II
RECOMMENDED STRATEGY AND OPPORTUNITIES FOR NEAR-TERM AND LONG-TERM RESEARCH

One of the primary objectives of this study was to identify an overall strategy that addresses the Air Force's aging aircraft needs. From the discussion of the aging aircraft problem in Chapter 2, including the assessments of the force management process, it is apparent to the committee that the recommended overall strategy must encompass several engineering and management issues as well as the near-and long-term research opportunities. The committee believes that there are a number of engineering tasks that do not require additional research and that should be accomplished in the near future.

Also, to be effective, the strategy must address the three Air Force objectives that are noted in Chapter 1:

  • identify and correct structural deterioration that could affect safety of flight

  • prevent or minimize structural deterioration that could become an excessive economic burden or adversely affect force readiness

  • predict, for the purpose of future force planning, when the maintenance burden will become so burdensome, or the aircraft availability so poor, that it will no longer be viable to retain the aircraft in the inventory

To provide a comprehensive approach that addresses these challenges, 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. This overall strategy is illustrated in Figure II-1.

Engineering and management tasks are near-term actions (within three to five years) to improve the maintenance and force management of aging aircraft. Each of the three aging aircraft challenges are shown on the left side of the figure connected to the primary engineering and management task that addresses each challenge. It should be noted, however, that this is not exclusively true. For example, the engineering task of obtaining improved corrosion control programs is connected to the challenge to minimize maintenance costs and improve readiness, since corrosion is currently the major contributor to maintenance costs and does not normally affect structural safety. However, corrosion could become a safety issue if not brought under control. Likewise the primary focus of the engineering tasks of updating durability and damage tolerance assessments, force structural maintenance plans, and tracking programs is to protect the structural safety, but they also impact maintenance costs and force readiness. The task of estimating the economic service life of an aircraft weapon system involves both engineering and management; engineering predictions of structural deterioration need to be coupled with a number of cost and operational considerations to arrive at the most probable time that the Air Force should plan on replacing the system. The last three tasks in Figure II-1 deal primarily with the Aircraft Structural Integrity Program and postproduction force management concerns discussed in Chapter 2 and further expanded in Chapter 5.

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 are shown to extend over a five-year period. Also, it is envisioned that some of the tasks should have periodic updates about every five years as indicated in the figure. The background justification and specific recommended actions for each of the eight engineering and management tasks are included in Chapter 5.

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 the Air Force's laboratories either in-house or by supporting contractors and academic institutions. Also, the Air Force laboratories should utilize the results from complementary near-term R&D efforts that are under the direction of other government agencies (i.e., the National Aeronautics and Space Administration, the Federal Aviation Administration, and the Navy). Figure II-2 illustrates the basic elements of both the near-term and the long-term R&D programs. The near-term program includes those efforts that reasonably can be expected to provide results that will assist in the performance of the near-term engineering tasks during the next five years. 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 aircraft maintenance organizations, but nevertheless should be initiated now, or continued if they already have been initiated. These efforts are typically higher



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Aging of U.S. Air Force Aircraft: Final Report II RECOMMENDED STRATEGY AND OPPORTUNITIES FOR NEAR-TERM AND LONG-TERM RESEARCH One of the primary objectives of this study was to identify an overall strategy that addresses the Air Force's aging aircraft needs. From the discussion of the aging aircraft problem in Chapter 2, including the assessments of the force management process, it is apparent to the committee that the recommended overall strategy must encompass several engineering and management issues as well as the near-and long-term research opportunities. The committee believes that there are a number of engineering tasks that do not require additional research and that should be accomplished in the near future. Also, to be effective, the strategy must address the three Air Force objectives that are noted in Chapter 1: identify and correct structural deterioration that could affect safety of flight prevent or minimize structural deterioration that could become an excessive economic burden or adversely affect force readiness predict, for the purpose of future force planning, when the maintenance burden will become so burdensome, or the aircraft availability so poor, that it will no longer be viable to retain the aircraft in the inventory To provide a comprehensive approach that addresses these challenges, 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. This overall strategy is illustrated in Figure II-1. Engineering and management tasks are near-term actions (within three to five years) to improve the maintenance and force management of aging aircraft. Each of the three aging aircraft challenges are shown on the left side of the figure connected to the primary engineering and management task that addresses each challenge. It should be noted, however, that this is not exclusively true. For example, the engineering task of obtaining improved corrosion control programs is connected to the challenge to minimize maintenance costs and improve readiness, since corrosion is currently the major contributor to maintenance costs and does not normally affect structural safety. However, corrosion could become a safety issue if not brought under control. Likewise the primary focus of the engineering tasks of updating durability and damage tolerance assessments, force structural maintenance plans, and tracking programs is to protect the structural safety, but they also impact maintenance costs and force readiness. The task of estimating the economic service life of an aircraft weapon system involves both engineering and management; engineering predictions of structural deterioration need to be coupled with a number of cost and operational considerations to arrive at the most probable time that the Air Force should plan on replacing the system. The last three tasks in Figure II-1 deal primarily with the Aircraft Structural Integrity Program and postproduction force management concerns discussed in Chapter 2 and further expanded in Chapter 5. 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 are shown to extend over a five-year period. Also, it is envisioned that some of the tasks should have periodic updates about every five years as indicated in the figure. The background justification and specific recommended actions for each of the eight engineering and management tasks are included in Chapter 5. 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 the Air Force's laboratories either in-house or by supporting contractors and academic institutions. Also, the Air Force laboratories should utilize the results from complementary near-term R&D efforts that are under the direction of other government agencies (i.e., the National Aeronautics and Space Administration, the Federal Aviation Administration, and the Navy). Figure II-2 illustrates the basic elements of both the near-term and the long-term R&D programs. The near-term program includes those efforts that reasonably can be expected to provide results that will assist in the performance of the near-term engineering tasks during the next five years. 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 aircraft maintenance organizations, but nevertheless should be initiated now, or continued if they already have been initiated. These efforts are typically higher

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Aging of U.S. Air Force Aircraft: Final Report FIGURE II-1  Recommended overall strategy to address Air Force aging aircraft challenges. Strategy includes near-term engineering and management tasks and near-term and long-term R&D programs. risk than the near-term R&D efforts, but the potentially high payoff justifies their pursuit. Included in Part II are descriptions of recommended near-term engineering and management tasks; assessments of current and planned research administered by the aging aircraft research program (detailed assessments are contained in the committee's interim report [NRC, 1997]); identification of near-term and long-term research opportunities in the areas of fatigue (low-cycle fatigue, high-cycle fatigue, and environmental effects), corrosion and stress corrosion cracking, and inspection and maintenance technology (nondestructive evaluation and maintenance and repair); and prioritization of recommended research. Although the investigation of structural aging phenomena is an inherently interdisciplinary endeavor, for convenience the recommended research is presented separately for individual topical areas. Chapters 6 (fatigue), 7 (corrosion and stress corrosion cracking), and 8 (nondestructive evaluation and maintenance) describe R&D opportunities focused on the aluminum structures that dominate the current aging aircraft problems. Chapter 9 provides prioritization of the near-term and long-term research recommendations. Finally, Chapter 10 describes issues related to composite primary structures that are becoming more common on newer aircraft that represent the next generation of aging aircraft.

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Aging of U.S. Air Force Aircraft: Final Report FIGURE II-2  Basic elements of the recommended near-term and long-term R&D programs.

