Print friendly version of this article (text only)

High seas, cold temperatures, strong winds, and corrosive salt spray create a harsh environment for humans and machines at sea. Aircraft carriers and amphibious assault ships deal with these conditions with brute force and sheer size. The aircraft operating off their decks, however, rely on finesse and ingenuity. As big as these ships are, deck space is at a premium, with every aircraft operation competing for its place.

The ability of an aircraft to operate effectively in these rigorous environments is called carrier suitability. A carrier-suitable aircraft has good launch and recovery performance in all-weather, day or night. It has increased landing gear and airframe strength and specialized systems for dealing with short takeoffs and landings. It has good deck handling attributes and compatibility with handling and support equipment. And it is reliable and easily maintained within the constraints of a flight deck and hangar deck.

Current Navy carriers are in the 90,000-ton displacement class, with an overall length of almost 1,100 feet. They have four steam catapults to launch their aircraft. Two catapults are located on the bow; two more abeam of the island on the ship’s angle deck. Four arresting wires or cables on the angle deck are used to recover aircraft. A webbed barricade can be rigged quickly for emergency landings. The carrier generally operates at sea with an air wing, which consists of seventy-six fixed-wing aircraft and six helicopters.

Carrier-based aircraft must meet certain "wind over deck" guidelines for launching with specified mission loads and for landing with specified bring-back loads. When necessary, the carrier must augment the natural wind with its own speed to achieve the wind velocities required to launch and recover aircraft. Carrier-suitable aircraft minimize these requirements to improve tactical flexibility of the carrier.

For launch, the aircraft attain flying speed with catapults pulling them along the deck with tow forces exceeding 200,000 pounds. For recovery, an arresting gear engages an arresting cable to stop the aircraft in less than 350 feet. Navy carrier aircraft use a nose gear launch system and high-strength landing gears to withstand the loads associated with launch and recovery. The airframe itself must withstand sink rates, the vertical speed at which the aircraft hits the deck, of up to twenty-six feet per second.

US Navy LHA and LHD amphibious assault ships, at roughly a 40,000-ton displacement and 820-foot length, are smaller than Navy carriers. These L-class ships normally carry six to eight aircraft with a short takeoff and vertical landing capability and about thirty helicopters.

These ships are designed to support multiple aspects of amphibious assault and rapid movement of combat troops and equipment. They also provide aircraft for close air support for ground troops. These operations are often conducted at the same time, so the relatively small decks of these ships must be used efficiently. Deck activity can get very busy, with fixed-wing aircraft competing for space with CH-53 Sea Stallion or CH-46 Sea Knight helicopters.

With no catapult or arresting gear, strength requirements for the Marine STOVL aircraft operating from these ships are not as severe as for carrier-based aircraft. Still, these aircraft must be designed to withstand landings in rough seas with sink rates up to fifteen feet per second.

When operating from the flat decks of L-class ships and from Navy carriers, USMC AV-8B Harrier pilots use a short takeoff procedure. Initially, the aircraft’s brakes restrain it from moving while the engine builds up thrust. The brakes are released and the aircraft accelerates down the deck with thrust directed primarily aft. When the aircraft gains sufficient airspeed, the pilot transitions to a combined airborne and thrustborne lift mode for takeoff. Although the ships have a maximum available flight deck length of 750 feet, the Marines prefer to use much less space for takeoffs to permit concurrent helicopter operations.

The Royal Navy has three Invincible class carriers, designated CVSGs. At approximately 20,000 tons displacement and 670 feet in length, these ships are smaller than US carriers and amphibious assault ships. They generally operate at sea with a complement of eighteen aircraft, which includes both Sea Harriers and helicopters. CVSGs can accommodate up to twenty-four aircraft.

The forward end of the deck of these Royal Navy ships has a 150-foot long "ski-jump" ramp. The increased elevation and twelve-degree exit angle of the ramp give the aircraft an upward boost, which translates into higher launch payloads by several thousand pounds. Sea Harriers, like their Marine AV-8B Harrier counterparts, are STOVL aircraft.

Whether on an L-class assault ship, Royal Navy CVSG, or on a Navy carrier, STOVL aircraft recover by transitioning from wing-borne flight to thrustborne flight for a vertical landing. This capability complicates aircraft design because the lift must be provided exclusively by propulsive force. Resulting gas temperatures, velocities, and the acoustic environment must be compatible with the ship and not endanger flight deck personnel.

Deck handling, or how compatible the aircraft is with operations on the flight deck and in the hangar bays, is another important feature of any sea-based aircraft. Space is scarce on all of these ships, and hangar bays have limited room for maintenance. Designers must consider the geometry of the overall configuration, landing gear design, and equipment placement to work within these small spaces. An aircraft with good deck handling characteristics is compact and light weight, resists tipping or turning over during heavy sea operations, clears both the deck and the handling equipment, and is easily fueled and serviced. These features allow it to be loaded and turned around quickly for another mission.

High reliability is a cornerstone of any carrier aircraft. However, when maintenance is needed, components must be repaired, removed, and replaced rapidly. Equipment must be easily accessed through quick-access panels or through natural access in wheel wells or weapon bays. Cramped carrier elevators and tight hangar bays demand lightweight and compact aircraft. Auxiliary power units inside the aircraft help by reducing the need for "yellow gear" specialized carts for electricity, air conditioning, and engine starting.

During initial design stages, configuration engineers consider all of the carrier-related constraints when developing the geometry of the aircraft. The design must clear all obstacles during launch with the most critical external store loadings and control surface positions. The same is true during landing and during deck operations. Carrier-suitability requirements, however, are not a simple matter to meet. Each dimensional change must be balanced against its effect in other areas. For example, wing fold locations are traded against internal fuel capacity (fuel is not carried in the outer wing), wing pylon placement, spot factor, and hangar bay clearances for folding and unfolding during maintenance.

Aerodynamics, performance, and handling qualities engineers work closely to provide low-speed performance necessary for launching and recovering at sea. The aircraft must have precise flight path stability and agility during carrier approaches. Since any new aircraft can be expected to operate for thirty or more years in the fleet, the design must include growth capacity as well.

Vehicle management system engineers combine the pilot controls, control surface effects, and the engine response to achieve these desired results. They develop the control law algorithms and convert them to software for use in flight control computers.

Sophisticated computer models of the total system account for geometry, mass properties, aerodynamic coefficients, propulsion effects, control surface actuator rates and time delays, and cockpit controls. These factors are combined for testing in manned flight simulators to verify aircraft flight characteristics before actual flight testing.

Company and military test pilots fly launch and recovery tasks with these models, which also simulate carrier and ship environments. The simulator includes the effects of turbulence from the island and ship motion due to varying sea states. These simulations help determine whether a design is carrier-suitable well before the aircraft ever launches from a carrier or an assault ship.

The design of carrier-capable aircraft is a unique challenge. Lockheed Martin is committed to ensuring that JSF variants operating on US Navy carriers and amphibious assault ships and on Royal Navy CVSGs meet the challenge.