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U.S. Supersonic Commercial Aircraft: Assessing NASA's High Speed Research Program APPENDICES

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U.S. Supersonic Commercial Aircraft: Assessing NASA's High Speed Research Program This page in the original is blank.

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U.S. Supersonic Commercial Aircraft: Assessing NASA's High Speed Research Program APPENDIX A List of Findings and Recommendations A complete list of the committee's findings and recommendations appears below. The major findings and recommendations from Chapter 6 are listed first. Other findings and recommendations are listed in the order they appear in the body of the report. MAJOR FINDINGS AND RECOMMENDATIONS Major Finding 1. The current HSR Program is making excellent progress. Achieving program objectives is a necessary precursor to the development of a U.S.-built HSCT and would develop important new technologies with broad applicability throughout the aeronautics industry. Major Recommendation 1. The HSR Program should adopt a new vision statement that emphasizes top-level requirements (i.e., safety, environmental acceptability, and economic viability) to encourage a more balanced technical approach to achieving aircraft performance goals (i.e., speed, range, and payload). The committee suggests the following: Develop high risk, critical, enabling technologies in conjunction with complementary industry investments to support the timely introduction of a Mach 2.0-plus HSCT. These technologies must lead to an environmentally acceptable, economically viable aircraft, with safety levels equal to or better than future subsonic transports. Successful completion of the NASA and industry programs will provide the technology foundation industry needs to proceed with the design, certification, and manufacture of an HSCT.

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U.S. Supersonic Commercial Aircraft: Assessing NASA's High Speed Research Program Major Finding 2. The goal of the HSR Program is to provide the technology foundation that industry needs to make an HSCT product launch decision. Without an extended period of technology maturation and advanced technology demonstration, the HSR Program will not achieve this goal. Major Recommendation 2. To accomplish HSR Program objectives, the program should be restructured in accordance with Figure 6-1. The recommended changes to Phase II should be implemented without waiting to determine if the technology maturation and advanced technology demonstration phases will be approved and funded. Major Recommendation 3. The HSR Program should increase the role of affordability (and related factors, such as inspectability, maintainability, and repairability) in evaluating the merit of alternate technology approaches, system design concepts, and vehicle configurations. Cost per available seat mile should be adopted as the key affordability metric. The technology management teams and ITD (Integrated Technology Development) teams should understand the impact of their technology and system choices on affordability/cost per available seat mile. Major Recommendation 4. The HSR Program should reevaluate the TRL goals assigned to each technology audit metric and ensure that they are coordinated across the program. NASA should conduct a QFD analysis (see Chapter 2) to validate the choice of metrics and their assigned TRL goals. Major Recommendation 5. The HSR Program plan should be revised to place more emphasis on the interdependencies inherent in the program. The revised plan should explicitly identify major interdependencies between the propulsion system, airframe, integrated aircraft systems, and process areas, such as certification and manufacturing. These interdependencies should be reflected in a time-phased, integrated plan that takes into account industry-funded research and development, as appropriate. Early emphasis should be put on technologies with longer development lead times. The plan should also specify a risk reduction approach for each program element with moderate or high risk. Major Recommendation 6. Industry participants in the HSR Program should grant NASA increased access to internal HSCT research, and the HSR Program should include appropriate information in its planning and oversight process. Major Recommendation 7. Instead of using surrogate materials, full-scale component tests should be delayed until ongoing material development efforts can supply the materials intended for use in those components. The HSR Program should work with industry to develop preproduction manufacturing processes to

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U.S. Supersonic Commercial Aircraft: Assessing NASA's High Speed Research Program manufacture test components. Implementing the recommended technology maturation phase would provide the time needed to implement this strategy. Major Recommendation 8. The HSR Program should demonstrate adequate materials durability and complete the development of manufacturing technology during the recommended technology maturation phase. In particular, the HSR Program should develop test methodologies suitable for validating an airframe service life of 60,000 hours. These methodologies would have wide application throughout the aeronautics industry. Major Recommendation 9. The HSR Program should develop a more comprehensive testing and risk mitigation strategy for the combustor that extends into the recommended technology maturation phase and includes combustor testing in a full-scale demonstrator engine. Because satisfactory combustor performance is essential to program success, additional resources should be devoted to development of combustor technology during Phase II, if necessary. Major Recommendation 10. As a high priority, the recommended technology maturation phase should fabricate and test two fully instrumented, full-scale engines in static sea-level conditions. In order to bring the engine to a TRL of 6, this effort should include aerodynamic and aeromechanical testing, 1,000 hours of accelerated mission endurance testing, acoustic tests, and a 150-hour simulated mission profile test. Major Recommendation 11. The HSR Program should form an integrated product team to develop a plan for identifying and resolving APSE (aero/propulsive/ servo/elastic) issues.1 This team should investigate the critical interrelationships among the following: airframe structural stiffness; the level of aerodynamic instability; the feasibility, performance, and complexity of the flight and engine control systems; and aircraft structural weight. Major Recommendation 12. The HSR Program should ensure that structural materials suitable for lower cruise speeds (Mach 2.0 to 2.2) will be available in case a lower-speed design turns out to be more economically viable than a Mach 2.4 design, based on technical and economic factors. Major Recommendation 13. The HSR Program, industry, and the FAA should determine if reserve fuel requirements for an operational HSCT in airline service will significantly reduce the effective range of a TCA-like HSCT. If so, the HSR Program should assess how to address that shortcoming in the context of an economically viable HSCT concept. The HSR Program should also determine the sensitivity of payload and range to key HSR technologies. 1    APSE phenomena are associated with the highly interactive, dynamic nature of the HSCT airframe, propulsion, and flight control systems. See Chapter 5.