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Aging of U.S. Air Force Aircraft: Final Report 5 Engineering and Management Tasks UPDATE OF DURABILITY AND DAMAGE TOLERANCE ASSESSMENTS As noted in Chapter 4, a number of aircraft failures resulting from fatigue crack growth from preexisting flaws or defects, which were introduced during material processing or manufacturing, caused the Air Force to extensively revise their Aircraft Structural Integrity Program (ASIP) in the early 1970s to include damage tolerance requirements. These requirements were defined in MIL-A-83444 (DOD, 1987) and MIL-STD-1530A (DOD, 1988) and were incorporated into the designs of the new aircraft then under way (e.g., the B-1A, F-16, and A-10). However, to protect the structural safety and assess the durability of the vast majority of Air Force aircraft that were not designed to these requirements, the Air Force and the aircraft contractors performed durability and damage tolerance assessments (DADTAs) on the aircraft models that were already in the operational inventory. By the early 1980s DADTAs had been performed on the F-4C/D/E, A-7D, C-5A, C-141, F-111, B-52D, E-3A, F-5E, T-38, T-37/A-37, KC-135, SR-71, T-39, KC-10, C-130, and F-15. Also, because of changes in use conditions, the durability and damage tolerance of both the A-10 and F-16 had to be revisited after only a short time in operational service. From the standpoint of safety, the most important outputs from these assessments were the identification of fatigue-critical areas, the determination of safety limits for these areas, and the development of safety inspection requirements. In addition, for some of the larger transport aircraft, estimates were made of the onset of widespread fatigue damage (WFD) and risk analyses were performed (e.g., on the C-5A, KC-135, and C-141). Where appropriate, lower-bound estimates were made of the major component modification or replacement times and modification options were defined. The overall approach or methodology used in conducting the DADTAs is illustrated in Figure 5-1. As can be seen in this figure, the four primary tasks in the assessments are (1) the identification of fracture-critical areas;1 (2) the development of the operational stress spectra for these areas; (3) an assessment of initial flaw distributions and/or the maximum probable initial flaw sizes; and (4) the determination of the safety limits, inspection intervals, and, for fail-safe designs, the estimated onset of WFD. The results were then used to update the individual aircraft tracking programs and the force structural maintenance plans for the aircraft, both of which are key elements of ASIP. Air Force-Supported Aircraft To obtain improved visibility of the actions that will be necessary to protect the structural safety of the Air Force's aging aircraft listed in Table 3-1 throughout their projected operational lives and to obtain the best estimates as to when the aircraft will likely be facing the economic impacts of major modifications or replacements, the committee strongly recommends that the DADTAs of these aircraft be updated periodically. In general, an update about every five years is appropriate. The urgency to perform these updates varies among the different aging aircraft types, depending on several factors: (1) whether the aircraft structure is designed to be fail-safe or is largely of a single load-path design, where missing a critical area could lead to the loss of an aircraft; (2) whether a replacement aircraft has been identified and the older aircraft are being phased out of the inventory; (3) the extent and nature of fatigue cracking problems the aircraft are currently encountering; and (4) whether there has been a recent independent review of the aircraft and corrective actions are already under way. Table 5-1 summarizes these different factors for each of the Air Force's aging aircraft types shown previously in Table 3-1. Also shown in Table 5-1 is the committee's assessment of the priority that should be assigned to performing the DADTA update for each type of aircraft. Those of greatest concern, based on the highest potential for structural safety problems, were given a number 1 priority and those with the least immediate concern were given a number 3 priority. However, it is recommended that the DADTA update be performed on all of the aircraft within the next five years and updated at approximately five-year intervals. The committee recognizes that the level of effort involved in performing these updates will vary significantly between the different types of aircraft as a function of aircraft complexity, variations in use, the numbers and types of cracking 1    If rapid crack propagation and part failure could lead to the loss of the aircraft, it is defined as a fracture-critical area.

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Aging of U.S. Air Force Aircraft: Final Report FIGURE 5-1  Overall approach to durability and damage tolerance assessments. TABLE 5-1 Prioritization of DADTA Update Needs for Air Force-Supported Aircraft Aircraft Fail-Safe Design Additional Years in Inventory Replacement Aircraft Identified Current Fatigue Cracking Recent Structural Reviewa Review Actions Under Way Priority KC-135 yes 25+ no limited yes yes 3 C-5A yes 10–25 no no report no no 2 C-141B yes 0–8 yes (C-17) yes yes yes 3 A-10 no 25+ no yes no no 1 B-52H no 25+ no yes no no 3b B-1B no 25+ no yes yes (horizontal tail) yes 2 F-15 no 5–25 yes (F-22) limited no no 2 F-16 no 10–25 yes (JSF) yes yes (fuselage bulkhead) yes (bulkhead) 1 C-130E/H yes 25+ some (C-130J) limited yes (fuselage) unknown 2 E-3 (AWACS) yes 17–25 no limited no no 3 E-8 (JSTARS) yes 15–20 no yes yes (wings) unknown 2 EC-135 yes 25+ no limited yes yes 3 U-2c no 25+ no unknown no no 1 EF-111 no <5 no limited no no noned T-37B no 0–12 yes (JPATS) limited no no 3e T-38 no 25+ no yes no no 1 a Within the past three years. b The lower priority is because a DADTA update was performed in 1995. c This aircraft was developed for the government and is maintained by the manufacturer rather than by an air logistics center. d Based on the assumption that all aircraft will be retired in less than five years as planned. e DADTA is currently being performed by Southwest Research Institute. Update suggested within five years.

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Aging of U.S. Air Force Aircraft: Final Report problems encountered, and how well the different air logistics centers (ALCs) and the airframe contractors have been performing the applicable ASIP tasks on a continuing basis during the aircraft's past operational use. As a minimum, the effort may merely require a summary of available data (e.g., critical areas, safety limits, inspection requirements, estimates of the onset of WFD, estimated future modification and replacement times, and possible future fatigue test needs) and a detailed review by the proposed Aging Aircraft Technical Steering Group discussed later in this chapter. For other aircraft it will require further identification of critical areas, stress spectra development, crack growth calculations and tests, and perhaps some tear-down inspections and/or full-scale fatigue testing. Contractor Logistics-Supported Commercial-Derivative Aircraft In a similar manner to the criteria for Air Force-supported aircraft (previous section), priorities are suggested for contractor logistics-supported commercial-derivative aircraft. In addition to the criteria described in the previous section for Air Force-supported aircraft, the experience with the commercial-equivalent aircraft can be taken into account. The KC-10 and C-27 have previously had DADTAs. It is recommended that they be updated within the next five years. Because there is no immediate safety concern, a priority 3 is suggested. The E-4, T-43, and C-9 have average ages of 23, 24, and 26 years with plans to keep them in the inventory for many more years. It is recommended that the Air Force form an independent team to review these aircraft. This team should consist of a small number of structures and materials experts chartered to assess the current condition of the aircraft, review the current use spectra, and determine if the current contractor database is sufficient to estimate the onset of WFD and the probable major component modification or replacement times or if DADTAs should be performed. The committee suggests that these reviews be performed within the next five years. Because of the much higher use, it is recommended that the C-9 be addressed first. A priority 2 is suggested for the C-9 and priority 3 for the E-4 and T-43. The C-18, C-22, and the VC-137 aircraft have quite high utilization times. There is some concern about the possible onset of WFD for the C-18, C-22, and possibly the VC-137. Thus, the committee recommends that an independent structures review be conducted by a team of structures and materials experts in the near future. If the high-use VC-137s are replaced by the C-32, they of course could be dropped from the review. Because of the potential safety implications, a priority 1 is suggested for these reviews. For the utility and commuter class commercial-derivative aircraft (i.e., the C-12, T-1A, C-21, C-23, C-26, C-20, E-9, UV-18, and T-3), the committee recommends that the Air Force initiate damage tolerance surveys, by a small team of structures and materials experts, similar to those conducted during this past year by the Federal Aviation Administration (FAA) on a number of other types of aircraft in this size class. These surveys should provide a preliminary assessment of the aircraft's damage tolerance, current structural health and estimated longevity, and the potential need for a detailed DADTA. The surveys should be conducted first on the older aircraft or aircraft where structural problems may have already been identified.2 UPDATE OF FORCE STRUCTURAL MAINTENANCE PLANS AND INDIVIDUAL AIRCRAFT TRACKING PROGRAMS The fourth and fifth tasks of the Air Force's ASIP (shown in Table 2-1) deal with force management. It is here that the results of design, analysis, and full-scale test activities in the previous parts of ASIP (including subsequent DADTAs) come together to define the specific actions that must be taken to protect the safety of the individual aircraft and allow for the timely and cost-effective structural modifications. The two key force management activities in ASIP are the development of the force structural maintenance plan (FSMP) and the individual tracking program (IATP). Force Structural Maintenance Plan During the initial design, the intent was to minimize the amount of structural maintenance that would be needed throughout the life of an aircraft, assuming that the aircraft is used as planned (i.e., it is flown to the design use spectrum). However, full-scale fatigue testing to the design spectrum will uncover critical areas missed during design and analysis, which then necessitates additional damage tolerance analysis, in-service safety inspections, and perhaps in-service modifications. It is the definition of when, where, how, and the estimated costs of these inspections and modifications that constitute the basis for the initial FSMP. Recognizing that the actual service use of military aircraft often differs from the original design use spectra, ASIP requires that a loads/environment spectra survey be conducted during the first two or three years of operational service to obtain actual use data that can be used to update the original design spectrum. These surveys generally consist of instrumenting 10 to 20 percent of the fleet and using a 2    For example, a potential fatigue cracking problem has been reported (by the Air Force's contractor logistics support office) to exist in some C-12 wing spars. This is a potential safety concern, since the structure is not a fail-safe design.

