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Suggested Citation:"Air Force Aging Aircraft Program." National Research Council. 2001. Small Business Innovation Research to Support Aging Aircraft: Priority Technical Areas and Process Improvements. Washington, DC: The National Academies Press. doi: 10.17226/10092.
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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

Suggested Citation:"Air Force Aging Aircraft Program." National Research Council. 2001. Small Business Innovation Research to Support Aging Aircraft: Priority Technical Areas and Process Improvements. Washington, DC: The National Academies Press. doi: 10.17226/10092.
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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).

Suggested Citation:"Air Force Aging Aircraft Program." National Research Council. 2001. Small Business Innovation Research to Support Aging Aircraft: Priority Technical Areas and Process Improvements. Washington, DC: The National Academies Press. doi: 10.17226/10092.
×

FIGURE 2-1 Solving aging aircraft problems with cost-focused methods —the AATT process. Figure courtesy of Air Force Aeronautical Systems Center.

Suggested Citation:"Air Force Aging Aircraft Program." National Research Council. 2001. Small Business Innovation Research to Support Aging Aircraft: Priority Technical Areas and Process Improvements. Washington, DC: The National Academies Press. doi: 10.17226/10092.
×

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

Suggested Citation:"Air Force Aging Aircraft Program." National Research Council. 2001. Small Business Innovation Research to Support Aging Aircraft: Priority Technical Areas and Process Improvements. Washington, DC: The National Academies Press. doi: 10.17226/10092.
×

FIGURE 2-2 Strategy for managing the aging aircraft fleet. Figure courtesy of Air Force Aeronautical Systems Center.

Suggested Citation:"Air Force Aging Aircraft Program." National Research Council. 2001. Small Business Innovation Research to Support Aging Aircraft: Priority Technical Areas and Process Improvements. Washington, DC: The National Academies Press. doi: 10.17226/10092.
×

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.

Suggested Citation:"Air Force Aging Aircraft Program." National Research Council. 2001. Small Business Innovation Research to Support Aging Aircraft: Priority Technical Areas and Process Improvements. Washington, DC: The National Academies Press. doi: 10.17226/10092.
×

TABLE 2-1 FY01 ASC Aging Aircraft Acquisition Programs

  1. Corrosion quantification for structural integrity analysis

  2. Detection and quantification of hidden corrosion using ultra-image system

  3. Corrosion prediction management

  4. AGILE for new landing-gear technologies

  5. MAUS ultrasound eddy current wing-skin corrosion detect transition

  6. Improvement of wire system integrity for legacy aircraft

  7. Quality control of composite/bonded repair surface preparation

  8. Material substitution for aging systems

  9. 2nd layer inspection of F-15 lower wing-spar areas

  10. AGILE for brake system and overhaul process improvement

  11. Aging aircraft software library

  12. Exfoliation effects on buckling strength

  13. 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.

Suggested Citation:"Air Force Aging Aircraft Program." National Research Council. 2001. Small Business Innovation Research to Support Aging Aircraft: Priority Technical Areas and Process Improvements. Washington, DC: The National Academies Press. doi: 10.17226/10092.
×

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/>.

Suggested Citation:"Air Force Aging Aircraft Program." National Research Council. 2001. Small Business Innovation Research to Support Aging Aircraft: Priority Technical Areas and Process Improvements. Washington, DC: The National Academies Press. doi: 10.17226/10092.
×

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.

Suggested Citation:"Air Force Aging Aircraft Program." National Research Council. 2001. Small Business Innovation Research to Support Aging Aircraft: Priority Technical Areas and Process Improvements. Washington, DC: The National Academies Press. doi: 10.17226/10092.
×
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

Suggested Citation:"Air Force Aging Aircraft Program." National Research Council. 2001. Small Business Innovation Research to Support Aging Aircraft: Priority Technical Areas and Process Improvements. Washington, DC: The National Academies Press. doi: 10.17226/10092.
×

FIGURE 2-3 Damage-tolerance approach to the prediction of fatigue life. Figure courtesy of Air Force Aeronautical Systems Center.

Suggested Citation:"Air Force Aging Aircraft Program." National Research Council. 2001. Small Business Innovation Research to Support Aging Aircraft: Priority Technical Areas and Process Improvements. Washington, DC: The National Academies Press. doi: 10.17226/10092.
×

important to the onset of WFD characterized by the simultaneous presence of small cracks in multiple structural details. The onset of WFD, which is the life-limiting condition, is defined as the simultaneous presence of small cracks in multiple structural details; when the cracks are of sufficient size and density, the structure can no longer sustain the required residual strength load in the event of a primary load-path failure or a large partial damage incident. When the onset of WFD occurs, the airframe has reached its operational life limit. However, the life of the aircraft can sometimes be extended if parts can be changed.

The development of cracking in joints has long been associated with fretting, which is defined as small-scale, relative sliding motions that occur between contacting surfaces. Fretting and associated concentrated stresses are known to lead to fatigue of joints and could well be a mechanism for the onset of WFD. In lap joints, fretting fatigue can lead to cracking at the rivet-skin interface and at the skin-skin interface, known as the faying surface (Szolwinski et al., 2000).

