UK – Firefighters and British troops spent eight hours battling the blaze on Epynt mountain in Powys, mid Wales.
An Osprey helicopter was taking part in an exercise as the fire broke out, destroying 150 hectares of grassland.
Mid and West Wales Fire and Rescue Service said it was caused by a “hot downdraft” in 20 C temperatures. The Ministry of Defence is investigating.
A fire service spokesman said: “The cause of the fire was attributed to a down draft from an American Osprey helicopter that was hovering above ground to deploy troops during an exercise at the military range.”
It happened at about 15:35 BST on 22 May, and was eventually extinguished at about 23:30 BST.
Osprey helicopters, which use upright rotors allowing them to land and take off, are used by the US Marine Corps, the US Air Force and to transport President Donald Trump.