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Aging of U.S. Air Force Aircraft: Final Report multichannel recorder (or more recently, microprocessor systems) to record such data as vertical and lateral road factor; roll, pitch, and yaw rates; roll, pitch, and yaw accelerations; altitude; mach number; rudder and aileron position; and selected strain measurements. These data are then used to generate a new baseline operational spectrum, and new damage tolerance analyses are performed to update the safety inspection and modification requirements with the results added to the FSMP. This updated FSMP then forms the basis for planning and scheduling the structural fatigue maintenance for the overall aircraft weapon system. The damage tolerance analysis should be updated and the results used to update the FSMP any time that there are significant changes in use; when operation is extended beyond the original life goal; or new analysis, test, or service experience indicate a growth in the number of fatigue-critical areas. Individual Aircraft Tracking Program In addition to the force-wide baseline operational use spectra being different from the original design spectra for military aircraft, the individual aircraft use within the force may be either more or less severe than that represented by the baseline spectrum. These variations from the baseline spectrum can be quite large, particularly for the high-performance combat type aircraft. Accordingly, the Air Force has included the requirement for individual aircraft tracking as part of the ASIP. The IATPs for the various types of aircraft within the Air Force inventory vary with regard to data acquisition and processing procedures. For the larger tanker, transport and bomber aircraft (e.g., the KC-135, B-52, and C-141), where the excursions in the flight spectra are relatively small, flight logs and pilot use forms (i.e., Air Force technical order form 16 and tactical maneuver supplemental forms) have been found to be satisfactory to track the aircraft use. For the fighter and attack aircraft the use of counting accelerometers and VGH (velocity; ground range and height) recorders were commonplace in the past, but are limited because they are not able to accommodate critical areas of the structure that are sensitive to asymmetrical loading. The use of multichannel recorders (e.g., the MXU-553), which record many more flight parameters, overcomes this limitation. More recently, the older tape systems are being replaced (as funding will allow) by microprocessor systems, further expanding data capture. Computerized methods have been developed and are used to reduce the measured flight data and to adjust the crack-growth-based damage rates and inspection intervals for each of the critical areas in the airframe for individual aircraft use. As the aircraft ages, the number of critical areas and inspections increase. When this happens, the IATPs must be updated to accommodate these changes. Although there has been some discussion about upgrading the Air Force's IATP to track potential corrosion damage and/or corrosive environments as well as fatigue damage, the committee believes that the application of sensor devices and data analysis and processing equipment in existing aircraft is currently impractical because of the large number of aircraft involved, the large sizes of affected areas in the aircraft most prone to corrosion damage (i.e., the large transport, tanker, and bomber aircraft), and the cost and intrusiveness of system installation. However, developments in multifunctional chemical and physical sensors (NRC, 1995), microelectromechanical systems, and smart diagnostics do provide some hope that long-term research in on-board health monitoring can be productive. Following the completion of the updates of the DADTAs, which were recommended above, the committee recommends that the inspection and modification requirements in the FSMPs be updated to reflect any changes in the baseline operational spectra and any additional critical areas that were identified, which in turn will increase the inspection requirements and possibly necessitate new modifications the IATP for each aircraft weapon system be updated to reflect additional critical areas that need to be tracked recording equipment, or analysis procedures that may be deemed necessary to protect the structural safety of the aircraft. In particular the Air Force should push for the force-wide use of the microprocessor-based recorders because of their improved reliability and the expanded data capture. STRESS CORROSION CRACKING ASSESSMENTS Although the environmental protection measures and material substitutions to eliminate corrosion-susceptible materials that take place as part of an aircraft's corrosion prevention and control program (CPCP) also apply to the prevention of stress corrosion cracking (SCC), there are some unique aspects about SCC that make this structural deterioration mechanism much more dangerous than other forms of corrosion. Thus, the committee believes that SCC deserves special attention. Stress corrosion cracks are characteristically intergranular and can occur with little or no evidence of corrosion products and as a result are often difficult to detect visually. Although they generally have not caused flight-safety problems, because of their orientation with respect to the applied flight stresses (see Chapter 4), this cannot always be considered to be a certainty. If large in-plane stress corrosion cracks or delaminations go undetected they could cause a loss in shear strength and trigger failure modes other than the tensile mode normally associated with crack propagation. Also, in thick sections (e.g., complex machined fittings) where there may be irregular grain flows and three-dimensionally applied

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Aging of U.S. Air Force Aircraft: Final Report stresses, it is often difficult to predict if a stress corrosion crack will turn normal to the largest component of stress and result in a tensile fracture. For fail-safe structural designs, a part or component failure caused by a stress corrosion crack is much less of a concern than in safe crack growth designs because of the second line of defense provided by the surrounding intact structure. In fact, over the years there have been many part failures caused by SCC in both commercial and military aircraft. When this occurs, the parts are generally replaced, ideally with new parts made from more-stress-corrosion-resistant materials. For safe crack growth designs, which are generally associated with high-performance combat aircraft, it is important that the stress corrosion cracks be prevented from occurring or that they be detected before failure, since failure of the parts or components may lead to the loss of the aircraft. As an aircraft ages and protective finishes and coatings break down, concern over part failure caused by SCC becomes more acute. As a result the committee believes that there is a need for the Air Force to periodically assess the susceptibility of their aging aircraft to SCC and take actions to diminish the occurrence of SCC and prevent future part failures. Particular attention should be given to structures that are not designed to be fail-safe. 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 DADTA updates proposed in this chapter. Specifically, the committee recommends that 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 the engineers performing the DADTA update make an evaluation of potential failure modes and consequences of failure for each stress-corrosion-critical area protection, inspection, modification, and replacement alternatives be developed as necessary (see recommended short-term research in Chapter 7) IMPROVED CORROSION CONTROL PROGRAMS The 1988 accident of the Aloha Airlines 737 aircraft (NTSB, 1988) resulted in much attention being paid to the aging aircraft issue both by the commercial and the military aviation sectors. Although this accident was primarily the result of WFD,3 it focused attention on all of the factors that can contribute to structural deterioration, including corrosion. Both the commercial and the military sectors have since taken actions to reduce corrosion and the very high associated maintenance costs. In the commercial sector, the Air Transport Association and the Aerospace Industries Association in cooperation with the FAA, established the Airworthiness Assurance Task Force to evaluate potential deficiencies in current commercial practices and to provide recommendations and guidance to the FAA and the airline industry on maintaining the structural integrity of 11 different aging aircraft models, including the Boeing 707, 727, 737, and 747; the Airbus A-300; the BAC 1-11; the Fokker F-28; the Lockheed L-1011; and the Douglas DC-8, DC-9, and DC-10. In 1992 the Airworthiness Assurance Task Force was incorporated into the FAA's Aviation Regulation Advisory Committee as the Airworthiness Assurance Working Group (AAWG), shown schematically in Figure 5-2. The AAWG proposed a mandatory CPCP to be tailored to each aircraft and operator and implemented by the FAA by airworthiness directives. The need for this program stemmed from fleet surveys, maintenance cost reviews, and comments from operators, all of which pointed to the fact that corrosion resulted in the single largest investment in time and resources in aircraft maintenance programs, and that, in some cases, the aircraft were being maintained in conditions below the manufacturer's expectations. On the other hand, operators that already had comprehensive CPCPs in place experienced much lower amounts of corrosion than those that did not. In fact, if the programs were implemented early in the aircraft's life, the aircraft remained essentially corrosion free. Also, it was noted that operators who utilized liberal applications of corrosion-preventive compounds showed significantly reduced corrosion damage. The essential elements of the AAWG overall CPCP are inspection of all primary structures initial and repeat inspection intervals based on calendar time rather than flight hours or number of flights performance of basic maintenance tasks, including exposure of the corroded area, cleaning, inspection, rework as required, reapplication corrosion-preventive treatments adjustments in the aircraft's overall maintenance program to maintain a corrosion severity of Level I or better (as described below) The CPCP for each specific type of aircraft was developed by that aircraft's structures task group, which was made up of representatives from the manufacturer, the operators and maintainers, and the FAA. In the development of CPCPs, the commercial aircraft industry has established severity classification criteria to guide maintenance programs. Corrosion severity is considered to fall into one of the following three classes: 3    Loss of adhesion in the cold-bonded fuselage lap splice contributed to the early fatigue cracking at knife-edged countersunk fastener holes.