Recently, several investigators showed that conventional mechanics-based models of fatigue can be used to model fretting fatigue. Thus, the wealth of fracture mechanics technology that has been developed as part of ASIP can be applied directly to predicting the effects of WFD on residual strength. This R&D has been supported by the Air Force both as part of its aging aircraft programs and as high-cycle fatigue initiatives, primarily associated with aircraft engines.

Newman and Piascik (2000) used a mechanics-based fatigue modeling of fretting of joints based on the notion of using equivalent initial-flaw size (EIFS) to predict the initial damage. Thus, fatigue-growth models could be used to predict fatigue lives for lap joints. The EIFS is indeed comparable to that found in microstructural features characterized by microscopy.

The predictions described above rely on small-crack theory and predictions of total life based on back calculation of the EIFS for life data. The most promising analytical approach is to use EIFSs based on experimental data. The 1997 NRC report suggested that an EIFS database, correlated with full-scale structural test articles, be developed for cracks that initiate because of fretting, very small defects, scratches, dings, and corrosion damage. AFRL and NASA continue to work on this problem through testing and inspection of full-scale test articles of lap joints. SBIR could be used to develop full-scale, finite-element models that include the details of friction and accompanying stresses in joints in the fatigue-life calculations.

For the ASIP to accomplish this, it must have a robust means of calculating stresses once loads are known. Evaluations of primary sources of loads are described below in the section on structural dynamics and aeroelasticity. Many groups have all-encompassing, finite-element capabilities for calculating stresses. Primary tools for the implementation of stresses into structural integrity methodology are Air Force Grow (AFGROW, a software code) and NASGROW (developed by NASA). AFGROW is maintained and constantly upgraded by

Suggested Citation:"Air Force Aging Aircraft Program." National Research Council. 2001. Small Business Innovation Research to Support Aging Aircraft: Priority Technical Areas and Process Improvements. Washington, DC: The National Academies Press. doi: 10.17226/10092.
×

AFRL to increase the accuracy of structural life assessments. AFGROW presently has the capability of analyzing multiple cracks at holes to assess WFD. SBIR could assist in the incorporation of finite-element results into AFGROW.

Corrosion Fatigue

The damage-tolerance approach to the prediction of fatigue life requires a definition of structural loads, the determination of critical stresses and their locations, and the determination of crack growth as a function of the number of loading cycles for various mission profiles (see Figure 2-3). Fracture mechanics has provided a theoretical framework for relating the crack growth rate, the increase in crack length per cycle, and the stress intensity factor. A major unresolved challenge is how to include the effects of corrosion in this theoretical framework for predicting fatigue life. Including corrosion effects will require both basic and applied R&D. SBIR efforts might incorporate present knowledge of corrosion effects into existing fracture-mechanics-based models for predicting fatigue life.

Both the AFRL and ASC aging aircraft programs are developing new capabilities for an improved structural integrity tool set (both for cracks and corrosion). The AATT has major programs in each of the corrosion fatigue building block areas shown in Figure 2-4. The AFRL Corrosion Fatigue Structural Demonstration Program and companion ASC Corrosion Management Program, the core efforts in the corrosion fatigue strategy, are focused on adding corrosion effects to the baseline structural integrity analyses that have been the basis for the ASIP durability and damage-tolerance approach (and championed by the current Air Force technical leader for aging aircraft, Jack Lincoln). A successful shift from the find-and-fix approach to a more cost-effective anticipate-and-manage approach will depend on the quality and completeness of the analysis tool sets.

The key implications of corrosion damage for structural life and residual strength are shown in Figure 2-5. Corrosion degradation occurs in many forms and can occur in many structural areas; often the critical areas are hidden. Even though NDE/NDI techniques being developed are sensitive enough to discriminate among the forms of corrosion and can provide some estimates of hidden damage, better technologies are critical. Shortfalls in high-POD inspection for small cracks and corrosion may mean that inspection intervals should be shortened (which would increase costs and could decrease aircraft availability). Another continuing challenge for the NDE/NDI community is the transitioning of improved, but more sophisticated, technologies to use in the field and at depots, which could take many years.

Once the best NDE/NDI information has been provided, the effects of the observed damage on strength and remaining life must be established. Before these

Suggested Citation:"Air Force Aging Aircraft Program." National Research Council. 2001. Small Business Innovation Research to Support Aging Aircraft: Priority Technical Areas and Process Improvements. Washington, DC: The National Academies Press. doi: 10.17226/10092.
×

FIGURE 2-4 Management of structural damage in aging aircraft Figure courtesy of Air Force Aeronautical Systems Center

Suggested Citation:"Air Force Aging Aircraft Program." National Research Council. 2001. Small Business Innovation Research to Support Aging Aircraft: Priority Technical Areas and Process Improvements. Washington, DC: The National Academies Press. doi: 10.17226/10092.
×

FIGURE 2-5 Corrosion fatigue structural demonstration approach. Figure courtesy of Air Force Aeronautical Systems Center.

