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Aging of U.S. Air Force Aircraft: Final Report Executive Summary The U.S. Air Force has many old (20 to 35+ years) aircraft that are the backbone of the total operational force, some of which will be retired and replaced with new aircraft. However, for the most part, replacements are a number of years away. For many aircraft, no replacements are planned, and many are expected to remain in service another 25 years or more. To varying degrees, all of these older aircraft have encountered, or can be expected to encounter, aging problems such as fatigue cracking, stress corrosion cracking, corrosion, and wear. Through the Aircraft Structural Integrity Program (ASIP) and through durability and damage tolerance assessments (DADTAs) of older aircraft, the Air Force has already identified many potential problems, developed individual aircraft tracking programs, developed force structural maintenance plans, and taken maintenance actions to ensure safety and extend aircraft life. The Air Force has also initiated an aging aircraft research and development (R&D) program intended to support ASIP and address identified needs in the areas of widespread fatigue damage, corrosion-fatigue interactions, structural repairs, dynamics, health monitoring, nondestructive evaluation and inspection, and various aircraft subsystems. The National Research Council Committee on Aging of U.S. Air Force Aircraft was formed to (1) identify Air Force aging aircraft needs and an overall strategy that addresses these needs and (2) recommend and prioritize specific technology opportunities, complementary to the efforts of industry, the Federal Aviation Administration, the National Aeronautics and Space Administration, and international organizations. The topics asked to be considered by the committee include fatigue, corrosion-fatigue interactions, and stress corrosion cracking; corrosion prevention and mitigation; nondestructive inspection; maintenance and repair; and failure analysis and life prediction technologies. This report provides the committee's findings, including (1) a description and assessment of the Air Force aging aircraft problem and the force management process, (2) a detailed summary of the structural status of the aging force,1 (3) a discussion of key technical issues and R&D needs, (4) a recommended overall strategy to address the Air Force aging aircraft problem, (5) recommendations for near-term engineering and management actions, and (6) prioritized near-term and long-term research recommendations. The committee's primary focus was on the deterioration of the metallic alloys used in the Air Force aging aircraft. Emerging issues concerning polymeric composite primary structures used in the Air Force's newer aircraft (e.g., the B-2 and F-22) are discussed in the final chapter of the report. CONCLUSIONS The challenge to the Air Force management and technical community is to meet the following objectives related to aging aircraft: Objective A. Identify and correct structural deterioration that could threaten aircraft safety. Objective B. Prevent or minimize structural deterioration that could become an excessive economic burden or could adversely affect force readiness. Objective C. Predict, for the purpose of future force planning, when the maintenance burden will become so high, or the aircraft availability so poor, that it will no longer be viable to retain the aircraft in the inventory. Safety The structural safety of the Air Force's aircraft is vitally dependent on damage tolerance requirements that have been imposed through military standards and specifications as part of ASIP. These requirements allow the designer to use either of the following two design approaches: Fail-safe design. This approach, which relies on multiple, redundant load paths or crack arrest features, is used in commercial aircraft design and for most of the Air Force's large aircraft. Safe crack growth design. This approach has been used for much of the structure in high-performance combat aircraft where weight is a significant consideration. Engineering analysis must demonstrate that the maximum probable nondetectable initial manufacturing flaw will not grow to critical size (i.e., the size required to cause failure) in any critical structural area during the operational life of the aircraft. 1 Appendix A contains synopses of structural histories of Air Force-supported aircraft.