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U.S. Supersonic Commercial Aircraft: Assessing NASA's High Speed Research Program Major Recommendation 14. The HSR Program should develop risk abatement plans that include additional research in selected technology areas to counter possible shortfalls in L/D, specific fuel consumption, structural weight fraction, and other critical parameters (which the HSR Program should identify). Major Recommendation 15. The HSR Program should ensure that the flight deck system concepts under development can provide a level of safety superior to conventional systems used by subsonic transports. To ensure flight tests are successful, preliminary flight tests planned during Phase II of the HSR Program should be supplemented by additional flight tests during the proposed technology maturation and advanced technology demonstration phases. These additional flight tests should use displays with resolution equal to the resolution needed for an operational HSCT. Major Recommendation 16. The FAA, NASA (i.e., the HSR Program), and industry should support timely resolution of certification issues—including dedicated certification-related research—to ensure that technology under development by the HSR Program can be applied to development of an HSCT. CHAPTER 2 REQUIREMENTS ANALYSIS Finding 2-1. Industry forecasts of market demand indicate that an HSCT consistent with TCA performance specifications will have a market size large enough to be economically viable. The assumptions in these market forecasts appear to be reasonable, although not certain or risk free. Generalizations in the forecast assumptions may overstate the projected market size. Recommendation 2-1. Industry should conduct further market research and simulations to reduce the uncertainties associated with current forecasts and to validate that performance specifications used by the HSR Program to guide technology development are consistent with the design of an economically viable HSCT. Finding 2-2. From an airline scheduling perspective, an HSCT with a cruise speed as low as Mach 2.0 is likely to have productivity similar to a Mach 2.4 HSCT, assuming similar maintenance and servicing requirements. Finding 2-3. There is general agreement within industry and the HSR Program that a payload of about 300 passengers is required for an economically viable HSCT. A similar level of agreement does not exist regarding what design range (between 4,500 n.m. and 6,500 n.m.) will maximize economic viability. Recommendation 2-2. The HSR Program should conduct further market research and economic simulations to capture the impact of payload and range on HSCT

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U.S. Supersonic Commercial Aircraft: Assessing NASA's High Speed Research Program utilization and economics. These simulations should be based on a comprehensive analysis of specific city-pair routes rather than a top-down analysis. Finding 2-4. Achieving the range, payload, and MTOW goals established by the HSR Program (i.e., 5,000 n.m., 300 passengers, and 740,000 pounds) depends on full attainment of goals for supersonic cruise L/D, specific fuel consumption, and structural weight fraction. Recommendation 2-3. The HSR Program should establish design margins to allow for possible shortfalls in key performance parameters. The HSR Program should also establish a management system to perform trade-offs between these parameters to maintain an acceptable level of overall system performance. Finding 2-5. Europe has the technical expertise to compete in developing a next-generation supersonic commercial transport. Japan and other Pacific rim countries could contribute financially and, to a lesser extent, technically. Because of technical challenges and financial requirements, it seems unlikely that foreign interests will initiate a program to develop an economically viable supersonic commercial transport during the next 5 to 10 years. However, political factors could spur earlier action. Recommendation 2-4. NASA should continue to track the development of supersonic commercial transport technology worldwide. Finding 2-6. The key product and process characteristics with the highest risk are engine emissions, engine service life, airframe service life, range, affordability, community noise, APSE phenomena, and manufacturability.2 Finding 2-7. Most of the advanced technologies the HSR Program is developing to support an HSCT product launch decision are very process dependent, especially from the point of view of affordability. Recommendation 2-5. The HSR Program's Integrated Planning Team should use the HSR/HSCT QFD planning matrix in Figure 2-1 to examine the complex interdisciplinary nature of the HSR Program and the trade-offs that may be required among design requirements. Recommendation 2-6. The HSR Program should ensure that current and future efforts are properly focused on the most important, highest risk areas. The single most critical design requirement is affordability, and the HSR Program should adopt an affordability metric—such as average yield per available seat mile— 2    These characteristics are listed in the order they appear in Figure 2-1.