Suggested Citation:"Air Force Aging Aircraft Program." National Research Council. 2001. Small Business Innovation Research to Support Aging Aircraft: Priority Technical Areas and Process Improvements. Washington, DC: The National Academies Press. doi: 10.17226/10092.
×

correlations can be adopted by the ASIP, they must be validated, which usually requires full-scale testing. Even when reasonably accurate descriptions of corrosion phenomena have been incorporated into structural integrity analyses, a difficult additional challenge must still be met—a practical model describing the corrosion degradation as a function of time. The expected future deployment of the aircraft must also be factored in, so environmental severity index correlations can be introduced.

As corrosion models are developed, the scatter in the basic data is being continually scrutinized to guide improvements in correlations and to guide the development of more comprehensive NDE/NDI techniques. The practical successes of the durability and damage-tolerance approach for dealing with fatigue cracking have depended, at least in part, on the development of extensive databases with manageable scatter and uncertainty. This is still a challenge when corrosion degradation is involved.

The long-standing ASIP framework is sufficiently robust to accommodate new procedures. The inclusion of cost and economic features in the improved tool sets will be especially important. Gathering corrosion-degradation information, evaluating it, and taking appropriate actions could significantly increase the cost of aircraft tracking and maintenance. A key challenge will be to implement the anticipate-and-manage approach as cost effectively as possible. And corrosion prevention will continue to be a top priority area for Air Force aging aircraft S&T and acquisition programs. In the near term, the focus will be on completing the development of a first-generation tool set for corrosion-fatigue and structural-integrity analysis and training personnel to use it. Further development will be necessary, however. Given the complexity of corrosion phenomena generally and the current limitations of NDE/NDI technology to quantify corrosion degradation, especially when it is hidden from direct view, the first-generation tool set will have many empirical features.

Program Themes for the Future

Principal themes for the next-generation tool set and the longer term will include the following:

  • the development of more complete corrosion growth rate models

  • the development of improved NDE/NDI corrosion inspection techniques, especially for hidden corrosion, and the quantification of the effects of corrosion degradation on mechanical properties

  • the incorporation of cost parameters into structural integrity tool sets

  • the incorporation of new developments into multiple-phase, next-generation corrosion fatigue analysis tools

Suggested Citation:"Air Force Aging Aircraft Program." National Research Council. 2001. Small Business Innovation Research to Support Aging Aircraft: Priority Technical Areas and Process Improvements. Washington, DC: The National Academies Press. doi: 10.17226/10092.
×
  • the incorporation of cost-based, corrosion-fatigue analyses into SPO/MAJCOM cost-of-ownership and economic-service-life models

Structural Dynamics and Aeroelasticity

There are several sources of dynamic loads on an aircraft that can lead to crack propagation and fatigue failure. For transport aircraft and some bomber aircraft with large-span (high aspect ratio) flexible wings, the predominant loads may be the result of aerodynamic forces created by atmospheric turbulence or gusts, which often govern much of the structural design, from the standpoint of both maximum loads and loads leading to fatigue. For fighter aircraft, extreme maneuvers may be the most important factor in maximum stress conditions and in generating repeated loads that lead to fatigue. Some aircraft maneuvers may lead to qualitative and important changes in the aerodynamic flow field around the aircraft. For example, massive flow separation may occur (with or without accompanying shock-wave oscillations), and so-called buffet loads caused by the large-scale oscillating flow field may be induced and give rise to significant dynamic loads on the aircraft structure. Although engine structures per se are not treated in any detail in this report, similar considerations apply to them. For engines, the term “inlet distortions” rather than gusts is more often used. Indeed, the engine itself may induce significant acoustic loads on the airframe structure. Acoustically induced fatigue has occurred in the B-1 horizontal tail, and buffet-load-induced damage has occurred in the F-15 and F-18A vertical tails. Collectively, these oscillations and the resulting structural failures are called high-cycle fatigue.

As the 1997 NRC report noted (p. 32),

The committee believes that dynamic loading and the resulting high-cycle fatigue is a key aging aircraft issue as well as an initial design issue, particularly for high-performance combat aircraft. The key technical issues include:

  • identification, reduction, or elimination of sources of dynamic excitation

  • passive and active methods to reduce the response of aircraft structures

  • measurement and characterization of the threshold for fatigue propagation values for airframe materials, including the applicability of long crack thresholds to small crack behavior

  • in-flight monitoring of changes in dynamic behavior.

The first two issues are discussed next.

One effective approach to delaying, diminishing, or eliminating structural damage is to reduce the dynamic aerodynamic loads or the dynamic structural

Suggested Citation:"Air Force Aging Aircraft Program." National Research Council. 2001. Small Business Innovation Research to Support Aging Aircraft: Priority Technical Areas and Process Improvements. Washington, DC: The National Academies Press. doi: 10.17226/10092.
×

response of the aircraft. The loads may be diminished by using active or passive control devices. For passive control, various damping enhancements have been proposed. Both constrained-layer damping, in which a viscoelastic material is constrained between two elastic layers (usually one of them is the primary structure whose response is to be reduced), and dry-friction damping have been proposed. Devises for the latter include the so-called shroud dampers, which contact adjacent fan blades in a jet engine. Dry-friction devices have not been used in airframes, although there is some evidence that damping due to dry friction is an important, though unintended, consequence of traditional metal airframe construction.