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Aging of U.S. Air Force Aircraft: Final Report FIGURE 5-2  Organization of commercial aircraft industry aging aircraft working groups. Source: Hidano and Goranson (1995). Level I corrosion. (1) Corrosion damage occurring between successive inspections that is local and can be re-worked/blended-out within allowable limits as defined by the manufacturer; or (2) corrosion damage occurring between successive inspections that is widespread and can be reworked/blended-out well below allowable limits as defined by the manufacturer; or (3) corrosion damage that exceeds allowable limits and can be attributed to an event not typical of the operator's use of other airplanes in the same fleet (e.g., mercury spill); or (4) operator experience over several years has demonstrated only light corrosion between successive inspections but latest inspection and cumulative blend-out now exceed allowable limit. Level II corrosion. (1) Corrosion occurring between successive inspections that requires a single re-work/blend-out which exceeds allowable limits, requiring a repair/reinforcement or complete or partial replacement of a principal structural element, as defined by the original equipment manufacturer's structural repair manual, or other structure listed in the baseline program; or (2) corrosion occurring between successive inspections that is widespread and requires a single blend-out approaching allowable re-work limits. Level III corrosion. Corrosion found during the first or subsequent inspections, which is determined (normally by the operator) to be an urgent airworthiness concern requiring expeditious action. Note: When level III corrosion is found, consideration should be given to action required on other airplanes in the operator's fleet. Details of the corrosion findings and planned action (s) should be expeditiously reported to the appropriate regulatory authority (Boeing, 1994:1.1-1–1.1-2). A CPCP is considered effective if corrosion of identified critical structure is limited to Level I or better. The intent of these CPCPs is to ensure that corrosion is never allowed to progress to the point that it could become a safety issue (hence the emphasis on primary structure). The secondary benefit of the programs is to reduce the operators long-term corrosion maintenance costs. In the military sector, the Air Force established a Corrosion Program Office at the Warner-Robins Air Logistics Center to

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Aging of U.S. Air Force Aircraft: Final Report oversee and coordinate the Air Force's corrosion prevention and control activities. However, the development, implementation, and execution of specific weapon system corrosion control efforts is the responsibility of the specific system program director. Guidance is provided by Technical Order 1-1-691, which is a tri-service (Navy/Army/Air Force) coordinated manual entitled ''Aircraft Weapons Systems Cleaning and Corrosion Control," published January 1992. This manual provides detailed information on such items as preventive maintenance procedures, methods, and materials; inspection techniques; corrosion and paint removal methods and the application of surface treatments; and procedures for applying sealing compounds. Appendix E to this manual is for Air Force use only and contains additional information on aircraft cleaning procedures and intervals as a function of aircraft basing, shot peening and roto peening procedures, and chemical corrosion removal procedures. It is intended that this tri-service manual be used in support of Air Force aircraft manuals and, in the event of conflict, the aircraft manual would take precedence. The Air Force Corrosion Control Office along with the Naval Air Systems Command and the Army Aviation Systems Command are responsible for the maintenance of the manual. The tri-service manual has a great deal of detailed information on corrosion prevention and control, and a significant effort is being made by the Corrosion Control Office to reduce corrosion in the Air Force's aging aircraft. However, the committee believes that the Air Force does not have the type of comprehensive CPCP for each of its aging aircraft weapon systems on the level of those mandated for commercial airplanes. The committee does not believe that corrosion can or will be completely eliminated in the Air Force's aging aircraft, but with comprehensive programs similar to those established for commercial aircraft, corrosion can be reduced significantly. The committee recommends that the Air Force undertake the following actions to improve corrosion prevention and control in the aging forces: The Air Force's system program directors, in concert with the appropriate major commands and the Corrosion Control Office, should perform an internal audit of each of the Air Force's commercial-derivative aging aircraft (i.e., the E-3, E-8, E-4, VC-25, T-43, C-137, C-18, C-22, KC-10, and C-9) to ensure that the corrosion control programs are in full compliance with the CPCPs mandated for commercial counterparts. In addition to the primary structures covered by the commercial programs, the Air Force should ensure that adequate corrosion control measures are being applied to corrosion-susceptible secondary structures. The Air Force's system program directors, in concert with the appropriate major command and the Corrosion Control Office, should review the detailed corrosion control programs of each of the Air Force's aging aircraft listed in Table 3-1 that is not scheduled to be retired in the near future (i.e., the KC-135, C-5, A-10, B-52H, B-IB, F-15, F-16, C-130E/H, U-2, and T-38) and upgrade them as necessary to a level equivalent to or better than the CPCPs that are mandated for commercial aircraft. Again, corrosion-susceptible secondary structures as well as the primary structures should be included in the programs. The Air Force's ALCs, with the Corrosion Control Office, should evaluate the applicability and cost effectiveness of dehumidification, as described in Chapter 4, to reduce the likelihood of corrosion. ECONOMIC SERVICE LIFE ESTIMATION As discussed in Chapter 4, major economic impacts can be expected to occur with the onset of WFD in fail-safe-designed aircraft structures and with the rapid growth in the number of fatigue-critical areas in safe-crack-growth-designed aircraft structures. When either of these occur, the options are to modify the structure, replace major portions or components of the airframe, or retire the aircraft. If the economic impact is sufficient to justify retirement, this would constitute the economic service life of the aircraft. However, there are a number of other factors that also contribute to the economic service life, and this should be viewed from the broader perspective of the total cost to operate an aircraft system. There are several examples in which it has been cost effective to modify or replace major components of an airframe, even when they have experienced WFD. Some of these aircraft have continued in service for many more years (e.g., the KC-135, C-5A, and C-141). On the other hand, it appears quite possible that the economic burden of operating a given type of aircraft could become excessive before the onset of WFD or the rapid rise in fatigue-critical areas. For example, it was pointed out in Chapter 4 that corrosion (including SCC) is currently the most costly maintenance problem for the Air Force's aging aircraft. If not substantially diminished in the future through improved prevention and mitigation measures, corrosion damage, either by itself or in combination with fatigue cracking, could cause the Air Force to undertake major modifications, major component replacements, or perhaps aircraft retirement. Clearly, as was pointed out in Chapter 2, there is a need for an overall economic service life estimation model that integrates the estimates of structural deterioration caused by fatigue, corrosion, and SCC with all other operating cost elements. The current lack of such a tool inhibits Air Force planners from establishing a realistic time table to phase out a current system and to begin planning for replacement aircraft. Some examples of cost elements that should be tracked and projected for inclusion in such a model are related to

