An aircraft carrier is a warship designed to deploy and recover aircraft - in effect acting as a sea-going airbase. Aircraft carriers thus allow a naval force to project air power great distances without having to depend on local bases for land-based aircraft.
Modern navies that operate such ships treat aircraft carriers as the capital ship of the fleet, a role previously played by the battleship. The change, part of the growth of air power as a significant part of warfare, took place during World War II. This change was driven by the superior range, flexibility and effectiveness of carrier-launched aircraft.
Unescorted carriers are considered vulnerable to attack by other ships, aircraft, submarines or missiles and therefore travel as part of a carrier battle group (CVBG), which is now known as a Carrier Strike Group (CSG) in the US Navy or Expeditionary Strike Group (ESG) if the carrier is an LHD or LHA. Unlike other types of capital ships in the 20th century, aircraft carrier designs since World War II have been effectively unlimited by any consideration save budgetary, and the ships have increased in size to handle the larger aircraft: The large, modern Nimitz class of United States Navy carriers has a displacement nearly four times that of the World War II-era USS Enterprise yet its complement of aircraft is roughly the same.
As "runways at sea," modern aircraft carriers have a flat-top deck design that serves as a flight deck for take-off and landing of aircraft. Aircraft take off to the front, into the wind, and land from the rear. Carriers steam at speed, for example up to 35 knots (65 km/h), into the wind during take-off in order to increase the apparent wind speed, thereby reducing the speed of the aircraft relative to the ship. On some ships, a steam-powered catapult is used to propel the aircraft forward assisting the power of its engines and allowing it to take off in a shorter distance than would otherwise be required, even with the headwind effect of the ship's course. On other carriers, aircraft do not require assistance for take off - the requirement for assistance relates to aircraft design and performance. Conversely, when landing on a carrier, conventional aircraft rely upon a tailhook that catches on arrestor wires stretched across the deck to bring them to a stop in a shorter distance than normal. Other aircraft - helicopters and V/STOL (Vertical/Short Take-Off and Landing) designs - utilize their hover capability to land vertically and so require no assistance in speed reduction upon landing.
Conventional ("tailhook") aircraft rely upon a landing signal officer (LSO) to control the plane's landing approach, visually gauging altitude, attitude, and speed, and transmitting that data to the pilot. Before the angled deck emerged in the 1950s, LSOs used colored paddles to signal corrections to the pilot. From the late 1950s onward, visual landing aids such as mirrors provided information on proper glide slope, but LSOs still transmit voice calls to landing pilots by radio.
Since the early 1950s it has been common to direct the landing recovery area off to port at an angle to the line of the ship. The primary function of the angled deck landing area is to allow aircraft who miss the arresting wires, referred to as a "bolter", to become airborne again without the risk of hitting aircraft parked on the forward parts of the deck. The angled deck also allows launching of aircraft at the same time as others land.
F/A-18 Hornets on the flight deck of the Nimitz-class supercarrier Harry S. TrumanThe above deck areas of the warship (the bridge, flight control tower, and so on) are concentrated to the starboard side of the deck in a relatively small area called an "island". Very few carriers have been designed or built without an island and such a configuration has not been seen in a fleet sized carrier.
A more recent configuration, used by the British Royal Navy, has a 'ski-jump' ramp at the forward end of the flight deck. This was developed to help launch VTOL (or STOVL) aircraft (aircraft that are able to take off and land with little or no forward movement) such as the Sea Harrier. Although the aircraft are capable of flying vertically off the deck, using the ramp is more fuel efficient. As catapults and arrestor cables are unnecessary, carriers with this arrangement reduce weight, complexity, and space needed for equipment. The disadvantage of the ski jump - and hence, the reason this configuration has not appeared on American supercarriers - is the penalty that it exacts on aircraft size, payload and fuel load (and hence, range): Large, slow planes such as the E-2 Hawkeye and heavily-laden strike fighters like the F/A-18E/F Super Hornet cannot use a ski jump because their high weight requires either a longer takeoff roll than is possible on a carrier deck (even on the large Admiral Kuznetsov, lightly-loaded Su-33 'Flanker-D' air superiority fighters require almost the entire length of the deck to take off) or catapult assistance.
Aircraft carriers today
Four modern aircraft carriers of various types - USS John C. Stennis, Charles de Gaulle, HMS Ocean and USS John F. Kennedy - and escort vessels on operations in 2002Aircraft carriers are generally the largest ships operated by navies; a Nimitz class carrier powered by two nuclear reactors and four steam turbines is 1092 feet (333 m) long and costs about $4.5 billion. The United States has the majority of aircraft carriers with a dozen in service and a dozen in reserve, and its aircraft carriers are a cornerstone of American power projection capability.
Nine countries maintain a total of 25 aircraft carriers: United States, United Kingdom, France, Russia, Italy, India, Spain, Brazil, and Thailand. In addition the People's Republic of China's People's Liberation Army Navy possesses the former Soviet aircraft carrier Varyag, but most naval analysts believe that they have no intention to operate it, but instead are using Varyag to learn about carrier operations for future Chinese aircraft carriers. Canada, the People's Republic of China, Japan, Pakistan, Australia, Chile and Singapore also operate vessels capable of carrying and operating multiple helicopters.
Aircraft carriers are generally accompanied by a number of other ships, to provide protection for the relatively unwieldy carrier, to carry supplies, and to provide additional offensive capabilities. This is often termed a battle group or carrier group, sometimes a carrier battle group.
In the early 21st century, worldwide aircraft carriers are capable of carrying about 1250 aircraft. US carriers account for over 1000 of these. The United Kingdom and France are both undergoing a major expansion in carrier capability (with a common ship class), but the United States will still maintain a very large lead.