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Aging of U.S. Air Force Aircraft: Final Report The committee concludes that, with increasing age and with changes in operation (or aircraft configuration) that increase the severity of the operational stress spectrum, the primary threats to structural safety arise from the onset of widespread fatigue damage (WFD) in fail-safe-designed structures the inexorable increase in the number of fatigue-critical areas in safe-crack-growth-designed structures and the potential for missing new areas as they develop The primary technical needs for fail-safe designs are improved methods of predicting the onset of WFD in an accurate and timely manner. This involves the prediction of initiation and growth of small fatigue cracks (or the interpretation of full-scale fatigue test data and service fatigue data), the prediction of fail-safe residual strength, and the evaluation of the potential effects of environmentally induced corrosion on crack initiation and growth and residual strength. development and implementation of nondestructive evaluation (NDE) techniques that can rapidly detect small fatigue cracks over large areas of the structure prior to the onset of WFD. Methods to detect second-or inner-layer cracks and hidden corrosion that could lead to the initiation of cracks would be included. The primary technical needs for safe crack growth structural designs are to identify the next most probable fatigue-critical areas in the structure through careful evaluation of past full-scale fatigue test results, service experience, service loading data (including dynamic loads), design details (including potential areas for hidden corrosion), and the results of stress analyses and strain surveys to perform simulative testing and crack growth analyses to establish safety limits and safety inspection requirements for all critical areas to investigate the potential effects of corrosion on those factors that could affect safety limits and safety inspection requirements to continue to improve methods of identifying fatigue-critical areas and flight load conditions to continue to improve NDE techniques that are sensitive enough to detect small cracks in multilayered and hidden structures to support safety inspections Economics and Readiness The economic burden associated with the inspection and repair of fatigue cracks can be expected to increase with age until the task of maintaining aircraft safety could become so overwhelming and the aircraft availability so poor that the continued operation of the aircraft is no longer viable. In addition, corrosion detection, repair, and component replacement can add significantly to or, in some cases, dominate the total structural maintenance burden. The committee concludes that the major emphasis of the Air Force's technical and force management with regard to corrosion and stress corrosion cracking (SCC) should be focused on the early detection of corrosion and the implementation of effective corrosion control and mitigation practices so as to drastically reduce unscheduled repairs and replacement costs and aircraft downtime. Key technical issues and operational needs include the development of improved NDE techniques for the detection and rough quantification of hidden corrosion the classification of corrosion severity to provide guidance for maintenance the generalized application of corrosion-preventive compounds and the development of corrosion-preventive compounds that can be applied on external surfaces to protect unsealed joints and fasteners the development of a material and process substitution handbook and engineering guidelines for the replacement of components exhibiting corrosion and SCC with more-resistant materials and processes the development and application of materials and processes to inhibit SCC the development of technologies for the removal, surface preparation, and reapplication of surface finishes with improved corrosion-resistant finishes on existing aircraft the assessment of the potential use of the dehumidified storage of aircraft, where practical The committee believes that fatigue cracking will occur eventually on all aging aircraft as flight hours increase. From an economic standpoint, the major impact for a fail-safe-designed structure occurs with the onset of WFD. For safe-crack-growth-designed structures, the major impact occurs when the structure exhibits a rapid increase in the number of fracture-critical areas. In both cases, a choice must be made to undertake major modifications, structural replacement, or retirement. Although it may not be possible to avoid reaching this point for any given aircraft, operational changes such as fuel management, gust avoidance, active or passive load alleviation systems, reduced pressurization, and flight restrictions to minimize flight in severe mission segments can reduce the rate of fatigue damage and delay expensive repair-replace-retire decisions. For aircraft that are approaching their economic service limit, these options should be considered to allow time for modification or replacement acquisition programs.
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Aging of U.S. Air Force Aircraft: Final Report Force Management and Predicted Economic Service Life The Air Force modernization planning process includes the essential elements for force structure planning and management, but, to be completely effective, it should significantly improve estimates of the probable economic service life of aging aircraft systems. There is no clear definition of all of the cost elements that contribute to the economic service life of an aircraft, nor is there a precise methodology for estimating when the costs of operating and maintaining a system will be high enough to warrant replacement. The committee believes that the development of an estimate of economic service life with metrics that integrate the effects of structural deterioration (i.e., from fatigue and corrosion) with economic considerations is essential to force management. Future Structural Issues Metallic alloy structures make up the vast majority of the airframes in the Air Force aging aircraft. However, more-recent aircraft have significant quantities of primary flight controls (C-17) and primary airframe structures (B-2, F-22) constructed