Feature article from the September-October issue of Wire Rope Exchange.
One of the most fascinating uses for wire rope has to be in arresting gear—the well-placed cable lying across an aircraft carrier runway that keeps a $50 million fighter jet from speeding off the end of a $12 billion boat. It’s easy to forget about the range of uses for wire rope, especially within the grip of a job—when you need it to do one thing, and do it well. But then you snag a glimpse of it on TV or online, lying there across a carrier deck waiting to hook a 30,000–pound jet, and you realize that what might look simple in form is nothing short of miraculous in function.
Case in point: there’s a lot that goes into catching a plane going 150 mph, and bringing it to a dead stop in under 500 feet. The trigger point that makes this entire process possible is the wire rope within that arresting gear—a mechanical system designed to catch a tailhook on the underside of a jet and transfer the aircraft’s kinetic energy to hydraulic damping systems attached below the deck. These systems, though extremely high-tech and efficient today, took quite a while to perfect—the first shipboard landing of an aircraft using arresting gear was on the USS Pennsylvania (ACR 4), on January 18, 1911. Fast-forward over a century, and without it, our modern floating war machines are pretty much just drifting parking lots with extravagant security systems.
Dennis Scholl, a Process Quality Engineer at WireCo® WorldGroup, spent many years designing rope, reels, and plant tooling as a product engineer, and is very familiar with the major players in this expert space. WireCo®, the world’s leader in manufacturing, engineering, and distributing wire rope, synthetic rope, specialized assemblies, wire products, and electromechanical cable, is also the only company in the U.S. that makes carrier rope. The only other supplier in North America is Wire Rope Industries, out of Canada. Scholl describes a specialized relationship that covers over 70 years. “Our tradition with the military goes back to World War II, when we were under the name Broderick & Bascom Rope. In 1983, we were bought by MacWhyte, which eventually became Wire Rope Corporation of America—and eventually WireCo® WorldGroup.” MacWhyte is now an official WireCo® brand, and has been around since 1896—creating specialized assemblies for aircraft control cables and torque-balanced oceanographic ropes.
As Scholl indicated, carrier rope is a pretty specialized field. Without a doubt, arresting gear on aircraft carriers is one of the most crucial components of naval aviation. Mostly used on Catobar and Stobar carriers, the system will also occasionally pop up on land-based airfields—either as a means to accommodate commercial-military operations, or for emergency use. CATOBAR (Catapult Assisted Take-Off But Arrested Recovery) essentially describes a system that both launches and receives an aircraft, with the catapult sending the plane forward, and hopefully aloft, and the arrestor wires catching it upon landing. STOBAR (Short Take-Off But Arrested Recovery) combines several methods of short takeoff and vertical landing, as well as catapult-assisted and arrested recovery.
An aircraft carrier can launch a plane every 20 seconds from its four catapults. About 300 feet long, the catapults consist of a large piston beneath the deck—while only a small piece of the shuttle system engages the aircraft’s nose gear above deck. After all final checks are made, the pilot fires up the jet’s engines to full power. When the engines are steady at full, the catapult is fired, and the plane accelerates from 0–184 mph in less than two seconds. The shuttle is then pulled back down the catapult by a cable and pulley assembly, and readied for the next launch.
Taking off is ridiculously intense, but landing an aircraft on a flight deck is one of the hardest things a Navy pilot will ever have to do. Many more things can go wrong than right, with the primary objective being to hit one of four parallel cables—woven from high-tensile steel wire—with an extended hook attached to the plane’s tail. The wires are connected at both ends to the arresting engines—large hydraulic devices below deck designed to spool out tensioned wire and absorb the aircraft’s momentum. An arresting wire system can stop up to 60,000 pounds of aircraft—with the breaking strength of the cable at around 250,000 pounds.
Of the four wires, spaced fifty feet apart, a pilot is typically shooting for the third one—which provides the safest option. The first wire is, more or less, an accidental snag—considered far too near the edge of the deck for comfort—while the second wire is acceptable, in theory. The fourth wire obviously gets the job done, and is a last resort. But the third wire is what a good pilot aims to hit every time—the bull’s-eye. And as if this isn’t stressful enough, as soon as the pilot touches the deck, he or she actually has to push the engines to full power (the opposite of slowing down), because if the tailhook doesn’t catch, they need to be able to take off again using a terrifically short piece of runway—referred to as “bolting.”
