Yeates joined the Inns of Court Officers' Training Corps in 1916 and transferred to the Royal Flying Corps (later the Royal Air Force) in May 1917. Yeates married in July 1917, aged only 19 and while still training, to the total disapproval of his parents. Initially serving with No. 46 Squadron, to which he was posted in February 1918, by then he had logged twelve hours of dual flying instruction and fifty-three hours solo, of which the last thirteen hours were in a Sopwith Camel.