2

Air Force Aging Aircraft Program

This chapter provides (1) a discussion of the Aging Aircraft Technologies Team (AATT), which was formed in response to the 1997 NRC study on U.S. aging aircraft and (2) discussion of technical areas and interagency technical issues.

AGING AIRCRAFT TECHNOLOGIES TEAM

The AATT was formed in response to a recommendation of the Committee on Aging of U.S. Air Force Aircraft that the Air Force “appoint a single knowledgeable and experienced technical leader responsible for the oversight of the aging aircraft activities” (NRC, 1997, p. 48). The AATT provides the framework for management, programming, and technology development and transition. The team has established three program groups: science and technology (S&T), technology transition, and structural assessments. AATT is responsible for identifying R &D needs and opportunities to support the continued operation of aging aircraft and to implement that research to ensure flight safety and reduce maintenance and repair costs. To carry out its responsibilities, AATT coordinates with the major commands, depots, field operations, and airplane single managers. The structural assessment group does not manage program funds but does provide engineering expertise in structural analyses and systems engineering. The systems engineers work with the other two groups under a single technical leader from the Aeronautical Systems Center (ASC) to develop all S&T and acquisition programs for aging aircraft.

Program Scope And Objectives

The 1997 NRC report recommended that the Air Force adopt a three-pronged plan of action: (1) near-term action (3 to 5 years) to improve the maintenance and management of aging aircraft; (2) near-term R&D to support the near-term actions; and (3) long-term R&D. The highest-priority research issues were technologies that would lead to reduced maintenance costs, improved force readiness (by prevention and/or control of corrosion and stress corrosion), and



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SMALL BUSINESS INNOVATION RESEARCH TO SUPPORT AGING AIRCRAFT: Priority Technical Areas and Process Improvements 2 Air Force Aging Aircraft Program This chapter provides (1) a discussion of the Aging Aircraft Technologies Team (AATT), which was formed in response to the 1997 NRC study on U.S. aging aircraft and (2) discussion of technical areas and interagency technical issues. AGING AIRCRAFT TECHNOLOGIES TEAM The AATT was formed in response to a recommendation of the Committee on Aging of U.S. Air Force Aircraft that the Air Force “appoint a single knowledgeable and experienced technical leader responsible for the oversight of the aging aircraft activities” (NRC, 1997, p. 48). The AATT provides the framework for management, programming, and technology development and transition. The team has established three program groups: science and technology (S&T), technology transition, and structural assessments. AATT is responsible for identifying R &D needs and opportunities to support the continued operation of aging aircraft and to implement that research to ensure flight safety and reduce maintenance and repair costs. To carry out its responsibilities, AATT coordinates with the major commands, depots, field operations, and airplane single managers. The structural assessment group does not manage program funds but does provide engineering expertise in structural analyses and systems engineering. The systems engineers work with the other two groups under a single technical leader from the Aeronautical Systems Center (ASC) to develop all S&T and acquisition programs for aging aircraft. Program Scope And Objectives The 1997 NRC report recommended that the Air Force adopt a three-pronged plan of action: (1) near-term action (3 to 5 years) to improve the maintenance and management of aging aircraft; (2) near-term R&D to support the near-term actions; and (3) long-term R&D. The highest-priority research issues were technologies that would lead to reduced maintenance costs, improved force readiness (by prevention and/or control of corrosion and stress corrosion), and

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SMALL BUSINESS INNOVATION RESEARCH TO SUPPORT AGING AIRCRAFT: Priority Technical Areas and Process Improvements automated NDE/NDI methods. A properly focused SBIR program could address some of these critical needs. Aging affects every element of the aircraft system: airframe, engines, avionics, and subsystems. AATT originally limited its scope to airframes, but it is considering expanding its scope to include subsystems. Based on input and participation from the aging aircraft community, AATT identifies problems that have an R&D solution, matches these problems with a technology, and then supports development and transfer of the technology to the user. Companion programs in ASC and AFRL with substantial resources are addressing other component areas, such as propulsion systems and avionics. AATT has adopted the following guiding principles: (1) meeting the needs of Air Force aircraft; (2) improving flight safety, reducing maintenance costs, and enhancing availability of aircraft; (3) remaining output-oriented and cost-focused; (4) developing technologies that can be transferred; and (5) augmenting the capability in industry and government. AATT's specific objectives are (1) to develop and field technologies to extend the life and/or reduce the cost of aging systems; (2) to ensure flight safety and avoid catastrophic failures; (3) to reduce maintenance and repair requirements and their associated costs; and (4) to increase force readiness. Processes AATT has established several key processes to implement its programs and to develop the partnerships necessary for effective technology transition (see Figure 2-1). These key processes are: annual durability assessment surveys led by ASC establishment of the Aging Aircraft Working Group, led by ASC initiation of the aging aircraft Integrated Technology Thrust Program (ITTP), led by AFRL The annual surveys cover all aging aircraft systems. An ASC/AFRL team, led by the technical leader, visits all Air Force air logistics centers (ALCs) during the summer to review the status of structures and subsystems of all aircraft, whether they are maintained by the Air Force or by contractor logistics support. The results of these surveys are compiled and summarized in an issues and requirements document (ASC/AFRL, 1999).

