Arresting gear

Arresting gear, or arrestor gear, is mechanical systems used to rapidly decelerate an aircraft as it lands. Arresting gear on aircraft carriers is an essential component of naval aviation, and it is most commonly used on CATOBAR and STOBAR aircraft carriers. Similar systems are also found at land-based airfields for expeditionary or emergency use. Typical systems consist of a steel wire rope laid across the aircraft landing area, designed to be caught by an aircraft's tailhook. During a normal arrestment, the tailhook engages the wire and the aircraft's kinetic energy is transferred to hydraulic damping systems attached below the carrier deck. There are other related systems which use nets to catch aircraft wings or landing gear. These barricade and barrier systems are only used for emergency arrestments for aircraft without operable tailhooks.

History
Arresting cable systems were invented by Hugh Robinson and were utilized by Eugene Ely on his first landing on a ship - the armored cruiser USS Pennsylvania, on January 18, 1911. These early systems had cables run through pulleys and attached to dead weights, such as sandbags. More modern arresting cables were tested on HMS Courageous in June 1931.

Modern U.S. Navy aircraft carriers have the Mark 7 Mod 3 arresting gear installed, which have the capability of recovering a 50000 lb aircraft at an engaging speed of 130 knots in a distance of 340 feet (104 m). The system is designed to absorb theoretical maximum energy of 47,500,000 foot-pounds (64.4 MJ) at maximum cable run-out.

Prior to the introduction of the angled flight deck, two systems were used (in addition to deck cables) to keep landing aircraft from running into parked aircraft further forward on the flight deck: the barrier and the barricade. If the aircraft tailhook failed to catch a wire, its landing gear would be caught by a 3–4 foot high net known as the barrier. If the aircraft caught a wire upon touchdown, the barrier could be quickly lowered to allow aircraft to taxi over it. The final safety net was the barricade, a large, 15 foot (4.6 m) high net that prevented landing aircraft from crashing into other aircraft parked on the bow. Barriers are no longer in use, although ground based arresting gear are sometimes called "barriers". Barricades are still in use aboard carriers, but they are only rigged and used in emergencies.

Operation
A normal arrestment is accomplished when the arresting hook of an incoming aircraft engages one of the deck pendants. When a landing aircraft engages a deck pendant, the force of the forward motion of the landing aircraft is transferred to a purchase cable which is routed via sheaves to the arresting engine, located in a machinery room below the flight deck or on either side of the runway. As the deck pendant and the purchase cable are pulled out by the aircraft being arrested, the kinetic energy of the aircraft is transferred to mechanical energy of the cables, and the arresting engine transfers the mechanical energy of the cables to hydraulic energy. The arresting engine brings about a smooth, controlled stop of the landing aircraft. At the completion of the arrestment, the aircraft arresting hook is disengaged from the deck pendant, which is then retracted to its normal position.

Sea-based systems
Modern carriers typically have three or four arresting cables laid across the landing area. All U.S. carriers in the Nimitz-class, along with Enterprise, have four wires, with the exception of the USS Ronald Reagan and USS George H.W. Bush, which have only three. The Gerald R. Ford-class carriers will also have three. Aircraft coming in to land on a carrier are at approximately 85% of full throttle. At touchdown, the pilot advances the throttles to full power. In the F/A-18E/F Super Hornet and EA-18G Growler aircraft, the aircraft automatically reduces engine thrust to 70% once the deceleration of a successful arrestment is detected. This feature can be overridden by the pilot by selecting max afterburner. If the aircraft fails to catch an arresting cable, a condition known as a "bolter", the aircraft has sufficient power to continue down the angled flight deck and become airborne again. Once the arresting gear stops the aircraft, the pilot brings the throttles back to idle, raises the hook and taxies clear.

In addition to American CVNs, the French Charles de Gaulle, the Russian Kuznetsov, the Brazilian São Paulo, the Chinese Liaoning, as well as the Indian Vikramaditya are active or future aircraft carriers installed with arresting gear.

Land-based systems
Land-based military airfields operating fighter or jet trainer aircraft also use arresting gear systems, although they are not required for all landings. Instead, they are used for landing aircraft on short or temporary runways, or for emergencies involving brake failure, steering problems or other situations in which using the full length of the runway is not possible or safe. There are three basic types of land based systems: permanent, expeditionary, and overrun gear.

Permanent systems are installed on nearly all U.S. military airfields operating fighter or jet trainer aircraft. Expeditionary systems are similar to permanent systems and are used for landing aircraft on short or temporary runways. Expeditionary systems are designed to be installed or uninstalled in only a few hours.

Overrun gear consisting of hook cables and/or elastic nets known as barriers are commonly used as a backup system. Barrier nets catch the wings and fuselage of an aircraft and use an arresting engine or other methods such as anchor chains or bundles of woven textile material to slow the aircraft down. On some land-based airfields where the overrun area is short, a series of concrete blocks referred to as an engineered materials arrestor system is used. These materials are used to catch the landing gear of an aircraft and slow it via rolling resistance and friction. Aircraft are stopped by the transfer of energy required to crush the blocks.

The first use of a barrier on a military airfield was during the Korean War when jet fighters had to operate from shorter airfield where there was no margin for error. The system used was just a transplant of the Davis Barrier used on straight deck carriers to keep any aircraft that missed the arrest wires from crashing into the aircraft parked forward of the landing area. But instead of the more complex hydraulic system used on carriers to stop the aircraft when it hits the barrier the land based system used heavy ship anchor chains to bring the aircraft to a halt.

Components
The major systems that make up typical arresting gear are the hook cable or pendants, purchase cables or tapes, sheaves and arresting engines.

