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Arresting gear

Arresting gear (A.K.A. "A-Gear") is the term used for mechanical systems designed 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 cable 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 whatever is attached to the ends of the wire. 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 first 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. Henry Fancourt was the first pilot to land using the new system. 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 high net that prevented landing aircraft from crashing into other aircraft parked on the bow. Modern U.S. Navy aircraft carriers have the Mark 7 Mod 3 arresting gear installed, which have the capability of recovering a 50,000 pound aircraft at an engaging speed of 130 knots in a distance of 340 feet. The system is designed to absorb theoretical maximum energy of 47,500,000 foot-pounds at maximum cable run-out.

"Barriers", as described above, are no longer in use, although ground based arresting wires 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 arresting engine(s) 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 (aka arrestor cables, arresting wires, cross deck pendants) strung across the landing area. All U.S. carriers in the Nimitz Class have 4 wires, with the exception of the USS Ronald Reagan (CVN-76), which has only 3. 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. Once the arresting gear stops the aircraft, he brings the throttles back to idle, raises the hook and taxies clear. If the aircraft fails to catch an arresting cable, a condition known as a "bolter", the aircraft has sufficient power to simply continue down the angled flight deck and become airborne again.

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 over-run gear.

Permanent installation

Permanent systems are installed on nearly all U.S. military airfields operating fighter or jet trainer aircraft. Some of these systems can be raised or lowered by air trafic control tower personnel.

Expeditionary

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 and/or uninstalled in only a few hours.

Overrun gear

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.

Components

The major systems that make up typical arresting gear are the hook cable or pendant(s), purchase cables or tapes, sheaves, and engine(s).

Cross Deck Pendant

Also known as arresting cables or wires, these flexible steel cables 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 100 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 gear, 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 along side of the runway to the arresting engines. Damper sheaves act as hydraulic shock absorbers that provide for the increased landing speeds.

Arresting gear 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. Cable tension is set by the arresting gear engine operators for the type of aircraft being arrested. If tension is set too strong, it could severely damage the aircraft, if set too weak, it will not stop the aircraft before a "two-block" occurs. 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. Typically, these systems are able to stop a 50,000 lb aircraft traveling at 180 knots in 950 or 1200 feet.

Two-Block

Excessive runout during an arrestment is a condition known as a "two-block." This name is derived from the two wooden blocks, attached to the ends of the purchase cables, that meet when there is no more cable to pay out past the normal "arrestment complete" position. 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 removed from storage below deck and 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 high. 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 stations are lowered back into their recess slots. Barricade engagements are extremely rare.