What happened
On 12 July 2017, a Quest Q-200 unmanned aircraft (UA) was conducting a commercial survey of a foreshore near Hinkley Point, Somerset. The flight was executing a pre-programmed route of waypoints at an altitude of 200 ft agl and a speed of 35 kt. The flight plan was designed with tracks running north to south to utilize crosswind conditions for optimal imaging.
During the mission, the aircraft encountered unexpected changes in wind strength and direction. These environmental shifts caused the aircraft to perform much wider turns than anticipated. Specifically, the first four downwind turns at the southern waypoints overshot their targets by approximately 125 m. As the aircraft continued, the overshoots increased to roughly 200 m.
As the UA transitioned through a turn, it began a programmed descent from 400 ft back to 200 ft agl. During this descent, the aircraft drifted inland and struck the horizontal beam of a tower crane located approximately 250 m from the foreshore. The impact resulted in extensive damage to the aircraft. There were no injuries and no damage to third-party property.
The investigation
The AAIB examined the flight planning and the pilot's response to the changing weather conditions. The investigation established that while the pilot had performed pre-flight checks and a risk assessment, the location of the tower cranes was not sufficiently accounted for due to their distance from the primary survey area.
It was noted that the pilot observed the increasing overshoots and considered aborting the flight on the next leg. However, the pilot did not manually intervene to initiate a climb before the collision, partly because the height and distance of the crane structures were difficult to perceive from the control station.
Findings
- The primary cause of the accident was inadequate flight planning regarding the orientation of flight legs and the consideration of nearby obstacles.
- A change in wind conditions caused the aircraft to deviate significantly from its intended path.
- The flight plan prioritized crosswind legs for imaging quality over the safety margin required to avoid the construction site.
- The pilot failed to implement manual corrective maneuvers to prevent the encroachment over the construction area.