Obviously, arresting cables on deck (also called cross deck pendants) are incredibly strong ropes—that also maintain a surprising flexibility—each twisted around an oiled hemp core that provides cable lubrication and serves as a cushion for each strand. Cross deck pendants are replaced every 125 arrests, and common types vary from 1-inch (25mm) to 1 ¼-inch (32mm) to 1 ⅜-inch (35mm) diameter. Scholl personally designed a 1-7/16-inch rope in 1995 and WireCo® has been using it on awarded contracts since 1998. “We actually make two main types of arrestor cables for the U.S. The 1-7/16-inch is a special Flattened Strand that is very strong, and has a smooth surface to catch the tailhook. Our ground-based 1-¼-inch rope has a different design, and is used for training and emergency landings around the world.”
Because of their specialized design and use, both WireCo® ropes are tested to military specification, and are considered Critical Safety Items. “There are both Defense Contract Management Agency and Lakehurst Quality inspectors that witness portions of the manufacturing tests,” Scholl adds. “Testing of the components of wire, cordage, and the finished wire rope are done to qualify the product for government acceptance. You have to keep in mind that the lives of the crew and ground crew are depending on this rope performing safely.”
And if you’ve ever seen a carrier deck in action while planes are landing and taking off, a couple of things are immediately noticeable: this job is extremely dangerous for everyone involved, and this arresting system needs to be extremely precise in every way.
Essentially, the system is made up of the wire rope hook cables (pendants), purchase cables (tapes), sheaves, and arresting engines. The ends of the cable can actually be detached quickly, for replacement—equipped with terminal couplings (cables can be changed out on a carrier in two to three minutes). Wire supports—which are simply flexible, curved steel leaf springs—raise the pendants a few inches so they can be more easily hooked by a landing aircraft. A “donut” support is used to raise the cable on a land-based arrest system—usually a minimum of two inches.
Connected to the arresting cable is the purchase cable. These wires are much longer and are not easily removed. There are two purchase cables per arresting cable—on opposite ends of the wire. When the aircraft engages the pendant with its tailhook, the purchase cables “pay out” by transmitting the force of the landing aircraft from the arresting gear to the arresting engine. The purchase cables run through “sheaves,” either within the flight deck or along the runway—to the arresting engine. These sheaves serve as a hydraulic shock absorber that accommodates the landing speed of the aircraft.
Every pendant (deck cable) has its own engine system that absorbs and dispels the energy accumulated when an aircraft lands. Aircraft carriers use what is known as a hydro-pneumatic system—oil fluid is forced out of a cylinder by a ram connected to a purchase cable. The fluid is pushed through a control valve—a system described as the “constant runnout control valve”—whereby any aircraft, regardless of weight or speed, can be stopped with the same amount of runnout. The constant pressure for arresting gear is set at about 400 pounds per square inch, and this constant runnout valve actually stops the aircraft, rather than hydraulic pressure. As the aircraft assumes its approach, its weight is recorded by the arresting gear engine operator—having received it from Primary Flight Control. The constant runnout control valve is then adjusted to the setting for that particular aircraft, and the system all but takes care of itself.
Through the years, carriers, and their amazingly efficient systems, continue to evolve. Scholl has noticed an increase in rope size as jets get bigger and faster, and even sees the deck cable’s role changing in time. “Eventually, synthetic (polymer) rope may be used. The use of other types of planes that do not need a runway will also reduce the need for arrestors.”
Until then, the arresting system itself is about to get a major facelift. The USS Gerald R. Ford (CVN 78), is expected to join the Naval fleet in 2016, and will be fitted with the newest system—the Advanced Arresting Gear (AAG). This high-level system will employ the most technologically proficient digital arresting components and controls on the planet—and is designed to recover aircrafts of any size. The AAG’s benefits will be numerous, and will set a new standard with built-in test and diagnostic capabilities—providing for less operational maintenance and manpower, and increased safety and reliability.
In the modern age, perhaps nothing is more advanced than an aircraft carrier. And these floating cities of maintenance and might would certainly have an entirely different purpose if it wasn’t for a familiar piece of twisted steel that most of us have come to know simply as, wire rope.