By the end of 2016 the U.S. Navy will disband one of its ten carrier air groups. This will mean the remaining nine carrier air groups will spend more time at sea and away from their home bases. The military term for this is “less dwell time” (with family and largely ashore). This is bad for morale but if the carriers spend less time at sea they will be less prepared for war.
A carrier air wing has about 2,500 personnel and 60-65 aircraft and helicopters. The elimination of a carrier air group is the result of the 2011 decision to disband one of its ten Carrier Strike Groups (SCGs), leaving only nine of them for the eleven aircraft carriers in service. This was a money saving measure because nuclear powered aircraft (CVN) carriers spend twenty percent of their time out-of-service having maintenance done. Thus only 8-9 CSGs are needed at any one time for carriers that can go to sea.
The SCG is actually a complex organization. There is the CVN and its crew, and the CAW (Carrier Air Wing), which includes all the aircraft, pilots and support personnel. The CAWs do not stay with the same CVN, but move around. When a CVN goes in for maintenance, its CAW will move ashore and then to another carrier (usually one coming out of dry dock). Also part of a SCG are the escort ships (usually a destroyer squadron of 2-4 destroyers, cruisers or frigates) and one or two SSNs (nuclear attack subs). There are also one or two supply ships carrying spare parts and maintenance personnel for all ships, as well as fuel for the escort ships.
Until 1998 the U.S. had twelve carriers, but new ones are not being built quickly enough to replace the older ones that must retire (because of old age). By 2007 there were only eleven carriers and soon there will only be nine CVNs, and there will be cost cutting pressure to disband another SCG.
The most expensive down time for CVNs are those shipyard trips that involve refueling the nuclear reactors. Those operations can take nine months and include lots of repairs and upgrades all over the ship. There is a lot of maintenance involved with CVNs, enough to keep these carriers unavailable for over 20 percent of their career. Over its fifty years of service, each Nimitz class carrier has 17 planned trips back to the shipyard. There are twelve Planned Incremental Availability (or PIA) operations in which new gear is installed, worn or damaged stuff is replaced and any heavy duty work needed, is completed. Duration of a PIA varies with the amount of work to be done, but it can take several months, or a year or more.
Even more lengthily are the four Dry-docking Planned Incremental Availabilities (DPIA) operations, which are more extensive PIAs that include putting the ship into dry dock. These efforts can last a year or two. The one Refueling and Complex Overhaul (RCOH) is like the DPIA, except the ship is partially dismantled so that the spent nuclear fuel can be replaced. This takes a little longer than your usual DPIA, and often costs over half a billion dollars.