from carbon-fiber-reinforced polymeric composites. Although limited Navy and commercial aircraft service experience with composite laminate primary structures has indicated very few occurrences of damage in primary structures, the Air Force needs to continue to monitor the performance of their composite components. Potential degradation mechanisms to monitor in the future for composite structural applications include (1) the development of transverse matrix cracking resulting from mechanical, thermal, or hygrothermal stresses; (2) the growth of impact damage under fatigue loading; (3) the growth of manufacturing-induced damage, especially from fastener installation; or (4) the development of corrosion in adjacent metal structures. The committee recommends that the Air Force undertake long-term research to monitor potential deterioration of composite structures, including the development of improved NDE methods, and to develop or improve maintenance and repair technologies, especially for composite primary structures. RECOMMENDATIONS The committee recommends that the Air Force adopt a three-pronged strategy that includes (1) near-term engineering and management tasks, (2) a near-term R&D program, and (3) a long-term R&D program. Engineering and management tasks are near-term actions (within three to five years) to improve the maintenance and force management of aging aircraft. Supporting the near-term engineering and management tasks are the near-term R&D efforts that the committee believes should be performed under the direction of Air Force laboratories or by supporting contractors and academic institutions. The long-term R&D program includes those efforts that the committee believes will take longer than three to five years to achieve a mature technology that could be adopted by industry or the Air Force air maintenance organizations but nevertheless should be initiated now or continued if they already have been initiated. Near-Term Engineering and Management Tasks The Air Force postproduction force management process, involving the implementation of inspections and modifications derived from the ASIP tasks and the results of DADTAs, has been a huge success in protecting the structural safety of the force aircraft for more than two decades. However, the committee is concerned that the extended use of old aircraft, coupled with the potentially adverse effects of reduced military budgets; reduced manpower; grade structure limitations; increased reliance on contractor maintenance; the elimination or relaxation of military regulations, standards, and specifications; and possible complacency of Air Force management, may make this past success rather fragile. The committee believes that it will take aggressive Air Force management and engineering actions to counter this deterioration in capability and loss in ASIP oversight and to prevent further deterioration in the future. The Air Force should continue to enforce ASIP and maintain sufficient resources to track the force aircraft, to keep DADTAs up to date, and to keep corrosion and stress corrosion cracking from becoming a structural safety issue. Also, sufficient resources should be maintained in R&D to support and improve the aging aircraft engineering, inspection, and maintenance and repair activities. The committee identified the following engineering and management tasks. With the exception of the technology transition task, which is considered to be a continuous effort throughout the life of a weapon system, all of the near-term engineering and management tasks should be accomplished within five years. Update of durability and damage tolerance assessments (DADTAs). The committee recommends that the DADTAs of Air Force aircraft be periodically updated. In general, an update about every five years is appropriate. For Air Force-supported aircraft, the aircraft that should be given highest priority for DADTA updates include the A-10, F-16, U-2,2 and T-38. The contractor logistics-supported, large commercial derivative aircraft that should be given highest priority for structural review are the high-use C-18, C-22, and possibly the VC-137. In addition, the committee recommends that damage tolerance surveys be conducted for utility and commuter class commercial-derivative aircraft to determine the need for DADTAs. 2 The U-2 was developed for the government and is logistically supported by Lockheed-Martin.
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Aging of U.S. Air Force Aircraft: Final Report Update of force structural maintenance plans and individual aircraft tracking programs. Following the completion of the updates of the DADTAs (1) the inspection and modification requirements in the force structural maintenance plans should be updated to reflect any changes in the baseline operational spectra and any additional critical areas that were identified, and (2) an individual aircraft tracking program for each aircraft weapon system should be updated to reflect additional critical areas that need to be tracked. Stress corrosion cracking (SCC) assessments. The committee recommends that the Air Force include an assessment of the vulnerability of each of their aging aircraft to structural failure caused by SCC or SCC combined with fatigue as part of the proposed DADTA updates. Specifically, it is suggested that (1) stress corrosion critical areas be identified based on past service experience, the susceptibility of the materials to SCC, grain orientations, and probable levels of both applied and residual stresses; (2) an evaluation be made of potential failure modes and consequences of failure for each stress corrosion critical area; and (3) protection, inspection, modification, and replacement alternatives be developed as necessary. Improved corrosion prevention and control programs. The committee recommends that the Air Force (1) perform an internal audit of each of the commercial-derivative aging aircraft to ensure that the corrosion control programs are in full compliance with the mandated programs for the commercial counterparts; (2) review the detailed corrosion control programs of each of the Air Force developed aging aircraft and upgrade them as necessary to a level equivalent or better than the mandated programs for commercial aircraft; and (3) evaluate