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Aging of U.S. Air Force Aircraft: Final Report of joining methods (e.g., adhesive bonds, mechanical, etc.), environmental degradation at repair edges, and base metal and repair material integrity. Related and key to this question is the determination of the appropriate NDE accept-reject criteria to be applied with the NDE test to determine the state of the repair. Recommendation 34.  Develop signal and image processing techniques, applying such technologies as expert systems, neural networks, and database methods that could be used by aircraft maintenance facilities to interpret and track damage development and maintenance trends. Work should be pursued to explore and develop useful signal and image processing techniques, applications of expert systems, using, for example, neural networks or database methods that can be used conveniently in depots and other maintenance organizations to interpret and track damage development and maintenance trends. These improvements should be targeted both to single probe inspection procedures as well as to the hybrid multimode approaches. Recommendation 35.  Increase R&D efforts for the automation of successful inspection methods and for overall automation of extensive wide-area inspections. These efforts would include two principal components: a generic effort based on the broad-based enhancement of scanning technology including on-board transducer (probe) mountings, signal processing methods, display techniques to enhance operator interactions, and data fusion procedures an effort aimed at specific aging aircraft structures and the scanning geometry needed for their inspection The potential advantages of automated NDE include the enhancement of inspection reliability and speed through the removal of the human operator, the likelihood of reduced inspection times, and the likelihood of reduced costs. General features of scanners that need to be considered include portability, flexibility (i.e., ability to run on horizontal, vertical, and curved surfaces), ability to handle a variety of inspection modalities, and possibilities for handling hybrid multi-inspection techniques with associated signal processing and read-out procedures. The committee recommends that collaborative planning between the ALC users and researchers be in hand before and during work in this area. Recommendation 36.  Perform basic and applied research to develop suitable NDE techniques for the early detection of corrosion. Examples of specific tasks include (a) development of suitable NDE techniques to assess the quality and integrity of corrosion-resistant finishes exploration of the potential of using NDE methods to determine the initiation and level of corrosion damage Work in NDE development that is specifically aimed at the quality of corrosion-resistant finishes and coatings has been limited. Emphasis should first be placed on understanding the ways in which finishes and coatings protect the base metal from corrosion (as recommended in Chapter 7), and with that, techniques devised to measure the degradation and failure of the protective mechanism. The Air Force Office of Scientific Research had basic materials and NDE efforts in progress, but this effort is no longer funded. Efforts to develop NDE methods to detect the initiation of corrosion should be coupled to the development of a mechanistic understanding of corrosion and the corrosion process as presented in Chapter 7. Particular emphasis should be made to identify material parameters or properties that can be measured in service that relate to the level of corrosion. For example, the elastic constants may be sensitive to the presence of hydrogen in the material that contributes to the corrosion process. As these properties are identified, NDE sensors should be developed to provide the inspection tools. This NDE approach, if successful, would potentially provide early warning and large cost benefit to the aging fleet. This effort should be performed in collaboration with the corrosion prevention and control recommendations in this report. MAINTENANCE AND REPAIR Air Force research in repair technology includes R&D tasks over a broad range of topics. The primary emphasis is on the maturation of bonded composite patch repairs, especially for metallic structures. These repair methods have had successful application at the depot level (e.g., to repair fatigue cracks emanating from weep holes in C-141 lower wing skins). However, the common use of bolted repairs for both battle damage and fatigue cracking problems cannot be overlooked. In many cases bolted repairs are expected to perform well beyond their original intent, making the repair an aging structure much like the airframe itself. The current Air Force R&D program on repairs includes basic research involving modeling of composite patch repairs as crack arrestors in aircraft and design and analysis techniques for composite patch repairs a large amount of applied research, including projects related to bonded composite patch repairs—to investigate repair procedures, analysis methods, and design considerations—along with efforts to develop repair methods and design guide for composite structures; development of advanced life-extension techniques; development of structural life enhancement, force management, and internal and external loads handbooks;

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Aging of U.S. Air Force Aircraft: Final Report and in-service and experimental repair data. Also included are repair efforts, including projects related to bonded composite patch repairs—to develop improved materials and processing methods, investigate analysis methods, and develop repair technology handbooks—and an effort to develop repair technology for high-temperature composites exploratory research, including a broadly defined effort to evaluate and demonstrate repair concepts, an effort to explore the redesign of selected structural components using advanced materials and process technology, and an effort to demonstrate life-enhancement technologies for metallic structures a number of small short-term projects focused on optimization and demonstration of materials and processes, repair criteria, and analysis methods for bonded composite patch repairs at the depot level; also included are projects to evaluate methods to generate stress spectrum and to evaluate cold-expansion bushing repairs The committee believes that the focus on optimization of materials and processes and analysis tools for bonded composite repair of metallic structures is appropriate because the Air Force has unique expertise in this technology. The committee also supports the planned research focused on the redesign of components to take advantage of advances in materials and processing technology. Although the current R&D program in the area of repairs is well planned, there are no current programs in the repair task that consider the removal, surface treatment, and reapplication of corrosion-resistant finishes or protection systems. This is a particular shortfall considering the materials and process changes that will be necessitated by environmental regulations concerned with the elimination of heavy metals (e.g., chromium and cadmium) and limits on volatile organic releases. The Air Force is currently undertaking a great deal of research on environmentally compliant finish material and process development (Donley, 1996), but has not yet come to terms with the particular needs of aging aircraft in this area. In general, the committee believes that the concept of repairs should be expanded to include maintenance and repair. This change would require closer coordination of R&D tasks in repair with NDE tasks and an emphasis on implementation of developed technology through the development of generic repair design and processing handbooks and engineering analysis tools to broaden the application of new repair technologies. The committee recommends that the emphasis of the repair R&D programs be increased in the following areas: technologies for the removal, surface preparation, and reapplication of corrosion-resistant finishes evaluation guidelines for the relative lives of bolted repairs guidelines for taking advantage of advances in materials and processing technology in component replacement (including an examination of certification requirements to see if they can be waived or simplified without compromising safety); an example would be to reduce susceptibility to stress corrosion cracking through the use of improved aluminum alloys, tempers, and processes to reduce residual stresses repair and analysis methods for maintenance of structures susceptible to high-cycle fatigue maintenance and repair methods and guidelines for advanced composite structures Near-Term Research and Development Much has been learned in the past ten years concerning methods to analyze and repair damage in primary metallic and composite structures. Although the focus of much of the early work was on designing repairs for battle damage, the focus more recently has been on repairs for durability and life extension for current aircraft. The primary focus of the near-term programs for aging aircraft must be to identify the lessons learned from recent programs (e.g., C-141 and battle damage repair) and apply them at the maintenance organizations where they can be used to make the repairs that can extend the life of current aircraft. Recommendation 37.  Develop tools and guidelines to implement recent advances in bonded repair of primary structure for Air Force and contractor maintenance organizations. Examples of specific tasks include optimization and validation of materials and processes, including adhesive materials and surface preparation and bonding processes development of computational tools and guidelines for the design and analysis of design bonded repairs validation and documentation of analysis techniques to evaluate continuing damage growth beneath bonded repairs (CALCUREP) and bolted repairs (RAPID) To ensure that structural repairs have the best possible chance for success, the committee recommends that materials and processes that have been developed to join the repair to the structure, seal the repaired surface from further degradation due to adverse environments, and protect the repair from rapid deterioration in the flight environments be documented and made available to the maintenance organizations. Materials and processes to be considered include surface preparations, adhesives, and bagging materials used for successful repairs of the C-141. Advances in these material systems and any new, validated processes must be demonstrated by maintenance personnel with on-site consultation from the developing organization.

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Aging of U.S. Air Force Aircraft: Final Report There are a number of design and analysis tools for repairs that have been developed in the recent past (Bakuckas et al., 1996; Fredell et al., 1996). Once validated these methods will provide the ALCs far better and faster means to design reliable repairs than those currently in use. Design and analysis tools must include capabilities in the following areas to be used in the design of reliable and durable repairs: (1) continuing damage growth beneath the repair due to fatigue loads, (2) reliability and durability of bond or bolted joints, and (3) variations in repair materials and processes used to fabricate and apply the repair. Although analysis codes such as A4EI, PGLUE, and RAPID perform analyses of bonded or bolted repairs, they are very limited in the types of repair geometries to which they are applicable. A4EI applies only to a linear bonded repair, PGLUE to doubly symmetric bonded repairs, and RAPID to bolted repairs. There is much to be done to extend these methods to explicitly analyze realistic three-dimensional structures. The growth of damage beneath the repair is a critical concern. Bonded composite repairs are intended to provide sufficient stiffness and constraint of the structure so that the stress intensity factors for existing flaws are reduced to levels below threshold so that they cannot continue to grow. Analysis routines such as those in the current version of CALCUREP (for bonded repairs) and RAPID (for bolted repairs) need to be validated to ensure their accuracy and then be made available to the ALCs. Recommendation 38.  Develop analytical tools to take advantage of effective solid model interfaces developed for finite element modeling to model and simulate repair methods and geometric relationships for particular component repairs. Methodology has been developed, under Navy funding, that uses super-element technology to allow limited use of vehicle-level finite element model analyses on laptop PC hardware (Goering and Dominguez, 1992). With condensation techniques to reduce the degrees of freedom within the model, it is possible to design sophisticated large-scale repairs of damage to major structural members, to assess structural integrity before and after repair, and to assess the feasibility and capability of the repair to restore the structure to its original function. With the visualization possible on laptop PCs to provide a three-dimensional image of the area to be repaired, the loading conditions, and the damage to be repaired, the current capability to perform rapid repair analysis is remarkable. Unfortunately, the modeling of such repairs is still a time-consuming process. Work needs to be performed to make automatically generated repairs for a number of typical damage scenarios available. Although this initial effort might be limited in what it can provide, it could be a valuable tool for maintenance organizations. Recommendation 39.  Develop and validate guidelines for the relative lives of bolted repairs. Specific tasks include testing to evaluate crack stopping by cold working, peening, laser shock treatment, stop drilling, or hole filling testing to evaluate repair designs, including issues such as protection systems, taper ratios, fastener patterns, and fastener types testing to evaluate innovative fastener concepts such as single-shank blind fasteners and hole-expanding blind fasteners Bolted repairs are the most common repair applied to aircraft structures. Their capability to extend lives is limited because bolted joints tend to loosen up and the load transfer occurs away from the damaged area. Like bonded repairs, bolted repairs provide the reduction in strain levels at the damage site. However, neither repair system is expected to provide restoration of strength in damaged structure to the original design loads for the life of the airframe. Bolted repairs are generally expected to extend lives of damaged structures to the next programmed depot maintenance cycle. However, experience indicates that the repairs are often called upon to remain effective in providing structural integrity far longer than a single depot maintenance cycle. In such cases, determination of the relative lifetimes for several bolted repair configurations is desirable so that any selection of repair configuration will take into consideration the lifetime requirement and repair capability. Bolted repairs are limited by the limited fatigue life of the blind fasteners typically used to install these repairs from one side of the closed box structures. The development of blind fasteners with improved fatigue lives, either through improved design or through interference in the hole, would provide significant benefits to the life of the repair attachment. In addition, there are a number of methods to extend the lives of the damaged structure: through cold working, peening, laser shock treatment, or hole filling. The ability of these treatments to provide extended lives must be verified and quantified by test. The techniques described above should be incorporated into design methods for repairs that assure, through damage tolerance analyses and verified by test, that the repair will retard or stop the flaw growth from previous damage. Moreover, the design must be sensitive to the potential for the development of flaws in the structure surrounding the repair since the load distributions nearby have been changed by the repair. Taper ratios and fastener pattern designs, along with fastener sizing for flexibility and strength, can provide significant life improvements for bolted patches, but test data must verify the projected improvements.

