A surveying drone struck the top of a tower crane at a construction site in Cheetham Hill, Manchester, in August and fell 120 metres to the ground, according to a bulletin from the Air Accidents Investigation Branch.
The AAIB said the unmanned aircraft was carrying out an aerial survey along a pre-selected flight path over a construction site in an urban area. Its obstacle avoidance feature was enabled, but the drone still hit the top of the crane.
The incident was reported to the AAIB, but no formal investigation was opened. The bulletin did not identify the site or the contractor involved.
An internal investigation by the construction company found that the company and its remote pilots might not have fully understood the limitations of the obstacle avoidance software, the AAIB bulletin said.
The bulletin describes the flight as taking place over an urban construction site, with the avoidance system active before the collision. It does not say that the software failed, nor does it set out a specific cause beyond the company's internal finding about possible gaps in understanding.
The aircraft is described only as an unmanned aircraft; no model or company name is given. The published account also leaves the site and contractor unidentified.