
- The pilot misjudged the height of the wind turbine by relying on an inaccurate internet search result rather than official aeronautical charts.
- The NATS Drone Assist app used during planning did not highlight the turbines as obstacles.
- The actual height of the turbine (413 ft) exceeded the pilot's planned flight altitude (400 ft).
Safety action
Following the investigation, the operator updated its flight planning and risk assessment protocols to ensure that aeronautical charts and obstacle-aware planning applications are specifically consulted.