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Aging of U.S. Air Force Aircraft: Final Report Recommendation 40.  Develop guidelines and procedures to reduce the cost of accepting new materials and structures as replacements for aging structural components. Since the design and manufacture of many of the aircraft that constitute the aging force, significant advances have been made in materials and processing technology to improve the resistance of aircraft components to aging degradation. For example, corrosion and stress corrosion cracking (SCC) resistance can be significantly upgraded through the use of substitute materials and heat treatments (e.g., more-corrosion-resistant 7050, 7150, or 7055 alloy for 7075, SCC-and exfoliation-resistant T-7X tempers for 7XXX-series aluminum alloys), improved protective finishes and corrosion prevention compounds, and incorporation of design features such as drainage and sealing to prevent corrosion. However, advances in materials and process technology have not been captured because of the excessive cost and time required to qualify them for service and because of the long lead times required for small-quantity procurement. Currently, material substitutions are handled on a individual part-by-part basis. The committee recommends that the Air Force develop guidelines to facilitate the force-wide implementation of the best materials and processing solutions while minimizing evaluation and qualification test requirements. Examples of specific tasks include substantiation of improved materials as preferred replacements for SCC-and corrosion-susceptible alloy components development of an approved alloy substitution matrix evaluation of test protocols for replacement materials and structures to allow for one-time approval of general materials substitutions This effort would reduce test costs for replacement structures, but would also act as an incentive to replace older, more-damage-prone materials with more-damage-resistant materials. Considered separately, the quantity of material required for validation efforts and support of replacement modification programs is small. However, quantities required for more general materials substitutions could be significant enough to enable reduction of long lead times associated with small-quantity procurement by stocking qualified substitutes. Recommendation 41.  Develop repair design guidelines for dynamically loaded structures. Examples of specific tasks would include documentation of repair materials and processes and design considerations based on an understanding of root causes, dynamic load conditions, and environmental factors develop and validate damped repair concepts based on currently available adhesive and composite repair technology Repairs for dynamically loaded structures offer the unique potential to significantly reduce load magnitudes or change the critical load frequencies while they serve to recover the integrity of the structure. The challenge for repairs of dynamically loaded structures is to recover the structural integrity and stiffness requirements while not moving critical dynamic modes into surrounding structures where damage can occur even more rapidly than in the initial failure. This is why knowledge of the dynamic modes and responses of both the original structure and the repaired structure are so important to the repair of dynamically loaded structures. Recently, adhesives that contain significant damping potential have become available with sufficient durability that they can be used in bonded repairs. These adhesives, combined with stand-off materials to maximize the shear transfer through the adhesive and composite skin materials to withstand low-velocity impacts and provide load-carrying capability, have provided the opportunity to design and fabricate repairs that damp the loads that cause high-cycle fatigue failures. Before these repairs can be used with confidence by the Air Force maintenance organizations, they must be verified to provide continuity of the structure while reducing the driving forces below that level which initiates failures in a part for the remainder of its design life. Long-Term Research and Development Recommendation 42.  Develop an expert system to aid in the assessment of damage, the need for repair, and the design and analysis of repairs. The committee believes that an expert system should be developed that has the capability to recall vehicle level loads and structural analysis, graphically isolate the region being repaired, and assess the viability, reliability, and durability of the repair. These systems would use databases developed and maintained by the recommended corrosion and fatigue working groups discussed in Chapter 5. Analysis methods should be developed that are capable of analyzing bolted or bonded joints for real repair configurations in which existing fastener patterns and other structural details need to be accommodated. Some of these more-flexible analysis tools have been developed, but are cumbersome and time-consuming to use. Simplifications in graphical interfaces and the ability to handle large data files representing complex three-dimensional geometries may permit better interfaces between structure and repair to be developed. It is possible to envision a virtual repair routine for a laptop environment that could lead the repair technician or analyst through the steps of the repair by

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Aging of U.S. Air Force Aircraft: Final Report providing both graphical and descriptive specifications of the repair processes and procedures. Recommendation 43.  Develop a common database of repair lessons learned, to be managed and maintained by the maintenance and repair working group, that would be available to the ALCs and would contain information on repair experience, including both adverse and successful results, problems in assessment, design, analysis, materials, or application of the repair. Recommendation 44.  Develop analysis methods and life prediction tools and methods for structural repairs and affected structure. There are several methods for the analysis of bonded patch repairs. They can be classified broadly as either analytical or numerical. The analytical approach of Rose (1981) is based on Hart-Smith's (1974) theory of bonds, elastic inclusion analogy, and on some simplifying assumptions. Fredell (1994) has extended this analysis to include thermal effects and has also carried out an evaluation of mechanical doubler repairs. Erdogan and Arin (1972) have used an integral equations approach to study bonded patch repairs. The assumptions of Erdogan and Arin were subsequently used by Ko (1978) and Hong and Jeng (1985) in an analysis of sandwich plates with part-through cracks. Jones and Callinan (1979), Mitchell et al. (1975), and Chu and Ko (1989) have used the finite element method to study bonded patch repairs. Park et al. (1992) have used an integral equation approach combined with the finite element alternating method to estimate the stress intensity factors for patched panels. Tarn and Shek (1991) have combined the boundary element method (for the plate) and finite element method (for the patch) to estimate the stress intensity factors. Other work in this area includes Atluri and Kathiresan (1978), Sethuraman and Mathi (1989), and Kan and Ratwani (1981). A comprehensive summary of the analytical and numerical work on composite patch repairs appears in a recent monograph (Atluri, 1997). In most of these approaches, only patches of infinite size, very narrow strip-type patches, or infinite sheet cases are considered. All of these cases are valid only for flat sheets. The loading for these analyses are hoop stresses evaluated from basic thin-shell theory. Although in most cases this is a good approximation, this does not take into account the stress redistribution due to curvature and to the presence of stiffeners. Specific capability improvements that are needed include the ability to analyze the following structural details: the joint between the repair and the original structure the damaged structure with the repair in place the surrounding structure affected by changes in load paths complex and curved structural details Recommendation 45.  Develop, characterize, and evaluate improved damping materials with improved environmental resistance and low-temperature performance for repair and modification of dynamically loaded structures. Examples of specific tasks include development of accelerated environmental test methods and criteria to evaluate resistance to aircraft service conditions, including thermal and fluid exposures development and validation of repair concepts that include improved damping materials Damping material systems currently in use have shown inadequate durability. The committee recommends that long-term research be conducted to develop improved damping material systems that provide low-temperature damping performance and better resistance to aircraft fluids and environmental exposure. Candidates should be tested under low-temperature conditions, with temperature cycling through realistic aircraft environments, including moisture and fuel, where necessary. Methods to accelerate this type of testing will be important for both the screening of developmental systems and for the characterization and acceptance of selected systems. Repair designs that use these improved damping systems must be validated to ensure that the improved performance translates into more-durable repairs. These systems may require additional care to ensure their durability. Damped composite repairs provide the potential to seal the stand-off material to prevent or delay moisture intrusion. Best practices must be incorporated into the repair system to ensure the integrity of the bond and the effectiveness of the damping materials.

