A survey drone struck the top of a tower crane and fell 120 m to the ground during an aerial survey over a construction site in the Cheetham Hill area of Manchester, according to the Air Accidents Investigation Branch.
The incident happened in August. It was reported to the AAIB, but the agency did not open a formal investigation, and neither the site nor the contractor was identified in the bulletin.
The AAIB said the unmanned aircraft was following a pre-selected flight path over an urban construction site. Its obstacle-avoidance feature was enabled, but the drone still hit the crane and dropped to the ground.
An internal investigation by the construction company concluded that the company and its remote pilots might not have fully understood the limitations of the obstacle-avoidance software, the bulletin said.
The account points to a gap between having a safety function switched on and understanding what it can reliably detect and avoid. In a survey flight planned over an active site, the pre-set route, the height of crane structures and the performance limits of onboard avoidance all affect the margin for error. The AAIB bulletin does not say whether the drone's route had been checked against the crane's position or what the remote pilots could see at the time.
No formal finding of cause was issued because no formal probe was launched. The details instead come from the AAIB's bulletin and the contractor's internal review, which described a possible gap in understanding of the software's limits.