A drone carrying out an aerial survey struck the top of a tower crane at a construction site in the Cheetham Hill area of Manchester and fell about 120 m to the ground, according to the Air Accidents Investigation Branch.
The incident happened in August. It was notified to the AAIB, but no formal investigation was launched, and neither the site nor the contractor was identified in the branch's account.
In its latest bulletin, the AAIB said the unmanned aircraft was flying a pre-selected survey route over a construction site in an urban area. The obstacle avoidance feature was enabled at the time, yet the aircraft still hit the top of the crane and dropped roughly 120 m.
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, according to the bulletin.
The account sets out the flight profile, the height of the fall and the internal finding, but gives no detail on the size of the aircraft, damage to the crane or whether anyone on the ground was affected.
The finding places part of the explanation in the gap between what operators assume automated avoidance will handle and what it actually covers around the dense vertical structures typical of city sites. It also spreads responsibility beyond the software itself, pointing to how well the people flying the aircraft understood its behaviour before the flight was planned.