A drone carrying out an aerial survey over a construction site in the Cheetham Hill area of Manchester struck the top of a tower crane and fell about 120 metres to the ground, according to the Air Accidents Investigation Branch.
The incident happened in August. It was notified to the AAIB, but the agency did not launch a formal investigation, and its bulletin does not identify the site or the contractor involved.
In its latest bulletin, the AAIB said the unmanned aircraft was conducting an aerial survey on a pre-selected flight path over a construction site in an urban area. The route had been fixed in advance rather than flown manually. The obstacle avoidance feature was enabled at the time, yet the aircraft struck the top of the crane and dropped to the ground.
An internal investigation by the construction company found that the company and its remote pilots may not have fully understood the limitations of the obstacle avoidance software, the bulletin states.
The AAIB record sets out the event without naming the parties; its account rests on the notification it received and the company's own internal review.
Taken together, the findings point to a gap between reliance on automated obstacle avoidance and what that software can actually handle on a built-up site. The company's conclusion was framed as a possibility rather than a definitive cause.