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SMALL BUSINESS INNOVATION RESEARCH TO SUPPORT AGING AIRCRAFT: Priority Technical Areas and Process Improvements FIGURE 2-1 Solving aging aircraft problems with cost-focused methods —the AATT process. Figure courtesy of Air Force Aeronautical Systems Center.

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SMALL BUSINESS INNOVATION RESEARCH TO SUPPORT AGING AIRCRAFT: Priority Technical Areas and Process Improvements At the beginning of each calendar year, ASC launches a dialogue with the ALCs, the system program offices (SPOs), the Major Commands (MAJCOMs), AFRL, and industry to obtain specific PE 6.5 program recommendations for the next fiscal year. This dialogue also includes the small business community and others (such as academia) who may have innovative ideas but may not be aware of aging aircraft issues. The results are brought to the Aging Aircraft Working Group in the spring, where a prioritized list of acquisition programs is developed and approved by ASC leadership. By designating aging aircraft as an ITTP within the sustainment integrated technology thrust, AFRL has enabled the coordination of management and programming among the AFRL directorates, principally the AFRL/Materials and Manufacturing Directorate (ML) and the AFRL/Air Vehicles Directorate (VA). The ITTP and directorate staffs participate in the processes described above to develop the S&T program each year along the same time line used by the ASC to develop the PE 6.5 acquisition program. All of these processes are timed so customer requirements can be updated by the beginning of the calendar year. According to the schedule, requirements-driven program recommendations are developed during the spring, leadership approval processes are completed, and budgets are finished by early summer in time to begin implementation at the beginning of the fiscal year in October. Program Strategy and Road Maps The Air Force technology strategy for managing the aging aircraft fleet is shown in Figure 2-2. The warfighters that manage the aircraft have a formal plan for keeping the structure healthy, the Force Structural Maintenance Plan (FSMP), which specifies what must be done to the aircraft structure during maintenance and how it must be maintained when returned to service. Road maps for resource allocation are developed for each technical topic area. The road maps, along with a high-level strategy, summarize the funding of AFRL and ASC programs, the program interrelationships, key program milestones, and scheduled product deliveries to the warfighter and sustainer customers. The principal interface between the supplier and customers occurs through the FSMP, which is used to guide aircraft maintenance and the development of structural-assessment tool sets by the technology community. The structural-assessment tool sets include structural integrity analysis techniques and supporting technologies for the prevention, identification, repair, and maintenance of structural degradation caused by cracking and corrosion. Cost-effectiveness analyses are being incorporated into the tool sets. The Air Force envisions that the implementation of new technologies will lead to a cultural change in the sustainment philosophy for aging aircraft. Instead

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SMALL BUSINESS INNOVATION RESEARCH TO SUPPORT AGING AIRCRAFT: Priority Technical Areas and Process Improvements FIGURE 2-2 Strategy for managing the aging aircraft fleet. Figure courtesy of Air Force Aeronautical Systems Center.

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SMALL BUSINESS INNOVATION RESEARCH TO SUPPORT AGING AIRCRAFT: Priority Technical Areas and Process Improvements of the old find-and-fix culture, which is conservative, reactive, and often costly, the new culture will incorporate a proactive philosophy of anticipating and managing problems. This new culture is much like the prevention-and-control strategy that has been very effectively implemented by the commercial aircraft industry. The new culture will enable the Air Force to anticipate and correct problems and manage its workload more effectively. The major needs identified by AAAT are as follows: developing economic-service-life and cost-of-ownership models determining the onset of widespread fatigue damage preventing, assessing, and controlling corrosion reducing the inspection burden and improving inspection capability standardizing bonded repair improving maintenance business practices The ASC Aging Aircraft Product Support Group has programs in all of these areas. Since 1996, these programs have been the principal source of new resources. ASC programs funded for FY01 are shown in Table 2-1 in order of priority. Note that some PE 6.5 programming is being initiated in high-priority subsystems areas such as electrical wiring and landing gear. Future programs may focus on NDE/NDI, repair, corrosion control, and structural integrity (see Table 2-2). As Table 2-1 and Table 2-2 show, corrosion (prediction, detection, and control), repair, and NDE/NDI are, and will continue to be, major areas of emphasis for aging aircraft. Table 2-1 and Table 2-2 also indicate many opportunities for SBIR projects. SBIR programs are currently not emphasized on road maps for future research (or in the programming strategy these road maps represent). One reason for this is that engineers cannot count on being awarded a Phase I topic when it is needed. Even if they are awarded one, there is no certainty that a Phase II award will be made following a successful Phase I. Many engineers attribute the problem to the large number of topics that are submitted initially to higher levels for approval, the very low percentage actually approved, and the lack of full-SBIR-cycle resource commitments. Finding. The current planning process does not encourage the identification of the SBIR program on the road map; thus, many Air Force engineers do not see the SBIR program as an opportunity to address issues in a timely fashion.