the applicability and cost effectiveness of dehumidification to reduce the likelihood of corrosion. Economic service life estimation. The committee recommends that the Air Force make a concerted effort to develop a credible service life estimation methodology, analogous to the cost and operational effectiveness analysis that is done early in a weapon system acquisition cycle, as the authoritative guide for supporting replacement decisions and budget inputs. Continued enforcement of ASIP. ASIP, as enforced through MIL-STD-1530 and supporting specifications, will no longer be placed on aircraft acquisition and modification contracts because of initiatives to reduce the use of government specifications in acquisition programs. The committee recommends that the Air Force take the lead in pursuing the development of a National Aerospace Standard to establish enforceable consensus industry standards for ASIP. Technical oversight and retention of technical capabilities. Reductions in technical capabilities and technical oversight should be addressed by (1) forming an aging aircraft engineering resources group to examine and develop solution options to engineering skill deficiencies (quantity and quality) in each of the aging aircraft disciplines, (2) forming an aging aircraft technical steering group to monitor and provide guidance to the various recommended near-term engineering and near-and long-term research activities discussed in this report, (3) forming five technical working groups (i.e., one for each of the five topical areas in the proposed near-term and long-term R&D programs) to provide the technical link from basic research through implementation, and (4) appointing a single knowledgeable and experienced technical leader responsible for the oversight of the aging aircraft engineering and the near-term and long-term R&D activities. Technology transition into aging aircraft. The committee recommends that generic aging aircraft technology programs with potential for wide application not be approved through the Air Force technology master process unless there is a clear link to an appropriate technology implementation program. It is critical to the success of the aging aircraft program that a seamless funding-budgeting link be created from development through application. Near-Term and Long-Term Research The committee developed prioritized recommendations for near-term (to support near-term engineering actions in the next five years) and long-term (more than five years until implementation) R&D in five program areas: Fatigue (including low-cycle fatigue, high-cycle fatigue, and corrosion/environmental effects): despite efforts by the committee to develop research initiatives to improve the current approach to identify new fatigue-critical areas (i.e., analysis of full-scale fatigue test data and service experience), no viable near-term or long-term R&D activities were identified. The committee emphasizes the extreme importance of using all available data and up-to-date stress analysis methods to accomplish this task during the recommended DADTA updates, particularly for safe-crack-growth- (i.e., non-fail-safe-) designed aircraft. in the area of low-cycle fatigue of fail-safe designed aircraft, the committee recommends near-term R&D to assess the validity of (and if necessary, develop improvements to) the current approach to estimate the onset of WFD and longer-term R&D to analytically predict the initiation and growth of cracks to the sizes at onset of WFD
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Aging of U.S. Air Force Aircraft: Final Report in the area of high-cycle fatigue, the committee recommends near-term R&D to improve methods to determine dynamic response and long-term research to characterize threshold crack growth behavior, develop an analytical prediction of dynamic response, develop expert systems for the design and analysis of repairs, and develop dynamic load monitoring and alleviation in the area of corrosion/environmental effects, the committee recommends near-term R&D to assess the effects of prior corrosion on the fatigue crack growth and fracture behavior of airframe structural components and long-term fundamental research to provide an understanding of corrosion degradation mechanisms Corrosion prevention and control. The committee recommends near-term program emphasis on corrosion detection and maintenance technology (i.e., how to deal with existing corrosion) and longer-term emphasis on the fundamental understanding of corrosion and characterization of corrosion rates and the development and institutionalization of corrosion prevention and control practices. Stress corrosion cracking. The committee recommends that near-term R&D focus on developing data and documenting results that would lead to affordable upgrades in SCC prevention and component repair and modification procedures. The recommended focus of the long-term R&D is on establishing fundamental materials and microstructural effects on SCC susceptibility and a basic understanding of SCC mechanisms to support efforts in SCC prevention. Nondestructive evaluation. The committee recommends that near-term R&D emphasize the implementation of advances from related government and industry programs and an evaluation of NDE reliability for current methods as they apply to aging aircraft. Long-term R&D would emphasize the development of new NDE equipment and the application of computational methods and simulations in the development and evaluation of inspection techniques. Maintenance and repair. The committee recommends that the primary focus of the near-term programs be to apply the lessons learned from recent programs (e.g., C-141 and battle damage repair) for use at Air Force maintenance organizations. The recommended long-term focus is on the development of analytical design, structural assessment, and life prediction tools for repairs and repaired structures and to develop improved damping materials for repair of structure prone to high-cycle fatigue. TABLE ES-1 Priority-1 Near-Term and Long-Term Research Recommendations Recommendation Description Objective Timing Fatigue None Corrosion Prevention and Control Evaluate durability of new protective coatings Page 58 B Near term Basic research in corrosion prevention and control Page 59 B Long term Basic research in coating durability Page 60 B Long term Stress Corrosion Cracking Affordable upgrades in SCC prevention Page 60 B Near term Evaluation of SCC protection systems Page 60 B Near term Fundamental research in SCC prevention Page 61 B Long term Nondestructive Evaluation Evaluate, validate, and implement NDE equipment and methods Page 64 B Near term Develop integrated quantitative NDE capability Page 66 B Long term Automation of wide-area NDE inspections Page 68 B Long term Maintenance and Repair None