Although not always recognized by structural designers, the aerodynamic flow that gives rise to the excitation of the structure can also contribute restoring forces, including damping, to the structure. That is, the structural response changes the aerodynamic forces on the structure, giving rise to aerodynamic damping. It is well known that the aerodynamic forces caused by structural motion can lead to a dynamic instability called “flutter” when the damping in an aeroelastic mode sinks to zero at some critical flight speed, the flutter speed. (An aeroelastic mode is a dynamic mode that represents a coupled oscillation of the structure and surrounding aerodynamic flow.) It is also true that for flight velocities below the flutter speed (at which aircraft are designed to fly), these aerodynamic forces may provide a damping to the structure that exceeds that damping inherent in the structural material or configuration, or both.

A recent observation of aeroelastic dynamic loading phenomena in operational aircraft (the B-2, the F-16, and the F-18) is that of limit cycle oscillations. Although these are thought to occur as a result of an interaction between the structural motion and the induced aerodynamic forces, the specific physical mechanism is not yet well understood and is a subject of current PE 6.1 and PE 6.2 research. The nature of limit cycle oscillations is that the motion is bounded in amplitude and is often a near-sinusoidal motion of nearly fixed peak amplitude and dominated by a single frequency. Although this motion is not immediately catastrophic (as is the classical flutter oscillation), a limit cycle oscillation can lead to structural damage and, potentially, to fatigue failure.

Another approach to reducing structural response is through active control devices. An early successful demonstration of this technology was in a B-52 aircraft in which a reduction of gust response was achieved. In this and many early efforts, the motion of the aircraft was sensed and an existing aerodynamic control surface was driven in response to the sensed motion. This feedback loop was shaped to achieve the desired reduction in aircraft motion and structural loads. In recent years, the research community has turned its attention to so-called smart structures, which have smaller, localized sensing and controlling elements made of (for example) piezoelectric materials. A demonstration of this technology on full-scale vehicle structures is currently being attempted. A combination of smart

Suggested Citation:"Air Force Aging Aircraft Program." National Research Council. 2001. Small Business Innovation Research to Support Aging Aircraft: Priority Technical Areas and Process Improvements. Washington, DC: The National Academies Press. doi: 10.17226/10092.
×

materials and more traditional control actuators may prove to be an effective way of reducing structural response.

The greatest benefits can be derived from active and passive control devices incorporated into the structural design process as part of the initial aircraft design. Nevertheless, active or passive damping and control devices may also be attractive for modifying existing aircraft. For example, a damping device was used to modify F-15 aircraft to reduce dynamic response due to buffeting.

As the 1997 NRC report also noted, “Near-term research opportunities include efforts to improve methods to determine dynamic response” (p. 52). This committee agrees but notes that improvements in the computational efficiency of mathematical models for time-dependent or unsteady aerodynamic flow fields that accurately describe the dynamic fluid forces acting on the aircraft structure will require both near-term and long-term R&D. Promising advances have been made recently in reduced-order modeling of unsteady aerodynamic flow fields. This model uses a global, modal description of the flow field rather than a local description as in traditional computational fluid dynamics models based on finite differences or finite elements.

The 1997 NRC report recommended the development of “load monitoring and alleviation technologies that take advantage of recent advances in sensors and controls and computational capabilities ” (p. 53). This committee heartily concurs with that recommendation and with the observation that intelligent control systems have been developed and demonstrated to suppress flutter and buffet load using both conventional control surface actuators and piezoelectric actuators.

Another recommendation of the 1997 report with which this committee concurs is 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 ” (p. 72). In this regard, dry-friction damping induced by adjacent sliding structures merits further investigation and exploitation in airframe systems.

It has been suggested that impact damage due to discrete sources, such as landing loads or bird strikes, is often a more critical design condition for composite structures than fatigue induced by crack propagation (NRC, 1996). Improved modeling and measurement of structural damage due to impact loads offer attractive opportunities for near-term and long-term R&D to predict and reduce structural response using modern computational and experimental methodologies.

Opportunities for SBIR-funded projects in this area of technology include active control to reduced dynamic loads and response, smart structural concepts to monitor and shape dynamic response, and improved computer modeling and prediction of aerodynamic and structural loads to more accurately estimate fatigue life, time between inspection cycles, effectiveness of active control devices, and smart structural elements.

Suggested Citation:"Air Force Aging Aircraft Program." National Research Council. 2001. Small Business Innovation Research to Support Aging Aircraft: Priority Technical Areas and Process Improvements. Washington, DC: The National Academies Press. doi: 10.17226/10092.
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Corrosion

Corrosion of airframe structures has been identified as the most costly maintenance problem for Air Force aging aircraft (SAB, 1994), and these costs are rising steadily (Cooke et al., 1998). The Air Force Scientific Advisory Board Materials Degradation Panel cited estimates of the costs associated with corrosion-related detection and repair in the range of $1 billion to $3 billion per year (SAB, 1996). Corrosion occurs in many forms, most of which are routinely detected in aging aircraft. Forms of corrosion are typically divided into general or uniform attacks and localized attacks, such as pitting, crevice corrosion, intergranular corrosion (including exfoliation), galvanic (two-metal) corrosion, de-alloying (selective leaching), hydrogen attack, erosion-corrosion, and stress-corrosion cracking (SCC).