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SMALL BUSINESS INNOVATION RESEARCH TO SUPPORT AGING AIRCRAFT: Priority Technical Areas and Process Improvements TABLE 2-1 FY01 ASC Aging Aircraft Acquisition Programs Corrosion quantification for structural integrity analysis Detection and quantification of hidden corrosion using ultra-image system Corrosion prediction management AGILE for new landing-gear technologies MAUS ultrasound eddy current wing-skin corrosion detect transition Improvement of wire system integrity for legacy aircraft Quality control of composite/bonded repair surface preparation Material substitution for aging systems 2nd layer inspection of F-15 lower wing-spar areas AGILE for brake system and overhaul process improvement Aging aircraft software library Exfoliation effects on buckling strength Wiring maintenance data analyses Table courtesy of Air Force Aeronautical Systems Center. TABLE 2-2 Future Technology Programs Nondestructive investigation (NDI) Corrosion-focused tools Multilayer inspection Hidden damage Health monitoring NDI through paint Repair Smart patch repair Advanced mechanical repairs Composite patch total transition Corrosion control/suppression technologies Surface preparation for field/depot Materials substitution Cadmium/chromium replacement Corrosion prediction/structural integrity modeling Paint-for-life corrosion system Selective stripping Piece part counting/repair technologies Structural integrity Add corrosion prediction to the structural integrity code Affordability framework Table courtesy of Air Force Aeronautical Systems Center.

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SMALL BUSINESS INNOVATION RESEARCH TO SUPPORT AGING AIRCRAFT: Priority Technical Areas and Process Improvements Recommendation. Under the current funding process for SBIR, at least one contract can be funded for each topic. These agency-approved and laboratory-approved SBIR topics should be shown on road maps systemwide and should be built into the overall road map programming strategy. If the focus topics approach (described in Chapter 5) is implemented, SBIR funding used to support the development of innovations needed can be accorded attention when a new research or development focus is being planned or is just beginning. Resources The AFRL baseline funding for R&D on aging aircraft includes funding for projects focused on structural integrity, repair, NDE/NDI, and corrosion. Table 2-3 shows the funding profiles for those four areas from FY99 through FY05. ASC funding for the new PE 6.5 acquisition program in aging aircraft managed by SMA is shown in Table 2-4. TABLE 2-3 AFRL Funding Profiles for Aging Aircraft Programs (million $)   FY99 FY00 FY01 FY02 FY03 FY04 FY05 Structural integrity 8.5 11.0 9.3 13.3 16.4 12.7 11.5 Repair 4.2 5.0 5.0 3.7 1.6 0.2 0.2 NDE/NDI 3.6 2.8 1.8 2.5 3.5 1.7 0.7 Corrosion 5.5 2.9 1.6 1.5 1.4 1.4 1.5 TOTAL 21.8 21.7 17.7 21.0 22.9 16.0 13.9 Table courtesy of Air Force Aeronautical Systems Center. TABLE 2-4 ASC Funding for Aging Aircraft (million $)   FY99 FY00 FY01 FY02 FY03 FY04 FY05 Approximate Approximate PE 6.5 funding 4.6 4.9 14.2 28.2 42.1 42.9 43.7 Source: Defense Technology Information Center, <www.dtic.mil/rdds/>.

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SMALL BUSINESS INNOVATION RESEARCH TO SUPPORT AGING AIRCRAFT: Priority Technical Areas and Process Improvements Other acquisition programs managed by ASC/SMA also have aging aircraft programs (see Table 2-1). These include the Commercial Operations and Support Savings Initiative, a DOD initiative for the insertion of cost-saving commercial technologies into fielded military systems; overall funding for this initiative is projected to be approximately $20 million per year through FY05. ASC's Productivity/Reliability/Availability/Maintainability Program also includes work on structures to facilitate the transition of off-the-shelf and emerging technologies; funding is projected to increase from $9.4 million in FY00 to $31.2 million in FY05. These significant funds are an important potential source of Phase III funding for SBIR innovations. An AFRL-directed analysis of the technology recommendations in the 1997 NRC report indicated that additional S&T investments would be appropriate, particularly in the areas of NDE/NDI and corrosion. The results of this analysis are shown in Table 2-2. AFRL did not increase its overall investments significantly; however, investments were focused in the areas recommended by the NRC (NRC, 1997) and the AATT. TECHNICAL ISSUES The 1997 NRC report described many technical challenges involved in maintaining a large fleet of aging aircraft; in this section, those technical challenges are summarized and areas that can be addressed by the SBIR program are identified. This section also provides (1) background on other technical issues facing the Air Force and (2) a description of some R&D undertaken in response to recommendations in the 1997 NRC report. Key technical issues are listed below (NRC, 1997; Lincoln, 2000): adequacy of damage-tolerance derived NDI programs determination of the time of onset of widespread fatigue damage (WFD) prevention and tracking of corrosion and incorporation of the effects of corrosion into structural integrity analyses high-reliability repairs adequacy, completeness, and retention of flight data and field and depot maintenance information flight beyond design life ability to make repair, replacement, and retirement decisions: support of cost-of-ownership and economic-service-life determinations These issues, and the issue of structural dynamics and aeroelasticity, are discussed below.

