Chain Reaction: The Barrier Between Life and Death

The Chain Reaction article, published in The MATS Flyer, August 1955, highlighted the first time a land-based aircraft arrestment occurred. Following lessons learned during the Korean War, a need for a reliable land-based aircraft arresting system (AAS) arose.

The Korean War presented tremendous challenges for USAF engineers. Between World War II and the Korean War, several new aircraft were introduced into the inventory. The newer aircraft required longer and wider runways, larger taxiways and parking aprons, and more stringent design criteria for gradients, clear zones, and pavement thickness. Larger fuel storage and munitions facilities, as well as more maintenance and support facilities, were needed. Engineers had to accommodate ever-larger cargo aircraft and jet aircraft.

Even with longer and wider runways, jet aircraft continued to be lost during takeoff and landing operations. The USAF decided to test the application of an aircraft arresting barrier system similar to barriers used on aircraft carriers. The system adopted by the USAF, the MA-1A, consisted of retractable stanchions that held the barrier in position and a heavy anchor chain to decelerate the aircraft when it engaged the barrier. During the first 6 months, 36 engagements were recorded with little or no damage to the aircraft. This success did not last long, however, as heavier and faster aircraft were put into service.

THE B-52S UNDERGROUND

No, we are not talking about unreleased tunes from the 80s pop band. This B-52 is the B-52 Stratofortress! What do they have to do with arresting systems?

John Sherman Strance was a U.S. Navy engineer who worked on shipboard arresting gears during WWII. The U.S. Navy contracted with E.W. Bliss Company in 1953 to assemble a group of experienced Catapult and Arresting Gear technicians under the direction of John J. Byrne and Robert W. Cruger in Philadelphia. The group provided support aboard ships, at shipyards, and at various Navy facilities. In 1954, Strance was hired as the resident engineer at the Puget Sound Naval Shipyard, where one of the first steam catapults was being installed and tested onboard the USS Hancock. After 1 year, Strance was returned to Philadelphia to work with Byrne and Cruger on a new venture to develop a safe and reliable, land-based arresting system. “We felt that world tensions would push the Air Force to base many new jet-powered aircraft at WWII airfields considered too short for safe operation. It was further believed that extensive experience in naval aircraft launching and recovery would provide a strong base from which to explore emergency aircraft overrun equipment.†-John Strance.

“BAK†TO THE FUTURE

Strance is credited for inventing the (Model 200) BAK-9 rotary friction arresting gear system, patented in 1961. The BAK-9 replaced chains as the energy absorber for the MA-1A to accommodate hook engagements. This innovation was followed by the BAK-12 system, with increased energy capacity from 1962 onward. The BAK-12 remains the standard USAF aircraft arresting system today. It consists of two hydraulically actuated rotary friction energy absorbers (B-52 brakes), one on each side of the runway, which are connected by a cross runway hook cable or a net barrier. The systems are designed to safely stop an aircraft in an emergency or at airfields where conditions are not present for conventional landings. If an AAS is required during landing, the pilot deploys the aircraft’s arresting hook to catch a cable suspended above the runway surface. As the aircraft’s hook grabs the cable and continues forward, the cable unspools a thick nylon tape from a storage reel on an AAS absorber secured on each side of the runway. These reels are keyed to a common shaft that utilizes two four-rotor hydraulic (modified B-52) brakes. Originally, BAK-12 energy absorbers were fitted with a 60-inch purchase-tape storage reel. This design allowed the maximum energy expected to be imparted during an aircraft engagement to dissipate within a runout of 290 meters (950 feet) plus the length of the aircraft. Bliss later improved the BAK-12 to meet the increased demands of heavier and faster aircraft.

The BAK-12 absorber uses a static pressure accumulator to hold 175 pounds per square inch of pressure on the brakes. When the aircraft’s pull exceeds the static pressure, the braking system switches to an integral hydraulic pump. As the aircraft slows to a stop, the reels turn more slowly, reducing the hydraulic pressure on the brake and preventing a “slingshot effect†at the end of the 1,200-foot runout. The engagement system (BAK­14 or Type H) is a bidirectional hook cable (pendant) support system used in conjunction with the BAK-12 or a comparable pair of arresting system absorbers to engage and safely stop a hook-equipped aircraft. The BAK-15 aircraft arresting barrier consists of a pair of electro-hydraulically powered steel masts that provide support and remote-controlled movement for a unidirectional nylon net barrier.

Other arresting systems in service within the USAF include textile brakes, which are specially woven textile tearing straps that absorb the kinetic energy generated during an engagement. Another option is a soft ground arresting system, such as the Engineered Material Arresting System, which Air Mobility Command (AMC) aircraft will encounter at commercial airports. It is a Federal Aviation Administration-approved soft ground system generally used for civil airports to mitigate short safety areas (less than 305 meters [1,000 feet] long) at runway ends.