Corrosion of aging aircraft results from a combination of the following factors:

  • older aluminum alloys and tempers that are more susceptible to corrosion than currently available alternatives

  • inadequacy or degradation of corrosion-protection systems

  • exposure to corrosive environments, such as humid air, saltwater, sump-tank water, and latrine leakage

Despite best practices of prevention and control, total elimination of corrosion is virtually impossible. Corrosion control in aging aircraft requires effective prevention, detection, and repair practices. The corrosion protection and control systems of aging airframes deteriorate over time. Consequently, maintenance costs increase as corrosion is identified and repaired. Based on current experience, the practice of repairing corrosion damage identified by visual inspection has seemed adequate for maintaining the integrity of aging structures. Unfortunately, a substantial amount of corrosion damage sustained by older Air Force aircraft is hidden from direct view; thus, a significant amount of material degradation can remain undetected. More importantly, the extent and severity of corrosion damage in similar aircraft can vary widely because of differences in mission cycle, environmental exposures, and the extent and type of maintenance.

Different forms of corrosion (i.e., corrosion caused by different mechanisms) exhibit different characteristics and consequences. For example, exfoliation corrosion (severe intergranular corrosion in which the buildup of corrosion causes flaking and surface blisters) and pitting can typically be readily detected, depending on the accessibility of the damaged surface. Although these two forms of localized corrosion are evident as surface deterioration, they may not be found if the surface is inaccessible to visual inspection. Moreover, intergranular corrosion that propagates along grain boundaries oriented away from exposed

Suggested Citation:"Air Force Aging Aircraft Program." National Research Council. 2001. Small Business Innovation Research to Support Aging Aircraft: Priority Technical Areas and Process Improvements. Washington, DC: The National Academies Press. doi: 10.17226/10092.
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surfaces may be indistinguishable from the surface, challenging the reliability of NDI techniques (Mindlin et al., 1996). Crevice corrosion, which occurs in lap joints, is particularly insidious because significant material loss can remain undetected. Such unexpected corrosion damage increases maintenance costs and time in the depot for maintenance. A more critical consequence is the increased risk that corrosion, in the presence of other forms of damage (e.g., fatigue), may cause a significant decrease in damage tolerance.

Although corrosion is very costly to repair, it has not yet been identified as the cause of any of the structural failures that have resulted in the loss of an Air Force aircraft. This is because it has been detected and repaired before becoming a flight safety problem. However, the Air Force admittedly has historically treated corrosion with a find-and-fix approach rather than an anticipate-and-manage approach. Current Air Force corrosion prevention and control programs are designed to change the culture so that corrosion is controlled using the latter rather than the former approach. Most notable among these programs is a PE 6.5 program, Corrosion Prediction Management, which is managed at the AFRL.

Corrosion prevention should begin during the acquisition stage with the selection of appropriate materials and manufacturing processes. The commercial aircraft industry has developed, as part of its structural maintenance programs, provisions to upgrade corrosion resistance through the use of substitute materials and heat treatments; improved protective finishes and corrosion prevention compounds (CPCs); and design features such as drainage and sealing to prevent corrosion. For example, the high-strength aluminum alloy 7075 has been replaced in many forging applications by the more corrosion-resistant alloys 7050, 7150, and 7055. These alternative alloys have been downselected based on studies such as the current PE 6.5 AFRL program, Material Substitution for Aging Aircraft. Similarly, the stress-corrosion-resistant and exfoliation-resistant T-7x tempers are now used for 7xxx-series aluminum alloys instead of the original design T-6x tempers, which have repeatedly shown inferior resistance to corrosion and SCC. The AFRL's recently developed retrogression and re-aging heat treatment is under study as a means for increasing corrosion resistance while maintaining the strength of existing 7075 components, the replacement of which would be costly. This two-stage technique, currently under development using a ZIMAC heating system, would locally boost corrosion resistance without sacrificing strength. Details of this heat treatment and its advantages can be accessed in a recent report, Stress Corrosion Cracking in Aging Aircraft (Shah et al., 1999). Similar engineering guidelines on substitute materials and processes with corrosion resistance better than those used in the original design have not yet been formally developed for Air Force aircraft.

To avoid costly component repair and replacement, much more emphasis should be given to early detection of corrosion and the implementation of

Suggested Citation:"Air Force Aging Aircraft Program." National Research Council. 2001. Small Business Innovation Research to Support Aging Aircraft: Priority Technical Areas and Process Improvements. Washington, DC: The National Academies Press. doi: 10.17226/10092.
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effective corrosion control and prevention practices. The 1997 NRC report identified, for example, the most important operations needs:

  • environmentally compatible protective coatings to replace the hazardous materials being phased out (e.g., chromates)

  • generalized use of CPCs that can be applied on external surfaces and that will penetrate and protect unsealed joints and around fasteners

  • guidance for the application of upgraded alloys and processes offering improved corrosion protection

  • improved NDE/NDI techniques to reveal and estimate hidden corrosion without requiring disassembly of the aircraft

  • classification of corrosion severity, similar to current commercial aircraft practice, to provide guidance for maintenance

Detection of corrosion is necessary for assessing the damage tolerance of affected structures and taking appropriate corrective actions. Current inspection methods require component disassembly, which increases the probability of maintenance-induced damage. Accurate detection and quantification of corrosion under paint, under multiple layers, under fastener heads, and on the interior surfaces of built-up structures would ensure that required repairs are made.

