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 in August, then fell about 120 metres to the ground, according to the Air Accidents Investigation Branch.
In its latest bulletin, the AAIB said the unmanned aircraft was flying a pre-selected flight path over a construction site in an urban area. The obstacle avoidance feature was enabled, but the aircraft still hit the top of the crane.
The incident was notified to the AAIB, which did not launch a formal investigation. The bulletin does not identify the site or the contractor involved.
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 AAIB said.
The bulletin does not describe injuries, damage on the ground or the type of aircraft involved, and it does not set out how the flight path was planned or what checks preceded the flight.
As described, the finding sits with the operator: the avoidance function was active, yet the aircraft collided with a structure inside the survey area. On the bulletin's account, the shortfall lay in understanding what the function could and could not handle, rather than in the feature being switched off.