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Aging of U.S. Air Force Aircraft: Final Report
3 Current Structural Status of the Aging Force
When discussing the Air Force's aging aircraft, it is helpful to consider the Air Force-supported aircraft separately from the Air Force commercial-derivative aircraft, which typically use contractor logistics support (i.e., contractor logistics-support aircraft). Both are discussed in the following sections.
AIR FORCE-SUPPORTED AIRCRAFT
Table 3-1 summarizes data on aircraft age and planned future replacements for aging aircraft that are maintained by the Air Force.1 During the course of this study the committee received briefings and written material on these aircraft. Some of the more significant structural problems encountered with these aircraft are discussed briefly below. Additional details on all the aircraft listed in Table 3-1 can be found in Appendix A. The B-2 bomber, the F-117 attack aircraft, and the C-17 airlifter are excluded because of their relatively recent introduction into the force. Also, because of time, budget, and technical considerations, the committee elected to exclude the H-1, H-53, and H-60 rotorcraft from this study. Helicopters are somewhat unique in that dynamic excitations in the rotor systems (i.e., causing combined high-and low-cycle fatigue) have been at the root of many past structural problems, and as such the committee suggests that this subject could best be addressed separately.
The Air Mobility Command's (AMC) airlifter and tanker aircraft listed in Table 3-1 were designed based on the fail-safe approach, and, as a result, the primary safety concern with regard to aging is the loss of this fail-safety from the onset of widespread fatigue damage (WFD).2 In fact, both the KC-135 and the C-5A had their original lower wing surfaces replaced in the 1970s and early 1980s because of WFD, and the wings of the C-141 have more recently undergone extensive modification because of WFD (i.e., use of boron composite doublers to repair and prevent further cracking at the weep holes in the lower wing surface risers). In addition, some C-141s are now experiencing WFD in the lower wing surface spanwise slices. Risk analyses performed by the aircraft manufacturer have shown that these splices reach the onset of WFD at about 37,000 flight hours. As shown in Table 3-1, the current plan is to retire all of the C-141s within the next eight years; however, this plan was based on an aircraft retirement time of 45,000 flight hours. For any aircraft that must be flown more than 37,000 hours before retirement, extensive and burdensome inspections are required to protect the structural safety. These inspections involve inspecting more than 6,000 fastener holes per aircraft every 120 days. The committee is not aware of any data to indicate that the KC-135 or the C-5 will experience the onset of WFD in the near future. For example, a blue ribbon panel reviewed the KC-135 during 1996 and concluded that the current data indicate that the aircraft could likely be flown to beyond the year 2040 before encountering WFD. However, the panel recommended some additional actions to improve this estimate and emphasized the need to control the present corrosion and stress corrosion cracking problems.
The Air Combat Command's (ACC) fighter, bomber, and attack aircraft and the Air Education and Training Command's T-37 and T-38 trainer aircraft were either designed to be damage tolerant using the safe crack growth concept or were later analyzed on the basis of crack growth to establish safety limits and inspection requirements. This was accomplished during the durability and damage tolerance assessments (DADTAs) that were performed on these aircraft. Although some of these aircraft have some inherent fail-safety resulting from redundancy in load paths and crack arresting features or because of battle damage requirements, they do not meet the fail-safe standards of the large transport aircraft. As such, with increasing age the primary threat to their structural safety is the growth in fatigue-critical areas and the potential of missing one or more of these areas. As noted in Chapter 4 and Appendix A, there already has been a significant increase in the number of critical areas in the F-16 since its introduction into service in 1979. Also, based on the number of cracking locations currently being reported in the T-38 and A-10 (see Appendix A), it appears that this is also true for these aircraft.
The T-38 is of particular concern because of its single-plank lower wing skin, its very small critical crack sizes (i.e., 0.20 to 0.40 in.), and the age of the aircraft in terms of both calendar years and flight hours. Wing failure and
An exception is the U-2, which was developed for the government and maintained by the contractor.
See Chapter 4 for a discussion of technical issues associated with aging aircraft, including widespread fatigue damage.