A surveying drone struck the top of a tower crane and fell 120 metres to the ground at a construction site in the Cheetham Hill area of Manchester, the Air Accidents Investigation Branch said in its latest bulletin. The incident happened in August.
According to the AAIB, the unmanned aircraft was carrying out an aerial survey using 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 notified to the AAIB, but no formal investigation was launched. 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 may not have fully understood the limitations of the obstacle-avoidance software, the AAIB said.
The account highlights the difference between having an automated safety feature switched on and understanding the conditions in which it can perform reliably around tall or complex structures. The bulletin does not specify which limitation of the software was relevant, nor does it identify the drone, software provider or remote pilot.
The AAIB's notice confirms the basic sequence of the accident but leaves the site, contractor and technical details undisclosed. It also makes clear that the internal finding was a possibility, not a concluded cause of the collision.