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SMALL BUSINESS INNOVATION RESEARCH TO SUPPORT AGING AIRCRAFT: Priority Technical Areas and Process Improvements Fatigue and Corrosion Fatigue Air Force Structural Integrity Program ASIP has developed a successful cradle-to-grave approach to ensuring the durability and safety (damage tolerance) of aircraft structures. In this damage-tolerance approach, a severe defect, flaw, or crack is placed at several critical locations in the structure where, if failure were to occur, loss of the aircraft might result. Crack-growth calculations, combined with known NDE/NDI high-probability-of-detection (POD) limits, are used to determine inspection intervals and the safety limits of the structure. Durability of an aircraft is established by assuming typical flight and structural conditions. The prediction of fatigue life is based on the identification of critical locations; definitions of structural loads, stresses, and stress spectra; the quality of the structure's manufacture; and the determination of crack growth as a function of the number of loading cycles for various mission profiles (see Figure 2-3). This information is then used in the development of the FSMP. The procedure for handling the structural integrity of aircraft structures is described in the 1997 NRC report, which also references the detailed military standards that are followed. Damage-tolerance assessments are the basis for maintaining flight safety. The basic principle of ASIP is that the damage-tolerance approach, in conjunction with a robust inspection and maintenance program, ensures flight safety. The current process, as institutionalized through ASIP, is working well. The 1997 NRC report also provides research recommendations for low-cycle and high-cycle fatigue. Two technical issues are related to low-cycle fatigue: the rapid increase in the number of fatigue-critical areas in safe-crack-growth-designed structures (structures designed to allow cracks that do not compromise safety) and the potential for missing new areas as they develop the onset of WFD in fail-safe-designed structures The committee that produced the 1997 NRC report concluded that it could not develop a research initiative that would improve on the current approach for identifying new fatigue-critical areas. Therefore, the Air Force has no current or ongoing research in this area. R &D in low-cycle fatigue is focused on WFD. R&D on high-cycle fatigue falls under the category of structural dynamics and aeroelasticity, described below. Much of the WFD in aging aircraft occurs in joints, where it is caused mostly by friction and wear associated with joint contact loads. These stresses are

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SMALL BUSINESS INNOVATION RESEARCH TO SUPPORT AGING AIRCRAFT: Priority Technical Areas and Process Improvements FIGURE 2-3 Damage-tolerance approach to the prediction of fatigue life. Figure courtesy of Air Force Aeronautical Systems Center.

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SMALL BUSINESS INNOVATION RESEARCH TO SUPPORT AGING AIRCRAFT: Priority Technical Areas and Process Improvements Coatings Coatings are an obvious operational requirement for implementing a cost-effective strategy to prevent and control corrosion damage to airframe structures. The integrity and durability of protective finish systems on aging aircraft is an important factor in corrosion prevention. Aircraft coatings must meet demanding design criteria, including ambient curing; adhesion to a wide variety of substrates; long-term corrosion protection against humidity, chemicals (e.g., hydraulic fluids, fuels, and solvents), and cleaning solutions; and mechanical durability under operating stresses and in fretting environments. Restoring coating integrity after maintenance and repair is extremely important. CPCs that can be applied to external surfaces to penetrate and protect unsealed joints and around fastener heads would be very beneficial. These compounds, which are a critical part of maintenance programs to prevent and control corrosion, are being increasingly used in new aircraft, especially in lower fuselage areas. As an aircraft ages and protective finishes and coatings break down, the danger of part failures caused by SCC increases, particularly in structures not designed to be fail-safe. The epoxy and polyurethane systems that have been the mainstay of aircraft coatings have been modified and will continue to change in response to environmental regulations that limit the release of volatile organic compounds (VOCs) and materials containing heavy metals such as chromium or cadmium, used to inhibit corrosion. Specific technical issues have been identified for CPC development: (1) the need for a topcoat with good optical properties (e.g. high pigmentation) and superior durability; (2) the need for a primer that is both a good inhibitor and a chromate-free barrier to corrosion; and (3) the need for a surface treatment that can densify the surface oxide, thus providing corrosion protection without adding chromates. A variety of coating technology programs are ongoing at the AFRL focused on near-term, medium-term, and long-term corrosion-prevention goals. The near-term programs are addressing the integration and transition of new coating materials and processes. Medium-term projects are focused on the development of high-durability, environmentally compliant (chromate-free and reduced VOCs) topcoats and selective stripping to the permanent chromated primer. Based on the promising results of current programs, the focus of long-term R&D has shifted toward discovering fundamental corrosion and degradation mechanisms. Many projects, such as the development of a permanent (30- to 40-year) primer or foundation layer, an 8-year mission-tailored topcoat that is easily removable, and effective NDE/NDI through coatings, have been established with the goal of minimizing maintenance over the system lifetime.

