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 and fell about 120 metres to the ground, the Air Accidents Investigation Branch has disclosed.
The AAIB said the incident had been notified to the branch, but that no formal investigation was opened, and neither the site nor the contractor was identified.
In its latest bulletin, the branch said the unmanned aircraft was following a pre-selected flight path over a construction site in an urban area. The obstacle avoidance feature was enabled, yet the aircraft struck 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, according to the bulletin.
The published account of the flight rests on that internal company review rather than on an AAIB field inquiry, so the circumstances surrounding the collision and the fall have not been documented in further public detail.
The finding points to a gap between relying on automated obstacle avoidance to keep a survey aircraft clear of site plant and knowing how far that software can be trusted — a question the company's review raised about its own operators.