A survey drone struck the top of a tower crane at a construction site in the Cheetham Hill area of Manchester in August and fell 120 m to the ground, according to the Air Accidents Investigation Branch (AAIB).
The AAIB was notified of the incident, but no formal investigation was launched. The site and contractor were not identified in the bulletin.
In its latest bulletin, the AAIB 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 obstacle-avoidance feature was enabled, but the aircraft still hit the top of the crane.
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 identify the drone operator, the crane owner or the construction company involved. It also provides no further account of damage, injuries or regulatory action.
The AAIB account is limited to the notified circumstances and the company's internal findings. It does not conclude that the software failed or assign responsibility for the incident.