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Aging of U.S. Air Force Aircraft: Final Report 9 Prioritized Research Recommendations Because of the budget pressures and difficult choices associated with conducting and managing a R&D program, the committee task included a charge to prioritize research recommendations. The committee developed criteria that were used to prioritize all of the research recommendations in Chapters 6 to 8. Priority levels for recommended R&D opportunities were established relative to the Air Force objectives introduced in Chapter 1 (i.e., safety of flight [Objective A], maintenance costs and force readiness [Objective B], and economic life estimates [Objective C]). Clearly, research that eliminates substantial threats to flight safety receives consideration for the highest priority to the Air Force. However, the escalation of maintenance costs and the impact on force readiness has become a pervasive issue that, if allowed to continue unchecked, could significantly hamper the ability of the Air Force to field a force that meets mission requirements for capability and readiness. The committee did not prioritize the recommendations with respect to Objective C because they found that research recommendations to develop technology to support economic life estimates related closely to the more important Objective B. Definitions of priority categories 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 recommendations 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 improvements 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) In addition, the committee assigned technical risk categories for long-term research recommendations. Technical risk is an assessment of the difficulty involved in achieving a technical objective. The committee designated technical risk associated with long-term research opportunities as low (validation and implementation of technology that is essentially mature), moderate (some further technology development and scaling required), and high (significant technology advancement required). The long-term research program should contain a mix of risk categories. Moderate-and high-risk programs should be monitored closely by the proposed aging aircraft technical steering group to ensure continued progress in clearing technical hurdles and continued need for the resulting technology for the maintenance of the aging force. Near-term opportunities were generally assumed to have low technical risk. CRITICAL PRIORITIES There are no research efforts identified at this time that are considered of sufficient magnitude to be categorized as critical priorities. However, the committee believes that it is possible that the durability and damage tolerance updates recommended in Chapter 5, and in particular the high-priority updates on the F-16, A-10, U-2, and T-38 aircraft, will identify critical priority near-term research and engineering tasks. These could include development of specific inspection instruments or procedures development of specific repair or modification designs or processes development and use of more sophisticated analysis procedures and additional full-scale fatigue testing to identify fatigue-critical areas obtaining additional flight loads and environment data for specific aircraft NEAR-TERM RESEARCH Prioritized recommendations for near-term R&D are shown in Table 9-1, including the recommendation number, a brief description of the recommendation, the page where the recommendation is discussed, the objective that is addressed primarily by the recommended research, and the suggested

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Aging of U.S. Air Force Aircraft: Final Report TABLE 9-1  Prioritized Near-Term Research Recommendations No. Recommendation Description Objective Priority   Fatigue (1) Fail-safe residual strength prediction methods Page 50 A 2 (2) Improve current methods to estimate the onset of WFD Page 50 A 2 (6) Methods to predict dynamic responses Page 52 B 2 (11) Effect of corrosion damage on material properties Page 55 A 3 (12) Effect of corrosion and corrosive environment on safety limits Page 55 A 3 (13) Effect of joint pillowing on fail-safety Page 55 A 2   Corrosion Prevention and Control (17) Laboratory test protocol for accelerated corrosion testing Page 57 B 2 (18) Evaluate durability of new protective coatings Page 58 B 1 (19) Methods for early detection of corrosion Page 58 B 2   Stress Corrosion Cracking (23) Affordable upgrades in SCC prevention Page 60 B 1 (24) Evaluation of SCC protection systems Page 60 B 1 (25) Residual stresses and their alleviation Page 61 A 2 (26) SCC susceptibility of Air Force alloys Page 61 A 2   NDE (29) Evaluate, validate, and implement NDE equipment and methods Page 64 B 1 (30) NDE automation, data processing, and analysis Page 66 B 2   Maintenance and Repair (37) Guidelines to implement advances in bonded repairs Page 69 B 2 (38) Solid model interfaces to simulate repair methods Page 70 B 2 (39) Guidelines on relative lives of bolted repairs Page 70 A 3 (40) Reduce cost of materials and structures substitution Page 71 B 2 (41) Repair design guidelines for high-cycle fatigue problems Page 71 B 2 priority. Priority 1 recommendations include (1) research to develop and implement corrosion prevention and control procedures and (2) evaluation and implementation of nondestructive evaluation techniques that address specific Air Force aging aircraft issues. LONG-TERM RESEARCH Prioritized recommendations for long-term R&D are shown in Table 9-2, including the recommendation number, a brief description of the recommendation, the page where the full recommendation appears, the objective that is addressed primarily by the recommended research, an assessment of technical risk, and the suggested priority. Priority 1 recommendations include (1) research to develop a fundamental understanding of corrosion and stress corrosion cracking to support the development of improved corrosion prevention and control procedures and (2) development and validation of rapid, wide-area nondestructive evaluation techniques to address specific aging aircraft needs.

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Aging of U.S. Air Force Aircraft: Final Report TABLE 9-2 Prioritized Long-Term Research Recommendations No. Recommendation Description Objective Technical Risk Priority   Fatigue (3) WFD crack formation and distribution relationships Page 50 A moderate 2 (4) Analytical prediction of WFD crack distribution functions Page 51 A high 2 (5) Validation of analytical WFD methods Page 51 A low 2 (7) Crack growth threshold behavior Page 52 B low 2 (8) Analytical methods to predict dynamic behavior Page 53 B moderate 2 (9) Expert system for high-cycle fatigue repairs Page 53 B high 3 (10) Dynamic load monitoring and alleviation Page 53 B moderate-high 2 (14) Effect of environment on growth of small cracks Page 55 A low 2 (15) Effect of flaw morphology on crack growth Page 56 A moderate-high 2 (16) Effect of hydrogen on fatigue crack growth Page 56 A moderate 3   Corrosion Prevention and Control (20) Basic research in corrosion prevention and control Page 59 B high 1 (21) Corrosion rates for major corrosion types Page 59 B moderate 2 (22) Basic research in coating durability Page 60 B moderate 1   Stress Corrosion Cracking (27) Fundamental research in SCC prevention Page 61 B moderate-high 1 (28) Life prediction methods for SCC Page 62 B high 2   NDE (31) Develop integrated quantitative NDE capability Page 66 B moderate-high 1 (32) Hybrid inspection technologies Page 67 B high 2 (33) NDE to assess composite repairs Page 67 B high 2 (34) Advanced technologies to track maintenance trends Page 68 B moderate-high 3 (35) Automation of wide-area inspections Page 68 B moderate 1 (36) NDE for early corrosion detection Page 68 B high 3   Maintenance and Repair (42) Expert system for design and analysis of repairs Page 71 B moderate 2 (43) Common database of repair lessons learned Page 72 B low 2 (44) Analysis methods for structural repairs Page 72 B moderate 3 (45) Damping material for dynamically loaded structures Page 72 B moderate 3