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SMALL BUSINESS INNOVATION RESEARCH TO SUPPORT AGING AIRCRAFT: Priority Technical Areas and Process Improvements Nondestructive Evaluation Methods For a fleet that is growing older and older and requires not only aircraft safety but also mission readiness, improved nondestructive inspection (NDI) methods are critical. As reported recently in the Nondestructive Testing Information Analysis Center Newsletter, “the reality of trying to maintain aircraft airworthiness over an unprecedented 50- to 80-year life span presents a whole new set of technical problems/issues the original design did not have to meet ” (Bartel, 2000). In concurrence with the 1997 NRC report, the AATT identified the detection of subsurface cracks and hidden corrosion as the two greatest concerns for deployed aircraft. The costs of repair for corrosion-related problems as estimated by the Air Force corrosion office survey (conducted periodically) exceeded $800 million in 1997 (Cooke et al., 1998). Not surprisingly, the critical nature of these two problem areas was overwhelmingly reinforced by operations and sustainment data from the Navy, as reported to the Joint Aeronautical Commanders Group. Accordingly, the DOD NDE/NDI community has focused its efforts on developing and implementing technologies to address these specific issues. Both the Air Force and the Navy have increased their use of SBIR funds to supplement their in-house efforts; however, those efforts have not yet made an impact at the field depot level. The FAA, which is primarily a regulatory agency, has focused more on validating and enforcing the implementation of existing inspection protocols and improving the training of airworthiness inspectors and maintenance technicians for commercial aircraft. Although method development is not a specific aspect of the FAA's mission, the agency is supporting the development of maturing NDE/NDI technologies for corrosion and crack detection through its SBIR program. It is also encouraging commercial airlines and aircraft manufacturers to find alternative, less costly ways to perform required inspections. However, the FAA's major focus at the moment is on the detection of aging and faulty wiring. Historically, the most common NDI method for detecting corrosion and cracking in aircraft structures has been visual inspection. Several drawbacks to this approach have been noted, the most significant being the amount of time it takes to inspect an entire airframe and all of its critical components visually and the inability to see beneath paint and inaccessible areas. By the time hidden corrosion is detectable visually—usually because the buildup of corrosion products between layers results in a bulging external surface (pillowing) —the degree of damage is so great (10 percent or greater material loss) that repair or replacement are the only viable options. For critical substructures, inspection often requires the costly removal of overlying components, which has the potential for causing damage. Also, some forms of corrosion damage, such as SCC, are not readily detectable visually, even at an advanced stage.

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SMALL BUSINESS INNOVATION RESEARCH TO SUPPORT AGING AIRCRAFT: Priority Technical Areas and Process Improvements Implementing proactive measures for aircraft sustainment (e.g., life-cycle management decisions to repair, replace, or fly-as-is and the establishment of inspection intervals) will require quantitative assessments of damage as opposed to simple damage or defect detection. Other traditional nondestructive methods and facilities (e.g., radiography, ultrasonics, eddy current) could be used to characterize hidden cracks and corrosion. The issue has most often not been the feasibility of the method but the practicality and the specifics of its implementation. Multilayer structures in particular can present immense difficulties to NDE/NDI methods, such as ultrasonics and thermal imaging. The form of the flaw, such as cracks under rivet heads, SCC, and pitting, can severely impact the efficacy of NDE/NDI methods. In addition, field depots responsible for aircraft inspection, maintenance, and repair are traditionally not as well outfitted or up to date as their production counterparts or research partners. Manual inspections and many portable units are tedious and potentially ineffective owing to human factors, such as fatigue, that arise simply because such a large area must be covered to do the job correctly. The difficulty of correcting this situation has been compounded by growing demands (i.e., increasing costs of corrosion repairs) on decreasing sustainment funds. These deficiencies have long been recognized by the Air Force, which sponsored an NED/NDI program in 1992 to evaluate commercially available NDE/NDI alternatives (Alcott et al., 1993). To the surprise of many researchers at the time, the enhanced visual method was the most effective of the portable, field-level methods surveyed for the detection of hidden corrosion. However, not all variations were represented in the study, and none of the techniques were performed at the levels desired by the Air Force. Most of the more advanced commercial equipment that had been successfully demonstrated in university or research laboratories was simply not field ready. In controlled experiments, these techniques were shown to be better in terms of sensitivity, but nonautomated field implementations were found to have the same drawbacks as existing techniques. In addition to corrosion-detection solutions for large accessible areas, such as fuselage and wing skins, corrosion in lugs, fittings, and landing-gear components (some of the most dangerous corrosion), especially those made of high-strength steel where cracks can propagate from a single corrosion pit, must also be addressed. In a recent survey on Boeing's 7XX series airplane models, fittings accounted for 45 percent of safety-critical Airworthiness Directive inspection procedures. Current technology-ready programs using the Mobile Automated Ultrasound System (MAUS) scanner can already scan fuselage and wing skins and detect thinning to within 5 percent. But for corrosion in fittings, other solutions, such as ultrasonic modeling techniques, methods that can detect cracks beneath bushings, embedded sensors, and small rotating scanners for areas with poor access, will be necessary. Emphasis should be on low-tech, inexpensive methods

