swilliam@dtoa1.dt.navy.mil (Williams) (02/04/91)
From: swilliam@dtoa1.dt.navy.mil (Williams) The B-52 Engineered To Endure Condensed from an article in the MECHANICAL ENGINEERING issue, January 1990 Part 2 of 3 A New Life ---------- The modifications performed to keep the B-52 not only flying, but also capable of performing ever more complex missions while being able to survive constantly more sophisticated enemy threats were rather impressive. The B-52 was originally designed with a life of 5000 flying hours. This was quite appropriate since prior to the B-52, bombers became obsolete long before they wore out. Through some extensive modifications, an ongoing structural integrity program, and many more minor modifications, B-52s have been able to accumulate many times the 5000 hours originally planned. Early on, it was discovered that the wings on the B-52G and H did not have a long structural life because these models used 7178 aluminum in lower wing to reduce weight. It was necessary to redesign the wing using more durable and damage-tolerant 2024 aluminum. Replacing the wing structure in all the G and H aircraft was a major operation that extended from 1959 through 1964. In the Pacer Plank program starting in 1972, 80 of the best B-52Ds in the fleet were chosen for intensive modification based on number of total flying hours, number of hours flown at low altitudes and during refueling, and battle damage sustained. These aircraft were essentially taken apart, with over 8000 parts removed. Major portions of the wings and fuselage were rebuilt using redesigned components. The wings were given new skins, structural components, and leading- and trailing-edge assemblies. The center portion of the fuselage was also reskinned. As part of other modification programs, major portions of the fuselage, tail empennage, and other critical parts of the structure were strengthened or replaced. One modification resulted in fairings called strakelets on the leading edge of each wingroot of all B-52Gs that carried Air Launched Cruise Missiles (ALCM). This modification allows Soviet satellites to easily keep count of B-52s equipped with ALCMs as part of the SALT II agreement. Since all B-52Hs carry cruise missiles and are easily distinguished from the G model, the strakelets are not needed. The average age of the 262 existing B-52s is 28 years. One of the key ways the Air Force is fighting the affiliations of old age - structural failure and corrosion - is through its Aircraft Structural Integrity Program (ASIP) managed by the Air Force Logistics Command at Wright-Patterson AFB, Ohio. While ASIP has been in existence for three decades, it is continually being updated as new structural analysis and inspection techniques and equipment are developed. ASIP is a two-pronged attack on the problem that includes advanced predictive models combined with inspection of individual aircraft components. The end result are reports prepared by the Aircraft Management Information System at the Oklahoma Air Logistics Center located at Tinker AFB, Oklahoma, that summarize the damage that the structure of a particular aircraft might have sustained. From this information, logistics managers can tell operational users when individual aircraft are approaching critical stress limits or when certain flight regimes could endanger the aircraft and its crew. Maintenance procedures can also be prescribed to extend the total number of flying hours. The ASIP has included extensive inspection for cracks and corrosion in the B-52. This has been augmented by both destructive and nondestructive testing that included loading wings until they collapsed and subjecting almost all B-52Ds to a 100 percent design limit proof loading test. Information from ASIP also plays an important role in determining when modifications are required. The information also helps project how much longer key structures will last under certain flying conditions. For example, ASIP information was used when the B-52 got new wings and strengthening of its structure. One key element of ASIP is the Individual Aircraft Tracking Program that monitors the detailed flight histories of all USAF aircraft. Much of this information is obtained through instrumentation installed on selected aircraft. In the fleet damage monitoring program, special measuring equipment was installed in aircraft to keep track of structural integrity. At one time, the USAF even had a "lead the fleet" program where selected B-52s were flown much more often than the rest of the fleet so future problems could be found before they affected the rest of the B-52 force. Nowhere has the need to modernize been more apparent than in the B-52's electronic systems. Modernization programs have updated "black boxes" and displays in an attempt to keep up with rapid advances in electronic technology as well as to maintain the ability to survive in an increasingly hostile threat environment. The B-52's original systems used vacuum tubes and analog computer technology. Vacuum tube electronics are almost impossible to maintain as the supply sources for their components have virtually disappeared. They are also prone to failure due to in-flight vibration and jolting. The first electronics improvements came with the expansion of the B-52's mission to low-level penetration and the need to fly at an altitude of 500 feet or less. A key element of this modification was the Advanced Capability Radar (ACR) for terrain avoidance that was added beginning in the early 1960s. In the early through mid-1970s, the G and H models received the Electrooptical Viewing System (EVS) that consisted of Low Light TV (LLTV) and Forward-Looking Infrared Radar (FLIR) that, when used with the ACR, meant much safer and effective low-level capability. A Stability Augmentation System was added in the G and H models in the early 1970s to improve controllability in turbulence. Other electronic improvements have included many upgrades to Electronic Countermeasure (ECM) and Defensive Avionics systems. The ability to communicate with the AFSATCOM communications satellite was added starting in the late 1970s. In the early 1980s, a Tail Warning System was added on the B-52G and H to improve radar warning of threat aircraft and missiles as well as automatically triggering chaff and flare dispensing to confuse enemy radar. In 1980, a 10-year $1.5 billion program was begun to replace the analog computer-based bomb navigation system with a modern digital, solid-state Offensive Avionics System (OAS) on the G and H models. The OAS not only reduces life cycle costs, but offers greatly improved performance while maintaining compatibility with other systems on the B-52. A similar though less ambitious improvement was also carried out on the D models.