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Aging of U.S. Air Force Aircraft: Final Report TABLE ES-2 Priority-2 Near-Term and Long-Term Research Recommendations Recommendation Description Objective Timing Fatigue Fail-safe residual strength prediction methods Page 50 A Near term Improve current methods to estimate onset of WFD Page 50 A Near term Methods to predict dynamic responses Page 52 B Near term Effect of joint pillowing on fail-safety Page 55 A Near term WFD crack formation and distribution relationships Page 50 A Long term Analytical prediction of WFD crack distribution functions Page 51 A Long term Validation of analytical WFD methods Page 51 A Long term Crack growth threshold behavior Page 52 B Long term Analytical methods to predict dynamic behavior Page 53 B Long term Dynamic load monitoring and alleviation Page 53 B Long term Effect of environment on growth of small cracks Page 55 A Long term Effect of flaw morphology on crack growth Page 56 A Long term Corrosion Prevention and Control Laboratory test protocol for accelerated corrosion testing Page 57 B Near term Methods for early detection of corrosion Page 58 B Near term Corrosion rates for major corrosion types Page 59 B Long term Stress Corrosion Cracking Residual stresses and their alleviation Page 61 A Near term SCC susceptibility of Air Force alloys Page 61 A Near term Life prediction methods for SCC Page 62 B Long term Nondestructive Evaluation NDE automation, data processing, and analysis Page 66 B Near term Hybrid inspection technologies Page 67 B Long term NDE to assess composite repairs Page 67 B Long term Maintenance and Repair Guidelines to implement advances in bonded repairs Page 69 B Near term Solid model interfaces to simulate repair methods Page 70 B Near term Reduce cost of materials and structures substitution Page 71 B Near term Repair design guidelines for high-cycle fatigue problems Page 71 B Near term Expert system for design and analysis of repairs Page 72 B Long term Common database for repair lessons learned Page 72 B Long term Priority levels for recommended R&D opportunities were established relative to the Air Force objectives (i.e., safety of flight [Objective A] and maintenance costs and force readiness [Objective B]). Definitions of priority categories include Critical priority: essential to flight safety (Objective A) (i.e., would eliminate a substantial threat to flight safety) Priority 1: essential to the reduction of maintenance costs and improvement of force readiness (Objective B) (i.e., would enable the Air Force to address significant technical problems) Priority 2: important to improved flight safety (Objective A) or reduced maintenance costs and improved force readiness (Objective B) (i.e., would represent significant improvement over current solutions) Priority 3: advantageous to improved flight safety (Objective A) or reduced maintenance costs and improved force readiness (Objective B) (i.e., would improve efficiency or reduce cost of current methods) There are no R&D efforts identified at this time that are of sufficient magnitude to be categorized as critical priority.
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Aging of U.S. Air Force Aircraft: Final Report TABLE ES-3 Priority-3 Near-Term and Long-Term Research Recommendations Recommendation Description Objective Timing Fatigue Effect of corrosion on material properties Page 55 A Near term Effect of corrosion and corrosive environment on safety limits Page 55 A Near term Expert systems for high-cycle fatigue repairs Page 53 B Long term Effect of hydrogen on fatigue crack growth Page 56 A Long term Corrosion Prevention and Control None Stress Corrosion Cracking None Nondestructive Evaluation Advanced technologies to track maintenance trends Page 68 B Long term NDE for early corrosion detection Page 68 B Long term Maintenance and Repair Guidelines on relative lives of bolted repairs Page 70 A Near term Analysis methods for structural repairs Page 72 B Long term Damping materials for dynamically loaded structure Page 72 B Long term However, the committee believes that it is possible that the recommended DADTA updates, and in particular the high-priority updates on the A-10, F-16, U-2, and T-38, will identify critical-priority near-term R&D or engineering tasks. For example, these could involve the need to develop a specific inspection technique or a specific type of modification for one or more aircraft. The committee's priority-1, -2, and -3 near-term and long-term R&D opportunities are summarized in Tables ES-1, ES-2, and ES-3, respectively. Each table contains reference to the pages in the report where the full description, background information, and justification for each recommendation can be found. As can be seen from the tables, there are a total of 9 priority-1 recommendations, 27 priority-2 recommendations, and 9 priority-3 recommendations. The priority-1 recommendations focus on reducing maintenance costs and improving force readiness (Objective B), particularly in the areas of corrosion prevention and control, SCC, and NDE. Many of the priority-2 recommendations address improving safety (Objective A) through development of improved methods to evaluate and analyze fatigue and stress corrosion cracking. The remainder of the recommendations deal with improvements in maintenance costs and force readiness (Objective B). Likewise, priority-3 recommendations address both objectives. The 45 recommended research opportunities, when coupled with the 8 engineering and management tasks (which the committee considers to be essential), will substantially enhance the ability of the Air Force to address the aging aircraft problem and to sustain the forces well into the next century.
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