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SMALL BUSINESS INNOVATION RESEARCH TO SUPPORT AGING AIRCRAFT: Priority Technical Areas and Process Improvements of inspecting small areas rather than on the development of expensive, complex systems built for a single purpose. In spite of a growing number of candidate techniques and adaptations, the number of new techniques implemented has not met growing needs. Although additional improvements or adaptations of existing systems are being made, no catchall solution is waiting in the wings. Air Force-funded systems, such as the MAUS, have helped overcome many of the implementation difficulties (AFRL, 1998). With continued improvements in hardware and software design, MAUS has become more useful at the depot level, and, with the incorporation of eddy-current sensors, a complementary inspection method particularly sensitive to near-surface cracks, MAUS can take advantage of the automated scanning platform. Current R&D on pulsed or low-frequency eddy-current methods is focusing on making them more effective for detecting cracks in multilayer structures (Buynak, 2000; Smith, 2000). For some applications, neural networks have been shown to increase the POD thresholds for traditional ultrasonics (Mullis et al., 2000). Many other adaptations of inspection methods (e.g., thermal imaging and real-time radiography) with varying degrees of promise and maturity are being investigated. All of these projects are moving in the right direction, but not at a pace that would meet the needs of the aging aircraft sustainment community. A strong SBIR program in this arena could have significant early payoffs. The 1997 NRC report recommended evaluation, validation, and implementation “of currently available NDE equipment and methods for use at Air Force maintenance facilities” as a near-term top priority (p. 64). The report also listed as a top priority the long-term need for the automation of successful inspection methods and the development of wide-area inspections. In addition, the report recommended a long-term top priority for an “integrated quantitative NDE capability,” indicating that the detection sensitivity requirements (i.e., percent corrosion, crack length) should be derived from structural analyses, including corrosion and crack geometry and local airframe structures, and that the NDE methods must have consistent, reliable POD and flaw sizing. The NDE/NDI development and insertion path the AFRL followed up to the time of the 1997 NRC report has since been validated by the report 's findings. Although the Air Force technical community obviously agrees with the recommendations in the report, AFRL admits that it has insufficient funds and staff to address them. Although AFRL has not redirected funds to cover this gap, plans are being made to strengthen collaborations with the Navy and the Coast Guard and with federal agencies such as the Defense Logistics Agency, NASA, and FAA to take better advantage of SBIR funding. More proactively, the ASC aging aircraft program (PE 6.5), under the guidance of AATT, is stepping up to cover some of the NDE/NDI needs. Nevertheless, the critical factor of time to technology insertion is not being met for several reasons. First, delays in getting SBIR-developed NDE/NDI

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SMALL BUSINESS INNOVATION RESEARCH TO SUPPORT AGING AIRCRAFT: Priority Technical Areas and Process Improvements technologies to field level are partly attributable to a lack of bridging funds after Phase II. Second, communication with the aging aircraft end users (e.g. ALCs) is not being done early enough or with enough follow-on commitment. Therefore, end users are not aware of how long it takes (and how much it costs) for new devices to mature. Although SBIR funds represent a significant investment by the government, those funds alone are insufficient to bring a new technology through prototype development to near-term implementation. Magneto-optic imaging of subsurface cracks, a new technology that resulted from SBIR funding, is a case in point (PRI R&D Corp, 1990). The current configuration of the imaging instrumentation is the result of two Phase I and Phase II R&D. In addition, the inventor required substantial venture capital, equivalent to about four more Phase I and Phase II funding cycles, to develop the instrument to its current state. It has been more than 10 years since the initial Phase I R &D, and no return on the investment has yet been realized. This time to commercialization is typical of most emerging technologies. Members of the committee have listened to several similar “success” stories in which the maturation of a new technology or concept took several cycles of SBIR Phase I and II funding to reach the technology insertion stage. Because small businesses must go back to the beginning of the SBIR process if Phase III funds or commercial partners cannot be found, they must contend not only with the delays associated with the Phase I and II selection and award processes but also with the possibility of not being selected in sequential cycles. Although innovation is traditionally interpreted as a new device or method, the term also applies to a novel adaptation, implementation, or integration of an existing technique. Adaptations of existing technologies can be performed relatively quickly and thus are better suited to addressing immediate operational sustainment needs. Integration with an existing platform (such as eddy-current probes with the MAUS scanning system) can significantly reduce development time. But success requires the collaboration and commitment of the “owners” (manufacturer or user) of the existing technology. This often requires discussions and negotiations of intellectual property or licensing agreements, or both, with the government or a DOD subcontractor, an effort that many small businesses are not prepared to handle. More often than not, a small business chooses to pursue an independent path that requires more effort and time to reach the implementation stage. Therefore, encouraging integration and collaboration in Phase I and II SBIR programs could make a significant impact. Commitment by the Air Force to provide continued support for SBIR NDE/NDI developments is critical to successful bilateral partnerships. Without a doubt, the development and validation of NDE/NDI methods for aging aircraft could benefit from SBIR programs, particularly those focused on implementation at the field depot level. Many new technologies and methods are

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SMALL BUSINESS INNOVATION RESEARCH TO SUPPORT AGING AIRCRAFT: Priority Technical Areas and Process Improvements being developed that have the potential to solve many aging aircraft problems but that lack funding and support for their timely implementation. Health Monitoring and Maintenance and Repair Issues The area of health monitoring and maintenance and repair is also undergoing a change in philosophy from the reactive find-and-fix approach to a more proactive predict-and-manage approach. Regardless of the overarching philosophy, damaged airplanes will still have to be repaired. Repair of damage resulting from in-service degradation mechanisms, such as fatigue, SCC, corrosion (when thinning requires structural repair), and discrete source damage (e.g., foreign object impact, handling damage, lightning attachment), is a critical maintenance activity. Repair of aging aircraft can add in bolted or bonded reinforcement doublers over damaged areas or can replace damaged components, preferably with materials that are not as susceptible to deterioration, especially corrosion and SCC. Health Monitoring For the last 30 years, the ASIP has been dealing with fatigue cracking of aircraft structures. ASIP's key management activities have been the development of the FSMP and the Individual Aircraft-Tracking (IAT) program. However, as certain aircraft systems age—such as the KC-135, which is more than 40 years old—corrosion is becoming a major maintenance item, and significant sums of money are being spent on the detection and repair of corrosion damage. Consequently, future health monitoring should include the tracking of corrosion damage as well as fatigue damage. Developments in multifunctional chemical and physical sensors, microelectromechanical systems (MEMS), and smart diagnostics offer some hope that long-term research in onboard health monitors will be productive. In addition, alternatives to existing tape recorder systems should provide an acceptable return on investment, a significant improvement in data capture, improved turnaround time in reporting, the potential of integration with corrosion monitoring, faster identification of usage changes, and acceptance by the users. IAT is intended to provide a limited amount of information on the flight loads experienced by all aircraft in the field. An Air Force goal is to tail-number-track every aircraft. The IAT is not yet universal for several reasons, both fiscal and technical. First, not enough funds are available for gathering and analyzing all the data. Second, the Air Force needs better, more automated, crash-survivable flight data recorders and reliable sensors for key parameters such as corrosive