Cross deck pendant
Also known as arresting cables or wires, cross deck pendants are flexible steel cables which are spanned across the landing area to be engaged by the arresting hook of an incoming aircraft. On aircraft carriers there are either three or four cables, numbered 1-4 from aft to forward. Pendants are made of 1 inch (25 mm), 1-1/4 (32 mm) inch or 1-3/8 inch (35mm) diameter wire rope. Each wire rope is made up of numerous strands twisted about an oiled hemp center core, which provides a "cushion" for each strand and also supplies cable lubrication. The cable ends are equipped with terminal couplings designed for quick detachment during replacement and are able to be rapidly detached and replaced (in about 2–3 minutes on aircraft carriers). On U.S. carriers, the arresting cables are removed and replaced after each 125 arrested landings. Individual cables are often removed and left "stripped" in order to perform maintenance on other components of the arresting gear during aircraft recoveries (using other, on line, systems). Wire supports raise the deck pendants several inches so that they may be picked up by the tailhook of a landing aircraft. The wire supports on carriers are merely curved steel leaf springs that can flex to allow an aircraft to taxi over the installed deck pendant. On land based systems, "donut" shaped rubber supports raise the cable off the runway surface a minimum of 2 inches (51 mm).

Purchase cables or tapes
The purchase cable is a wire rope that looks very similar to the arresting cable. They are much longer, however, and are not designed to be easily removed. There are two purchase cables per arresting cable, and they connect to each end of the arresting wire. Purchase cables connect the arresting wire to the arresting gear engines and "pay out" as the arresting wire is engaged by the aircraft. As an incoming aircraft engages the deck pendant, the purchase cable transmits the force of the landing aircraft from the deck gear to the arresting engine. On land based systems, heavy nylon tapes are used in place of purchase cables, but they serve the same function.

Sheaves
Purchase cables or tapes run through sheaves in the flight deck or alongside of the runway to the arresting engines. Damper sheaves act as hydraulic shock absorbers that provide for the increased landing speeds.

Arresting Engines
Each pendant has its own engine systems that absorb and dispel the energies developed when a landing aircraft is arrested. Carriers use hydro-pneumatic systems, wherein oil fluid is forced out of a cylinder by a ram (that is connected to the purchase cable) through a control valve. A major development in arresting gear was the constant runout control valve, which controls the fluid flow from the engine cylinder to the accumulator and is designed to stop all aircraft with the same amount of runout regardless of the weight and speed. The aircraft's weight is set by the arresting gear engine operator. The operator is given the weight of the aircraft by the air officer in Primary Flight Control. The operator then sets the Constant Runout Control Valve to the appropriate weight setting for that aircraft. The pressure setting for the arresting gear engine remains at a constant pressure of about 400 pounds per square inch. The constant runout valve (CROV) stops the aircraft (as opposed to hydraulic pressure).

Permanent and expeditionary land based systems usually consist of two arresting engines located on either side of the runway. The arresting engines apply braking force to reels holding the purchase tapes, which in turn slow the aircraft and bring it to a stop. The two most common methods used by land based arresting engines to apply the braking force are the rotary friction brake and the rotary hydraulic (water twister) systems. The rotary friction brake is simply a hydraulic pump coupled to the reel which applies a graduated pressure to multi-disc brakes mounted on the reel. The rotary hydraulic system is a turbine inside a water/glycol filled housing coupled to the reel. The turbulence generated in the water/glycol mixture by the turbine during the arrestment provides the resistance to slow the reel and stop the aircraft. Once the aircraft is released from the cable, the tapes and cable are retracted by an internal combustion engine or electric motor installed on the arresting engine.

Spray-type arrest gear
In 1957 the concept of a piston being pulled through a tube of water was first purposed as a cheap arrest gear system for land airbases. In the early 1960s, the British took this basic concept and developed a spray-type arrest gear system for both land and sea use. The engine had hydraulic cylinders that moved through water filled pipe, with a smaller pipe alongside that has holes of various size along its length. The Royal Navy claimed that there was no theoretical weight limit, but there was a speed limit.

Excessive runout during an arrestment is a condition known as a "two-block." This name is derived from naval parlance when all the line has been pulled through a pulley system, the two pulley blocks are touching, hence "two blocked". Excessive runout can be caused by improper arresting gear settings, excess aircraft gross weight, excess aircraft engagement speed, or excess airplane thrust applied during the arrestment.

Barricade
The barricade is an emergency recovery system used only when a normal (pendant) arrestment cannot be made. The barricade is normally in a stowed condition and rigged only when required. To rig a barricade, it is stretched across the flight deck between stanchions, which are raised from the flight deck. Rigging the barricade is routinely practiced by U.S. carrier flight deck personnel, and good crews can accomplish the task in under three minutes.

The barricade webbing consists of upper and lower horizontal loading straps joined to each other at the ends. Five vertical engaging straps, spaced 20 feet apart, are connected to each upper and lower load strap. The barricade webbing is raised to a height of approximately 20 feet. The barricade webbing engages the wings of the landing aircraft, wherein energy is transmitted from the barricade webbing through the purchase cable to the arresting engine. Following a barricade arrestment, the webbing and deck cables are discarded and the stanchions are lowered back into their recessed slots. Barricade engagements are rare, as tailhooks are designed to be extremely fail-safe, and an aircraft returning from combat with such severe damage would likely not be able to land. This device has been installed on all American aircraft carriers and on French Charles de Gaulle while Brazilian CATOBAR, Russian and Indian STOBAR aircraft carriers have conventional arresting gear installed.