Stress Corrosion Cracking

SCC is treated separately from other forms of corrosion because of its potential structural effects on aging Air Force aircraft. Some unique aspects of SCC render it much more dangerous than other forms of corrosion. SCC is an environmentally induced, sustained-stress (versus cyclic-stress) cracking mechanism that requires three components: (1) a susceptible microstructure; (2) a corrosive environment; and (3) local tensile stresses. Prevention requires elimination of any of these components. SCC, characteristically intergranular in aging aircraft environments, can occur with little or no evidence of corrosion products and is therefore often difficult to detect visually. SCC can also occur transgranularly in some material systems (most notably in steels, but also in aluminum alloys).

SCC is typically exacerbated by residual tensile stresses remaining from material heat treatment or component fit-up but can also be triggered by operational loads and forces from the buildup of corrosion by-products that act as wedges to open cracks. The poorer mechanical properties of forging and thick plate materials in the short-transverse-grain direction compared with those in the longitudinal-grain direction have been well documented, so structural components are designed for the primary load paths to be parallel to the principal grain

Suggested Citation:"Air Force Aging Aircraft Program." National Research Council. 2001. Small Business Innovation Research to Support Aging Aircraft: Priority Technical Areas and Process Improvements. Washington, DC: The National Academies Press. doi: 10.17226/10092.
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direction. In this case, the elongated grain boundaries are parallel to, rather than normal to, the applied operational stresses. Fortunately, when SCC occurs parallel to applied operational stresses, cracks can often be very large (as much as several inches long) before they become a flight safety problem. Conversely, loading stresses parallel to the short-transverse direction of a plate or extrusion, where the grain boundary density is far greater than that of either the rolling or long-transverse directions, result in a highly increased sensitivity to SCC.

Grain orientation with respect to the applied flight stresses has, in general, not caused flight safety problems, but this may not always be the case. If large inplane 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. In addition, in thick sections (e.g., complex machined fittings) where there may be irregular grain flow and three-dimensionally applied 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.

The costly replacement and repair of components necessitated by SCC could be reduced, or at least delayed, with appropriate maintenance. For example, improved CPCs and surface finishes would reduce the corrosion rates of susceptible materials; manufacturing processes could be modified to reduce exposed end-grain and residual stress effects that exacerbate SCC in large structural components; and repair procedures could be improved to maintain the integrity of the surface finishes. Programs such as these could be funded through SBIR.

In addition to preventive maintenance, the onset of SCC should be anticipated using statistical tools to predict the time to initiation of the cracks and their growth rates. Current NDE/NDI techniques, which are effective in the detection of surface-connected SCC, could be improved to detect cracks below coatings. A probabilistic approach, based on an evaluation matrix that includes factors, such as (1) material, (2) stresses and load, (3) manufacturing, (4) environment, and (5) surface finishes, has been developed and reported (Shah et al., 1999). Prediction of the onset of cracks would then be a complement to damage-tolerance analyses, which do not currently predict the occurrence of SCC. With continued and consistent improvements in prevention and control procedures, upgrading of susceptible materials with more corrosion-resistant alloys, and minimization of residual stress, SCC problems in aging Air Force aircraft should remain manageable and need not be a life-limiting damage mechanism.

Suggested Citation:"Air Force Aging Aircraft Program." National Research Council. 2001. Small Business Innovation Research to Support Aging Aircraft: Priority Technical Areas and Process Improvements. Washington, DC: The National Academies Press. doi: 10.17226/10092.
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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.

Suggested Citation:"Air Force Aging Aircraft Program." National Research Council. 2001. Small Business Innovation Research to Support Aging Aircraft: Priority Technical Areas and Process Improvements. Washington, DC: The National Academies Press. doi: 10.17226/10092.
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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.

Suggested Citation:"Air Force Aging Aircraft Program." National Research Council. 2001. Small Business Innovation Research to Support Aging Aircraft: Priority Technical Areas and Process Improvements. Washington, DC: The National Academies Press. doi: 10.17226/10092.
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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

Suggested Citation:"Air Force Aging Aircraft Program." National Research Council. 2001. Small Business Innovation Research to Support Aging Aircraft: Priority Technical Areas and Process Improvements. Washington, DC: The National Academies Press. doi: 10.17226/10092.
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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

Suggested Citation:"Air Force Aging Aircraft Program." National Research Council. 2001. Small Business Innovation Research to Support Aging Aircraft: Priority Technical Areas and Process Improvements. Washington, DC: The National Academies Press. doi: 10.17226/10092.
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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

Suggested Citation:"Air Force Aging Aircraft Program." National Research Council. 2001. Small Business Innovation Research to Support Aging Aircraft: Priority Technical Areas and Process Improvements. Washington, DC: The National Academies Press. doi: 10.17226/10092.
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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

Suggested Citation:"Air Force Aging Aircraft Program." National Research Council. 2001. Small Business Innovation Research to Support Aging Aircraft: Priority Technical Areas and Process Improvements. Washington, DC: The National Academies Press. doi: 10.17226/10092.
×

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

Suggested Citation:"Air Force Aging Aircraft Program." National Research Council. 2001. Small Business Innovation Research to Support Aging Aircraft: Priority Technical Areas and Process Improvements. Washington, DC: The National Academies Press. doi: 10.17226/10092.
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  • 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.