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SMALL BUSINESS INNOVATION RESEARCH TO SUPPORT AGING AIRCRAFT: Priority Technical Areas and Process Improvements environments. The LESS (loads and environmental severity survey) database gives more detailed and complete information on a few of the fielded aircraft (e.g., temperature and some corrosion indexes). A recommendation in the 1997 NRC report addressed the issue of evaluating and implementing the following methods to provide earlier detection of corrosion: (1) investigation of environmental sensors to allow aircraft maintenance organizations to anticipate when conditions are likely to lead to corrosion; (2) evaluation of the applicability of the Navy's condition-based maintenance program to Air Force needs; and (3) development of techniques to locate, monitor, and characterize defects and chemical and physical heterogeneity within coatings. The goal of the program would be to develop corrosion-tracking methods that can scan an aircraft rapidly, detect thinning to within 5 percent, and provide a permanent record of corrosion found and corrective actions taken. Another recommendation supports the development of signal and image processing techniques based on technologies such as expert systems, neural networks, and database methods that could be used by aircraft maintenance facilities to interpret and track damage development and maintenance needs. If these recommendations are implemented, the health of fleets of aircraft could be ascertained annually and plans could be made to address aging aircraft problems. Maintenance and Repair The Air Force recognizes that bolted metal repairs are a mature technology. Thus, the primary emphasis in R&D has been on bonded repairs for both metal structures and composite structures. The most pressing problem for aging aircraft is bonded repair of metal structures. The current Air Force R&D program includes design and analysis techniques for composite patch repairs, repair procedures, design guidelines, and surface preparation for bonding. The 1997 NRC report recommended that the emphasis of the repair R&D programs be increased in the following areas (p. 69): technologies for the removal, surface preparation, and reapplication of corrosion-resistant finishes evaluation guidelines for the lives of bolted repairs, which are often called upon to remain effective for longer than a single depot-maintenance cycle guidelines for taking advantage of advances in materials and processing technologies in component replacement (including a review of certification requirements to see if they can be waived or simplified without compromising safety); an example would be the reduction of susceptibility to stress-corrosion cracking through the use of improved aluminum alloys, tempers, and processes to reduce residual stresses

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SMALL BUSINESS INNOVATION RESEARCH TO SUPPORT AGING AIRCRAFT: Priority Technical Areas and Process Improvements repair and analysis methods for maintaining structures susceptible to high-cycle fatigue maintenance and repair methods and guidelines for advanced composite structures Several programs for repair technologies are either ongoing or planned. Those programs include the Composite Repair of Aircraft Structures (the development of bonded-repair design/analysis and validation tools); the Corrosion Repair of Metallic Structures (the development of bonded-repair design/installation guidelines); Sol-Gel Technologies for Metallic Surface Preparation; Durability Patch (damping/repair acoustic fatigue damage); RAPID (a software code developed by the FAA for metallic repair design and analysis); Development/Validation of Patch Inspection Methods; Commercial Aircraft Composite Repair (the development of repair techniques for conventional composite structures); Environmentally Friendly Adhesive Primer and Sealants; and High-Temperature Composite Structure Repair. Many technology gaps must be filled in the overall arena of structural repair. Programs to address those gaps for composite doublers and conventional repairs could focus on repair design and analysis methods for sonic fatigue, standard repairs for corrosion damage, self-monitoring/smart patches, cold working as a repair option for short-edge margin holes, and repair of honeycomb and laminate structures. A number of other issues must also be addressed, including issues associated with the conventional repair of composite structure, such as material degradation, design, and analysis; material supply management; improved processing for field-level repair; and damage tolerance versus NDI sensitivity. Other unresolved issues are associated with metallic structures, such as surface preparation; repair design and analysis; bondline durability prediction and accelerated testing; damage tolerance versus NDI sensitivity; documentation (procedures/guidelines) and certification of bonded repairs; repair material management; and smart patch technology. Future repair technologies should include standard repairs for corrosion damage; self-monitoring bonded repair patches; repair of aging composite structures; and incremental improvements in existing capabilities. In summary, the Air Force's repair technologies program includes R&D on mechanically fastened and adhesive-bonded repair technology, with an emphasis on bonded repair. The program is addressing the 1997 NRC recommendations, and current programs could deliver basic mechanically fastened and bonded repair capabilities to ALC customers by FY03. Future needs include simple repairs for corrosion and self-monitoring, bonded-repair patches for safety of flight-critical structures. Many of these needs could be met through SBIR projects.

