Article: Although provision is made in Title 10 of the United States Code for the Secretary of the Air Force to appoint warrant officers, the Air Force does not currently use warrant officer grades, and is the only one of the U.S. Armed Services not to do so. The Air Force inherited warrant officer ranks from the Army at its inception in 1947, but their place in the Air Force structure was never made clear.[citation needed] When the Congress authorized the creation of two new senior enlisted ranks in 1958, Air Force officials privately concluded that these two new "super grades" could fill all Air Force needs then performed at the warrant officer level, although this was not publicly acknowledged until years later.[citation needed] The Air Force stopped appointing warrant officers in 1959, the same year the first promotions were made to the new top enlisted grade, Chief Master Sergeant. Most of the existing Air Force warrant officers entered the commissioned officer ranks during the 1960s, but small numbers continued to exist in the warrant officer grades for the next 21 years.

Question: When did the USAF stop appointing warrant officers?
1959