On U.S. aircraft carriers, per Title 10 United States Code, both the captain (i.e., the commanding officer or CO) and the XO assigned to the ship are naval aviators or naval flight officers. Although not specified by 10 U.S.C., large, air-capable amphibious assault ships will have one of the two senior positions (CO or XO) occupied by a surface warfare officer and the other by a naval aviator or naval flight officer, alternating at each change of command. In naval aviation, in U.S. Navy squadrons (other than fleet replacement squadrons and the Navy Flight Demonstration Squadron {viz., Blue Angels}), the XO will eventually "fleet up" to become the CO of that squadron after twelve to fifteen months as XO. This fleet up model was also adopted in the early 2000s for XO and CO positions of both large amphibious assault ships (but not aircraft carriers) and "Arleigh Burke"-class guided missile destroyers. In addition to operational and tactical responsibilities, XOs also shoulder most of the CO's administrative burden, to include oversight of the command's administrative officer (if assigned) and administrative department.