A surveying drone struck the top of a tower crane at a construction site in the Cheetham Hill area of Manchester in August, according to the Air Accidents Investigation Branch.
The AAIB said the incident was notified to it, but no formal investigation was launched and neither the site nor the contractor was identified.
In its latest bulletin, the branch said the unmanned aircraft was carrying out an aerial survey on a pre-selected flight path over a construction site in an urban area.
The drone struck the crane's top despite its obstacle avoidance feature being enabled, then fell 120 m to the ground, the AAIB said.
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 internal finding points to a possible gap between the avoidance feature being switched on and the crew's understanding of what the system could and could not do during the survey flight, particularly near tall structures.
The AAIB bulletin did not identify the site or the contractor, and it provided no further details about the drone, the flight plan or the outcome of the fall.