Military


C-5

Ordered under a totally new procurement concept designed to control costs, the C-5A aircraft ended up costing a small fortune. Its purchase in 1965 depended on achieving an initial operational capability no later than 1969, but the transport did not appear in South Vietnam in a truly operational capacity until August 1971.

The 81st and final C-5A rolled out of Lockheed's Marietta plant on 31 January 1973, and the Military Airlift Command accepted delivery of the aircraft on 18 May 1973.

After the aircraft had been in service several years, a wing tear-down inspection on one aircraft with a high number of flight hours revealed significant cracks. Lockheed proposed several approaches to restore C-5 wing fatigue life to a specified level of 30,000 flying hours. These approaches included (1) an active aileron system to alleviate gust loads on the wing, (2) local wing modifications to improve fatigue, and (3) redistribution of fuel within the wing to reduce bending moments. Tests of the C-5 active lift distribution control system (ALDCS) in 1973 validated the use of active control technology for the minimization of aircraft aeroelastic response. Active ailerons were retrofitted to 77 C-5's in 1975 through 1977.

The active lift distribution control system was superseded by a redesign of the wing that included a new center wing, two inner wing boxes, and two outer wing box sections, which were manufactured from advanced aluminum alloys that were unavailable when the original wings were produced in the late 1960's and early 1970's.

In November 1973, work began on what become known as the H configuration. The design used stronger center and inner wing boxes with a modified outer wing box, while retaining only the leading edges, pylons, trailing edges, and flaps from the original wing. In August 1974, John L. McLucas, the new Secretary of the Air Force, approved the H modification for the C-5A. Though the on-going rebuilding program would not affect the basic aerodynamic shape of the original C-5A wing, changes required to reinforce the wing to the H configuration added 18,000 pounds to each aircraft. This increase amounted to less than five percent of the C-5A's empty operating weight-or tare weight-of 326,962 pounds, but it meant reducing the C-5A's payload weight and fuel capacity by an equivalent amount. Besides extending the wing's flying life to a minimum of 30,000 hours -- the primary objective of re-winging-the program ensured a degree of structural soundness that ended the previous operating restrictions. For example, the C-5A could now carry a payload 190,000 pounds rather than 164,000 pounds. Early in 1980 Air Force Secretary Hans M. Mark reported that tests of a C-5A with the new wings showed that, contrary to expectation, the use of unimproved landing strips could seriously damage the aircraft.

Into the 1980s the C-5A operated under increasingly stringent flying restrictions because the flawed wing structure deteriorated until it had to be replaced. While under these restrictions, the C-5A could carry only 174,000 pounds of cargo, about 100,000 pounds more than the C-141, but 46,000 pounds less than the Galaxy's design objective. Although installation of a heavier new wing would probably prevent the airplane from ever attaining the design capacity of 220,000 pounds, the Military Airlift Command was determined to extend the service life of the C-5A because its performance remained so impressive even with a reduced load. All C-5A aircraft were modified with the new wings. Avco began fabrication of the wing parts in August 1980, and machining operations for major wing panels commenced in September. The entire modification program progressed smoothly, and the 76th and last re-winged C-5A rejoined the operational fleet in mid-1987, as scheduled.



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