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Aging of U.S. Air Force Aircraft: Final Report 10 Future Structural Issues: Composite Primary Structures The issues and concerns identified by the committee during this study have concerned metallic alloy structures that make up the vast majority of the airframes in the Air Force aging aircraft. However, more-recent aircraft have significant quantities of the flight control (C-17) and primary airframe structures (B-2, F-22) constructed from carbon-fiber-reinforced polymeric composites. The purpose of this chapter is to describe service experience with composite structure—including Air Force and commercial experience with secondary structures and flight control structures as well as Navy and commercial experience with primary structure—and to recommend potential degradation mechanisms and actions to monitor and evaluate the aging of composite structures in the future. APPLICATIONS AND SERVICE EXPERIENCE Secondary Structures The application of polymeric composites has been an evolutionary process, with increased use as materials and processing technology matured and program needs dictated their use. First-generation, glass-reinforced composites, in the form of thin-face sheet honeycomb sandwich constructions, have been in general use for secondary structures (i.e., wing-to-body fairings, fixed-wing and empennage cover panels, and secondary control surfaces) on Air Force and commercial transport aircraft since the 1960s. During the 1970s, the commercial availability of carbon and aramid fibers, the performance enhancements made possible by weight savings, and uncertainty in fuel supply and costs provided an impetus for the development and application of structural composites for airframe applications. The Air Force conducted much of the pioneering research in materials, processes, and design of composite structures leading to the application of composites in secondary and flight control structures on the F-15, F-16, and B-1B. The materials used for these components included largely unmodified amine-cured epoxy resins (e.g., TGMDA/DDS) reinforced with aramid (Kevlar® 49), carbon (e.g., Amoco T-300, Hercules AS-4), and E-glass fibers. Structures were generally thin 0.6- to 1.5-mm (0.024- to 0.060-in.) facesheets co-cured or secondarily bonded to composite or aluminum honeycomb core. At about the same time, the commercial industry became interested in the application of composite structures. To encourage the use of composites in commercial production applications, NASA conducted technology development and flight service programs, including design, certification, and use in airline service. Carbon/epoxy, aramid/epoxy, and aramid-carbon/epoxy and glass-carbon/epoxy hybrid composites were first used on a production scale in the early 1980s for the generation of aircraft that included the Boeing 757, 767, and 737-300; Airbus A310 and A320; and McDonnell Douglas MD-80 series. Applications included secondary structures such as fairings, fixed-wing and empennage cover panels, and engine cowlings, as well as primary flight controls such as ailerons, elevators, rudders, and spoilers. The number of aircraft involved and the high use rates of commercial aircraft has led to a great deal of service experience with composites in commercial aircraft applications. For example, NASA has conducted flight service evaluations of 350 components with over 5.3 million total flight hours (Dexter and Baker, 1994). In general, the service experience with composites indicates that damage occurs because of discrete sources such as impacts, lightning strikes, and handling rather than progressive growth caused by fatigue conditions (NRC, 1996a). The types of damage to composite components include disbonds or delaminations, holes or punctures, cracks, and other damage. An especially difficult maintenance issue resulting from these types of damage is when perforation of the facesheet allows hydraulic fluids, water, and other liquids to move into the honeycomb core. Primary Structures Throughout the 1970s and 1980s, the Air Force was instrumental in the development of materials, processes, and design considerations for primary structural applications of polymeric composites. The Air Force has only recently made significant use of composite primary structure on the B-2 and will continue on the upcoming F-22. The Navy and the commercial aircraft industry have limited service experience for primary composite structures on the Navy F/A-18 and AV-8B and on the Airbus A320. The constructions are integrally stiffened carbon/epoxy laminate skin fabricated from

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Aging of U.S. Air Force Aircraft: Final Report materials similar to the first-generation materials previously used for secondary structure and primary flight controls. The further development of carbon fibers with improved strength and modulus (e.g., Hercules IM7 and Toray T-800H) and high-performance and toughened matrix polymers has led to application on the Boeing 777 empennage to expand the primary structural applications. Guidance for the selection, design, and analysis of composite structures for polymeric composites have been developed over the past 25 years (Whitehead et al., 1986; Vosteen and Hadcock, 1994). These methods, forming the basis for MIL-HNBK-17 (DOD, 1994), are based on static ultimate strength considerations and the effects of three primary structural degradation mechanisms: Impact damage. To verify impact tolerance, the structure is subjected to a low-velocity impact prior to the fatigue testing to substantiate inspection intervals and performance for the life of the structure under barely visible impact damage criteria. Mechanical fatigue. Fatigue is not generally a significant damage mechanism in fiber-dominated composite structures that meet impact damage tolerance requirements described above (Jeans et al., 1980). Components that experience significant interlaminar or out-of-plane loading can be susceptible to fatigue damage. Humidity (or fluid) exposure. Design properties based on coupon tests are typically generated in a fully saturated humidity condition (85 percent relative humidity). Real-time exposures, using flight service components and ground exposures, have verified this approach (Dexter and Baker, 1994). Consideration of these degradation mechanisms and the use of structural design verification testing, with evaluations on scales from coupon level to full scale, have successfully offset the limitations of design analysis methods in terms of prediction of interlaminar stresses, damage initiation, and delamination growth (NRC, 1996b). The final step of this approach is typically a full-scale component fatigue test on an impact-damaged structure. The limited experience of the Navy and commercial aircraft service with composite laminate constructions used for primary structures has been good. There have been very few occurrences of damage in primary structures, and in the few cases that have occurred, there have been no indications of progressive damage. Potential degradation mechanisms to monitor in the future for composite structural applications include (1) the development of transverse matrix cracking due to 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; and (4) the development of corrosion in adjacent metal structures. RECOMMENDATIONS FOR LONG-TERM RESEARCH The committee recommends that the Air Force undertake research to monitor potential deterioration of composite structures and to develop or improve maintenance and repair technologies, especially for composite primary structures. The recommendations are considered long-term research opportunities because they do not specifically support near-term engineering or management actions discussed in Chapter 5. Recommendation 10-1.  Develop, validate, and implement NDE equipment and methods to reliably detect degradative damage of composite structures, especially the development of transverse matrix cracks, impact damage, delamination associated with growth of manufacturing-induced damage around fasteners, moisture penetration near edges, and corrosion of adjacent structure. The committee recommends that the Air Force evaluate, adapt, and utilize NDE advances to develop methods and equipment capable of evaluating the key composite damage mechanisms. Emphasis should be placed on automated methods, compatible with depot-level application, to perform rapid, wide-area inspections. As described in Chapter 8, the committee recommends a life-cycle approach to evaluate and validate methods that considers detectability and inspectability, full-scale validation, material degradation mechanisms, technique reliability, inspection intervals, and cost. The most promising technologies that are currently available include ultrasonic methods (c-scan, scan imaging, and resonance techniques) and thermal methods (large-area impulse heat technique). There have been significant advances in automated inspection methods for production and in-process inspection of composite structures that could be adapted to the depot environment. Recommendation 10-2.  Develop tools and guidelines to standardize bonded repair methods for composite structures. Occasionally, temporary or permanent repairs of composite honeycomb structures can be performed by bonding or bolting a sealant-coated metal or precured composite overlay over the damage. However, most permanent repairs are accomplished with room-temperature curing wet lay-up, precured patch, and elevated temperature prepreg repair techniques. The Air Force has a unique capability, as described in Chapter 8, in the area of laminated composite patch repairs for metal structures. The techniques and tools developed for the design and evaluation of repair of aged metallic structures should be extended and validated for composite structures. Perhaps the most pressing problem in patch repairs of composite structures is that the structures are fabricated from a large number of resin/reinforcement systems from several

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Aging of U.S. Air Force Aircraft: Final Report qualified suppliers, requiring the repair depot to stock a variety of repair materials. There is a pressing need to standardize repair materials and processes across the Air Force inventory. The Commercial Aircraft Composite Repair Committee (CACRC) has been formed to address composite service and repair concerns of the commercial aircraft industry. The Air Force should monitor the activities of the CACRC and evaluate the applicability of the recommendations of the CACRC to Air Force aircraft. Recommendation 10-3.  Develop tools and methods for bolted repairs of composite primary structures. The thicker laminate construction used in composite primary structures, as well as the size and nature of discrete damage from typical aircraft service (e.g., impact damage, lightning attachment damage, delaminations), are not conducive to wet lay-up patch repair technologies. Thin facesheets on honeycomb panels are generally repaired using bonded scarf patches with a scarf taper of 20:1, which, if applied to thicker laminate constructions, would result in the removal of a large amount of undamaged material (Bodine et al., 1994). Much of the emphasis in the development of primary structure repairs has therefore been on fastened, precured composite or metallic splice plates, similar to current bolted repair techniques for metal structure. The issues that must be addressed in these types of repairs include (1) criteria for determining when repairs are required; (2) availability of standardized repair elements; (3) drilled hole quality; (4) ability to restore original strength, durability, and damage tolerance; and (4) ability to match existing contours. Recommendation 10-4.  Evaluate environmentally benign paint removal methods recommended in Chapter 7 for compatibility with polymeric composite substrates. Composites must be protected by finishes with resistance to fluid penetration and UV degradation. Maintenance of protective finishes represents significant operational costs to the Air Force. The removal of finishes from composites is a slow and expensive process. Because chemical strippers attack the polymer matrix, finishes generally are removed using mechanical abrasion processes. New paint removal processes such as laser, heat, frozen carbon dioxide blasting, and wheat starch blasting are being evaluated. Rapid, low-cost, on-aircraft paint removal techniques are needed to reduce the cost of maintaining composite structures and to preclude damage to the structure.