Suggested Citation:"Air Force Aging Aircraft Program." National Research Council. 2001. Small Business Innovation Research to Support Aging Aircraft: Priority Technical Areas and Process Improvements. Washington, DC: The National Academies Press. doi: 10.17226/10092.
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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

Suggested Citation:"Air Force Aging Aircraft Program." National Research Council. 2001. Small Business Innovation Research to Support Aging Aircraft: Priority Technical Areas and Process Improvements. Washington, DC: The National Academies Press. doi: 10.17226/10092.
×

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.

Suggested Citation:"Air Force Aging Aircraft Program." National Research Council. 2001. Small Business Innovation Research to Support Aging Aircraft: Priority Technical Areas and Process Improvements. Washington, DC: The National Academies Press. doi: 10.17226/10092.
×

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.

Suggested Citation:"Air Force Aging Aircraft Program." National Research Council. 2001. Small Business Innovation Research to Support Aging Aircraft: Priority Technical Areas and Process Improvements. Washington, DC: The National Academies Press. doi: 10.17226/10092.
×
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Suggested Citation:"Air Force Aging Aircraft Program." National Research Council. 2001. Small Business Innovation Research to Support Aging Aircraft: Priority Technical Areas and Process Improvements. Washington, DC: The National Academies Press. doi: 10.17226/10092.
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Suggested Citation:"Air Force Aging Aircraft Program." National Research Council. 2001. Small Business Innovation Research to Support Aging Aircraft: Priority Technical Areas and Process Improvements. Washington, DC: The National Academies Press. doi: 10.17226/10092.
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Suggested Citation:"Air Force Aging Aircraft Program." National Research Council. 2001. Small Business Innovation Research to Support Aging Aircraft: Priority Technical Areas and Process Improvements. Washington, DC: The National Academies Press. doi: 10.17226/10092.
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Suggested Citation:"Air Force Aging Aircraft Program." National Research Council. 2001. Small Business Innovation Research to Support Aging Aircraft: Priority Technical Areas and Process Improvements. Washington, DC: The National Academies Press. doi: 10.17226/10092.
×
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Suggested Citation:"Air Force Aging Aircraft Program." National Research Council. 2001. Small Business Innovation Research to Support Aging Aircraft: Priority Technical Areas and Process Improvements. Washington, DC: The National Academies Press. doi: 10.17226/10092.
×
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Suggested Citation:"Air Force Aging Aircraft Program." National Research Council. 2001. Small Business Innovation Research to Support Aging Aircraft: Priority Technical Areas and Process Improvements. Washington, DC: The National Academies Press. doi: 10.17226/10092.
×
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Suggested Citation:"Air Force Aging Aircraft Program." National Research Council. 2001. Small Business Innovation Research to Support Aging Aircraft: Priority Technical Areas and Process Improvements. Washington, DC: The National Academies Press. doi: 10.17226/10092.
×
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Suggested Citation:"Air Force Aging Aircraft Program." National Research Council. 2001. Small Business Innovation Research to Support Aging Aircraft: Priority Technical Areas and Process Improvements. Washington, DC: The National Academies Press. doi: 10.17226/10092.
×
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Suggested Citation:"Air Force Aging Aircraft Program." National Research Council. 2001. Small Business Innovation Research to Support Aging Aircraft: Priority Technical Areas and Process Improvements. Washington, DC: The National Academies Press. doi: 10.17226/10092.
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Suggested Citation:"Air Force Aging Aircraft Program." National Research Council. 2001. Small Business Innovation Research to Support Aging Aircraft: Priority Technical Areas and Process Improvements. Washington, DC: The National Academies Press. doi: 10.17226/10092.
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Suggested Citation:"Air Force Aging Aircraft Program." National Research Council. 2001. Small Business Innovation Research to Support Aging Aircraft: Priority Technical Areas and Process Improvements. Washington, DC: The National Academies Press. doi: 10.17226/10092.
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Suggested Citation:"Air Force Aging Aircraft Program." National Research Council. 2001. Small Business Innovation Research to Support Aging Aircraft: Priority Technical Areas and Process Improvements. Washington, DC: The National Academies Press. doi: 10.17226/10092.
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Suggested Citation:"Air Force Aging Aircraft Program." National Research Council. 2001. Small Business Innovation Research to Support Aging Aircraft: Priority Technical Areas and Process Improvements. Washington, DC: The National Academies Press. doi: 10.17226/10092.
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Suggested Citation:"Air Force Aging Aircraft Program." National Research Council. 2001. Small Business Innovation Research to Support Aging Aircraft: Priority Technical Areas and Process Improvements. Washington, DC: The National Academies Press. doi: 10.17226/10092.
×
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Suggested Citation:"Air Force Aging Aircraft Program." National Research Council. 2001. Small Business Innovation Research to Support Aging Aircraft: Priority Technical Areas and Process Improvements. Washington, DC: The National Academies Press. doi: 10.17226/10092.
