A surveying drone struck the top of a tower crane and fell 120 m to the ground while working over a construction site in the Cheetham Hill area of Manchester, the Air Accidents Investigation Branch has disclosed.
The incident happened in August and was notified to the AAIB, but no formal investigation was launched. Neither the site nor the contractor involved has been identified.
Pre-set flight path
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. Its obstacle avoidance feature was enabled, yet the aircraft still struck the top of the crane and dropped 120 m.
An internal investigation by the construction company concluded that the company and its remote pilots "may not have fully understood the limitations of the obstacle avoidance software", according to the bulletin.
Reported, not investigated
With no formal probe opened, the bulletin records the occurrence as it was reported rather than setting out findings on causes. The internal review's conclusion is the only explanation of the sequence of events given in the published account.
The case turns on the distance between switching on an automated avoidance function and knowing when that function's performance falls short — a distinction that matters for survey flights close to tall structures in built-up areas.