SAFETY FIRST

The Air Force mandates that barriers must be overhauled or replaced every 10 years or 500 engagements. The Civil Engineer Maintenance Inspection and Repair Team (CEMIRT) has been the Air Force’s premier center for the overhaul and repair of AAS equipment, including the BAK-12, BAK-14, and Mobile Aircraft Arresting Systems (MAAS). CEMIRT is a division of the Air Force Civil Engineer Support Agency, headquartered at Tyndall Air Force Base (AFB), FL, with operations at Tyndall and Travis AFB, CA.

WE GOT YOUR “BAKâ€

This proven system provides high reliability under all operational conditions and is used across AMC and at any location where arresting systems are required. Some of the latest systems are electronic/hydraulic computer-controlled systems that directly and continuously measure aircraft location and velocity for automatic braking adjustment for variable run-out conditions—ideal for joint exercises and operations. The BAK-12 AAS may be installed on concrete pads above or below grade or in pits to suit specific site requirements or be trailer-mounted for mobility and transportability.

MAAS can be deployed anywhere in the world where fighter operations may be required from short runways or even those damaged by airstrikes. The MAAS is a pair of mobile units, each unit consisting of a BAK-12 arresting barrier mounted on a mobile trailer. The MAAS was originally developed and tested to accommodate the recovery of fighter aircraft that had to return to a battle-damaged airfield. In its unidirectional configuration, with either a 90- or 153-foot cable, the MAAS can be installed in as little as 2 hours. It can stop fighter aircraft in a 990-foot or 1,200-foot run-out zone, respectively. Each trailer contains a fixed BAK-12-based arresting system and all the tools and hardware necessary for installation. The system is air transportable.

AIR MOBILITY COMMAND

Many AMC installations and Civil Engineer Squadrons maintain arresting systems, but why? AMC aircraft are not tailhook equipped. It is not like the KC-10 will be arrested by its refueling drogue during an overrun anytime soon. Runways that are primary divert facilities for bases operating tactical or training tailhook­ equipped aircraft will typically have an emergency system in each overrun and an operational system on each end of the primary runway. Bases that are occasional hosts to arrestment­ capable transient aircraft should have an emergency system installed in each overrun of the primary runway or an operational system on each end of the primary runway.

RAMSTEIN AIR BASE, GERMANY

An annual recertification of the barrier arresting kit was conducted by the 786th Civil Engineer Squadron (CES) and 435th Construction and Training Squadron (CTS) at Ramstein Air Base (AB), Germany, February 9, 2021.

An F-16 Fighting Falcon assigned to the 52nd Fighter Wing, Spangdahlem AB, Germany, provided the fighter aircraft to support the annual certification of the aircraft arresting system. The two-part system is there for an aircraft experiencing an inflight emergency issue that prevents the pilot from safely landing. The BAK­14 is the support system, whereas the BAK-12 is the braking unit attached to nylon tapes connected to the cable that catches tailhook-equipped aircraft. As the aircraft transits the runway, it pulls the tape off the reel, which slowly increases the system’s hydraulic pressure. This gradual increase in hydraulic pressure is what brings the aircraft to a safe and controlled stop.

“The system is capable of catching 65 million foot-pounds at about 180 knots,†said TSgt Robert Miller, 435 CTS command AAS supervisor.

“It will pay out about 1,200 feet of tape when engaged.†One foot-pound translates to the amount of kinetic energy it takes to move a one-pound object one foot. “It provides safety and security to any tailhook­equipped aircraft which may need to make an emergency landing,†said TSgt Bryce Skawski, 786 CES power production craftsman.

Ramstein serves as the gateway to the world and is identified as a stopping point, or divert base, for a variety of transient aircraft. The airfield is only one of several rated for fighter aircraft in the European theater. “It [BAK-12/14] enables any mission tied to a fighter aircraft by giving them an additional area to land if another runway is unavailable,†Miller said.

Certification of the BAK system reinforces Ramstein’s partnerships with other bases and provides opportunities for personnel to become more familiar with the equipment and reset procedures.

Emergency landings occur far more often than most would care to believe. Sometimes they are more precautionary or due to environmental system issues or warning lights. Other times they occur due to far more invasive problems, such as when the loss of hydraulics or engine problems raise the stakes and prompt the pilot to drop the arresting hook for a barrier-assisted recovery.

With BAK-12 systems installed worldwide, we do not know how many total aircraft and precious lives have been saved, but it would be an impressive number. Safran Aerosystems, the original equipment manufacturer, claims that, with 5,000 active systems deployed in 72 countries, the BAK-12 stops an average of five aircraft per day.