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Suggested Citation:"Air Force Aging Aircraft Program." National Research Council. 2001. Small Business Innovation Research to Support Aging Aircraft: Priority Technical Areas and Process Improvements. Washington, DC: The National Academies Press. doi: 10.17226/10092.
×
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Suggested Citation:"Air Force Aging Aircraft Program." National Research Council. 2001. Small Business Innovation Research to Support Aging Aircraft: Priority Technical Areas and Process Improvements. Washington, DC: The National Academies Press. doi: 10.17226/10092.
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Suggested Citation:"Air Force Aging Aircraft Program." National Research Council. 2001. Small Business Innovation Research to Support Aging Aircraft: Priority Technical Areas and Process Improvements. Washington, DC: The National Academies Press. doi: 10.17226/10092.
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Suggested Citation:"Air Force Aging Aircraft Program." National Research Council. 2001. Small Business Innovation Research to Support Aging Aircraft: Priority Technical Areas and Process Improvements. Washington, DC: The National Academies Press. doi: 10.17226/10092.
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Suggested Citation:"Air Force Aging Aircraft Program." National Research Council. 2001. Small Business Innovation Research to Support Aging Aircraft: Priority Technical Areas and Process Improvements. Washington, DC: The National Academies Press. doi: 10.17226/10092.
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Suggested Citation:"Air Force Aging Aircraft Program." National Research Council. 2001. Small Business Innovation Research to Support Aging Aircraft: Priority Technical Areas and Process Improvements. Washington, DC: The National Academies Press. doi: 10.17226/10092.
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Suggested Citation:"Air Force Aging Aircraft Program." National Research Council. 2001. Small Business Innovation Research to Support Aging Aircraft: Priority Technical Areas and Process Improvements. Washington, DC: The National Academies Press. doi: 10.17226/10092.
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Suggested Citation:"Air Force Aging Aircraft Program." National Research Council. 2001. Small Business Innovation Research to Support Aging Aircraft: Priority Technical Areas and Process Improvements. Washington, DC: The National Academies Press. doi: 10.17226/10092.
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Suggested Citation:"Air Force Aging Aircraft Program." National Research Council. 2001. Small Business Innovation Research to Support Aging Aircraft: Priority Technical Areas and Process Improvements. Washington, DC: The National Academies Press. doi: 10.17226/10092.
×
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Suggested Citation:"Air Force Aging Aircraft Program." National Research Council. 2001. Small Business Innovation Research to Support Aging Aircraft: Priority Technical Areas and Process Improvements. Washington, DC: The National Academies Press. doi: 10.17226/10092.
×
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Suggested Citation:"Air Force Aging Aircraft Program." National Research Council. 2001. Small Business Innovation Research to Support Aging Aircraft: Priority Technical Areas and Process Improvements. Washington, DC: The National Academies Press. doi: 10.17226/10092.
×
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Suggested Citation:"Air Force Aging Aircraft Program." National Research Council. 2001. Small Business Innovation Research to Support Aging Aircraft: Priority Technical Areas and Process Improvements. Washington, DC: The National Academies Press. doi: 10.17226/10092.
×
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Suggested Citation:"Air Force Aging Aircraft Program." National Research Council. 2001. Small Business Innovation Research to Support Aging Aircraft: Priority Technical Areas and Process Improvements. Washington, DC: The National Academies Press. doi: 10.17226/10092.
×
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Suggested Citation:"Air Force Aging Aircraft Program." National Research Council. 2001. Small Business Innovation Research to Support Aging Aircraft: Priority Technical Areas and Process Improvements. Washington, DC: The National Academies Press. doi: 10.17226/10092.
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Suggested Citation:"Air Force Aging Aircraft Program." National Research Council. 2001. Small Business Innovation Research to Support Aging Aircraft: Priority Technical Areas and Process Improvements. Washington, DC: The National Academies Press. doi: 10.17226/10092.
×
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Suggested Citation:"Air Force Aging Aircraft Program." National Research Council. 2001. Small Business Innovation Research to Support Aging Aircraft: Priority Technical Areas and Process Improvements. Washington, DC: The National Academies Press. doi: 10.17226/10092.
×
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Suggested Citation:"Air Force Aging Aircraft Program." National Research Council. 2001. Small Business Innovation Research to Support Aging Aircraft: Priority Technical Areas and Process Improvements. Washington, DC: The National Academies Press. doi: 10.17226/10092.
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Suggested Citation:"Air Force Aging Aircraft Program." National Research Council. 2001. Small Business Innovation Research to Support Aging Aircraft: Priority Technical Areas and Process Improvements. Washington, DC: The National Academies Press. doi: 10.17226/10092.
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