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SMALL BUSINESS INNOVATION RESEARCH TO SUPPORT AGING AIRCRAFT: Priority Technical Areas and Process Improvements Operational Issues Problems identified by the AATT that were not addressed above include lack of ownership cost models to facilitate repair, replacement, and retirement decisions and obsolete usage-monitoring methods. Ownership cost models that predict structural maintenance costs in out years would serve two purposes. First, the models would determine return on investment to support R&D. Second, they would provide data necessary for the modification, retirement, and replacement decisions. The development of these models will require detailed descriptions and a significant change in current business practices. Ownership cost models are also important to the commercial sector and thus present an opportunity for SBIR Phase III projects. Usage monitoring is currently done by tape recording systems. The alternatives must provide an acceptable return on investment, significant improvements in data capture, and improved turnaround time in reporting. A new usage monitoring system should be integrated with corrosion monitoring and identify usage changes more rapidly than current systems. The new systems will also have to be acceptable to the ALCs, which will have to address problems identified by these systems. As funding allows, existing tape recorder systems could be replaced with microprocessor systems that can record information on aging aircraft. Summary The 1997 NRC report presented a list of recommendations for near-term and long-term research in the following categories: fatigue; corrosion prevention and control; SCC; NDE/NDI; and maintenance and repair (NRC, 1997). Since 1997, the AATT has put into effect a plan to address those recommendations. Nevertheless two important areas, corrosion and NDE/NDI, are not being adequately addressed. INTERAGENCY ISSUES Prompted by the results of the 1997 NRC report, the AFRL aging aircraft ITTP, in partnership with the ASC, undertook a joint planning activity with NASA and the FAA that confirmed the problems that had been identified and highlighted a number of areas of mutual interest (AFRL, 1997). Building on this beginning, the Joint Aeronautical Commanders Group (JACG) formed an action team on aging aircraft that included all of the services, many agencies, and industry. The principal goals of this team were to identify common areas of

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SMALL BUSINESS INNOVATION RESEARCH TO SUPPORT AGING AIRCRAFT: Priority Technical Areas and Process Improvements interest and develop implementation plans to leverage resources. Workshops were held in 1998 and 2000, and the results were reported to JACG leadership. Corrosion was identified as a critical problem by NASA, the Air Force, the Navy, the Coast Guard, and commercial aviation. It was not identified as critical by the FAA. The Air Force and the Navy identified numerous common corrosion-related issues. Not surprisingly, however, some of the technical issues important to the Air Force differed from those important to the Navy. The Air Force manages the majority of DOD's large transport aircraft, and although these aircraft are used by all of the services, their aging is an issue mainly for the Air Force. The Navy's fighter aircraft have more robust structural designs for carrier landings, must operate in a more corrosion-aggressive marine environment, and require significant maintenance aboard ship. Overviews presented at the 2000 Aging Aircraft Conference provided excellent summaries of areas of mutual interest and some new topics (UTC, 2000). For example, corrosion is becoming a major technical issue for space shuttles, which will remain in service for some time. Many of the tools and products being developed overlap with other technical areas, especially NDE/NDI and structural integrity. A program identified by the JACG action team for near-term cooperation was the substitution of new materials for existing aluminum alloys and tempers. Programs addressing CPCs are of near-term interest to the Air Force and the Navy. One of the important issues that received the unanimous support of the services and industry was the need for fundamental research to provide a basic understanding of corrosion mechanisms and rates. Chromate-based coating replacement, smart coatings, and paint stripping were identified as important long-term issues by the Air Force and the Navy. In addition, the Air Force, the Navy, NASA, and industrial participants (Boeing, Lockheed Martin, and Northrop Grumman) all agreed that corrosion sensors, including fiber optics, for corrosion monitoring were of common interest. Appliqué technology is currently being investigated jointly by the Air Force and the Navy. Common structural integrity issues were widespread fatigue damage, corrosion, unitized structures, and dynamics (e.g., sonic fatigue, buffet, and vibration). The focus areas for structural integrity technology included determination of the onset of WFD using deterministic and probabilistic methodologies; the development of structural analysis methodologies to assess the impact of corrosion and corrosion repair on life and residual strength; improvements in structural-analysis and life-prediction codes for unitized structures (e.g., castings). The interagency focus areas for repair technologies included repair of metallic structures (conventional mechanically fastened repairs and bonded composite doublers); repair of composite structures (conventional epoxy and high temperature); and life-enhancement methods, including advanced laser, shot peening, and cold working applications.

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SMALL BUSINESS INNOVATION RESEARCH TO SUPPORT AGING AIRCRAFT: Priority Technical Areas and Process Improvements Several areas related to NDI were of common interest to agencies and industry. Crack detection was of interest to the Air Force, FAA, NASA, and industry. An information exchange has been initiated between an Air Force program that uses NDI to find cracks in fastener holes in thick structures (e.g., the B-l) and a NASA program on a low-frequency, self-nulling probe. The FAA's Airworthiness Assurance NDI Validation Center is coordinated with the Air Force study on POD protocol. A joint program between the FAA and the Air Force Commercial Aircraft Composite Repair Committee is addressing composite reference standards. The Air Force and NASA have a coordinated program on enhanced laser-generated ultrasound. In addition, the Air Force has an SBIR program on the development of a MEMS sensor for adhesion-bond degradation that will end in FY01. NASA will initiate a program on the same subject in FY01. In the area of NDI training, the Air Force, FAA, and NASA plan to initiate 1-year programs in FY01 on computer-based training radiography.