A report released by the Air Accidents Investigation Branch said the drone suffered a rapid loss of battery power above the Garsteng site.
The remote pilot initiated an immediate automatic landing from the drone’s operational 50 metre height but it stopped responding to controls.
The report stated: “The pilot described it as then ‘appearing to descend too fast, despite the propellers spinning’, before going out of sight behind some construction materials.
“The unmanned aerial drone struck a site worker on their arm, dropped into some cement, and fell to the ground. The site worker was not injured.”