The Assistant Chief of the Air Staff (ACAS) is a senior appointment in the Royal Air Force. The incumbent is in practical terms the deputy to the head of the RAF, the Chief of the Air Staff.
The ACAS post was established in 1985 by eliminating the Vice Chief of the Air Staff and combining the Policy and Operations two star assistant chiefs. In 1992, ACAS became a member of the Air Force Board. One of the many duties of the ACAS is to sit on the board of the Civil Aviation Authority.