Wings do many things. Their first purpose, of course, is to provide aerodynamic lift to get the aircraft off the ground; but they also carry heavy engines, bombs, and fuel pods. Still others carry undersigned-for external ordnance. One other common thing these all important wings do is get tired and fatigued and many times they develop dangerous cracks. Sometimes it happens much sooner than predicted. The Air Force has examples of each type. The first is the venerable 15- to 20-year-old B-52 and, surprisingly the relatively new C-5A.
The oldest of the giant eight-engined bombers still flying, the B-52D, was one of the most numerous of the giant Stratofortresses built. Extensively used in Vietnam, many of these late 1950s versions were sporting well over10,000 flight hours although they were originally designed for only half that number. In 1971, the planned phase-out year for the D-birds, the Air Force decided that the aircraft would be needed much longer. But something had to be done--and in a hurry, since the aircraft were showing their age, large structural cracks appearing with increasing regularity. So it was decided that a portion of the B-52D fleet would get a sizable face lift.
Starting in the mid-1970s, 80 of the D-birds were extensively modified by Boeing, the last modifications completed in February of 1977. These modifications entailed the scrapping of about 15 tons of parts from each aircraft. Then new leading edges and stiffeners were provided. wings were partially reskinned, wing pylons modified, and the fuselage partly reskinned at the wing root. The program, called Pacer Plank, resulted in a price tag of $2.6 million per copy--a large percent of the original cost of the aircraft when it was built two decades earlier.
In the modification process, Boeing removed the wings from the fuselage and worked on the fuselage and wings separately. Following modification, wings and fuselage were rejoined, tested, and sent on their way.
A surprising phenomenon resulted from the Pacer Plank modifications. The new wing skin is much cleaner aerodynamically than the skin it replaced, resulting in considerably less drag. Even though the modified plane weighs about 3400 pounds more, its cruise range has been increased by three percent. After three decades, this aircraft is getting older, but she is also getting better.
The later G and H models are also undergoing structural changes, although not as extensive as those on the B-52D* Not to be forgotten are the 615 KC-135 tankers that are also undergoing wing modifications to stretch their operational lifetime into the 1990s.
* "USAF is considering the use of winglets on its Boeing B-52Gs. The modification would not only enhance aerodynamic performance of the bomber, earmarked as the air-launched cruise missile carrier, but also would provide distinctive markings…of those B-52G models used for cruise missiles and thus avoid having all B-52s counted as potential carriers of this weapon." (Aviation Week & Space Technology, March 6,1978, p. 9).
But wing problems certainly have not been restricted to 20-year-old airplanes. The relatively new C-5 transport fleet is also having its problems. The first indication of C-5 wing structural deficiencies occurred in July1969, when the wing root failed in static test at 126 percent of design limit load. Then, in January1970, cracks in the rear beam cap of the third C-5A built were discovered.
The Air Force Scientific Advisory Board (SAB) was convened in February 1970, to review the C-5 structural failures and plan corrective actions. However, by this time, the fortieth airplane was in final assembly, and major wing parts were already machined for the sixtieth article. It was clearly impractical to incorporate in production a major redesign for improved fatigue life.
So, in 1975, the Air Force awarded a $28,454,000 contract to Lockheed for design of a modification to the C-5A wing. The wing modification involved redesign of the center, inner, and outer wing boxes. The existing leading and trailing edge structures, as well as all wing subsystems, were retained. It was hoped that these changes, coupled with the use of an improved wing material with better fracture toughness characteristics, would ensure a long life wing. The design contract awarded in 1975 was the first phase in a four-phase program and covered the necessary engineering analyses and tests to define the modification details.
The second phase of the modification program called for fabrication of two kits of new wing sections and associated hardware. A C-5 fuselage (ground test article) outfitted with one "kit" would receive extensive fatigue testing. The second wing kit will be installed on a selected C-5A aircraft and flight tested at the Air Force Flight Test Center in the fall of 1980. If approved Phase III, the production of wing modification kits, would be initiated in early 1980. Installation of the wing modification on the remaining C-5 force (76 aircraft) would begin in early 1982 and would constitute Phase IV of the program. The aircraft would be modified at a rate of 1.5 per month, with a downtime of eight months.
As part of Phase IV, an Active Lift Distribution Control System will be reinstalled on the C-5 wing as a fuel conservation measure. It is estimated that this system, which automatically reduces wing loads in flight will save six million gallons of fuel per aircraft over a period of 30 years, based on current usage rates.