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U.S. Supersonic Commercial Aircraft: Assessing NASA's High Speed Research Program that is more comprehensive than MTOW. The other areas of greatest importance, many of which are closely linked to affordability, are as follows: airframe service life (durability) dynamic interactions among the airframe, propulsion, and flight control systems (i.e., APSE effects) engine emissions (ozone depletion) engine service life manufacturing and producibility range Finding 2-8. The strong negative (adverse) relationships among high-priority design requirements and the risks associated with these requirements (especially with regard to affordability) support the committee's recommendation for a substantial effort beyond the current Phase II.3 CHAPTER 3 PROPULSION Finding 3-1. The HSR Program's turbine airfoil system development effort is a high risk endeavor that is unlikely to demonstrate the specified level of technology readiness (TRL 6) by the end of Phase II. Recommendation 3-1. The HSR Program should expand its efforts to develop suitable alloys and thermal barrier systems during Phase II to increase the probability that the airfoil system will satisfy durability and lifetime requirements and to prepare for the recommended technology maturation phase. Finding 3-2. The HSR Program's disk manufacturing development effort will not demonstrate a necessary level of technology readiness (TRL 6) by the end of Phase II. Recommendation 3-2. Early in the recommended technology maturation phase, which would follow Phase II, the HSR Program should manufacture and destructively test representative full-scale disk components to verify that manufacturing technologies are feasible and that measured material properties are consistent with design data generated from small samples. Disk performance should be demonstrated in a full-scale engine later in the technology maturation phase. Finding 3-3. Significant uncertainties regarding the viability of potential ultralow NOx combustor designs—and the materials needed to implement those designs—are likely to remain at the conclusion of Phase II, as currently planned. 3    See Chapters 1 and 6.

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U.S. Supersonic Commercial Aircraft: Assessing NASA's High Speed Research Program Recommendation 3-3a. During the recommended technology maturation phase, the HSR Program should test a full-scale demonstrator engine to reduce uncertainties regarding the viability of the selected ultralow NOx combustor design. Combustor development during Phase II should focus on preparations for full-scale tests. Recommendation 3-3b. In order to increase the potential market for silicon carbide CMC liners—and thereby ensure their availability for use in HSCTs—the HSR Program should encourage other engine research programs sponsored by NASA, the Department of Defense, and the Department of Energy to include more CMC materials. Finding 3-4. Development efforts for the exhaust nozzle may achieve the specified level of technology readiness (TRL 6) by the end of Phase II. Nonetheless, uncertainties about nozzle materials and manufacturing processes will require additional work during the recommended technology maturation phase. Recommendation 3-4. The HSR Program should fabricate and test full-scale nozzles during the recommended technology maturation phase to validate nozzle manufacturing technology, noise levels, and material performance. Finding 3-5. Fabrication and testing of full-scale engines are needed to validate engine technologies, particularly with regard to emissions and noise requirements. Early action leading to this goal is required to ensure that the propulsion system technologies will be ready for flight testing at the same time as airframe and integrated aircraft system technologies. Recommendation 3-5. It is critical that the HSR Program build and test two full-scale, instrumented engines during the recommended technology maturation phase. Testing of one engine should focus on aerothermodynamics and aeromechanical issues (e.g., thrust, emissions, noise, and vibration); testing of the other should focus on structures and materials issues (e.g., reliability, service life, and weight). The second engine would also reduce risk by ensuring a backup engine is available in case the first engine experiences a catastrophic failure. CHAPTER 4 AIRFRAME Finding 4-1. Different families of materials (e.g., resins, adhesives, sealants, coatings, and finishes) are required for use at sustained temperatures above 250°F (i.e., for aircraft designs with cruising speeds above Mach 2.2) than for use at temperatures below 250°F. Therefore, the focus of the HSR Program on a speed of Mach 2.4 critically influences materials technology development. General

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U.S. Supersonic Commercial Aircraft: Assessing NASA's High Speed Research Program classes of polymeric materials and manufacturing processes suitable for a Mach 2.0 to 2.2 HSCT are available but have not demonstrated the life requirement and require significant technology development. Recommendation 4-1. The HSR Program should retain a cruise speed of Mach 2.4 as an important baseline objective to encourage development of advanced materials and to develop a fundamental understanding of high temperature material responses and degradation mechanisms. However, the HSR Program, with appropriate support from the airframe manufacturers and material suppliers, should also identify and develop critical enabling technologies to protect the viability of developing a Mach 2.0 to Mach 2.2 HSCT. This effort should start during Phase II and continue until risks associated with a Mach 2.4 design are substantially reduced. Finding 4-2. The focus of the HSR Program on a single basic PMC system (PETI-5) is a major program risk that could have a catastrophic effect on the HSR Program if the development effort falls short in critical areas, such as processing, properties, or durability. This risk underscores the importance of developing alternative materials technologies for Mach 2.0 to Mach 2.2.4 Recommendation 4-2. The HSR Program should make development of PMC-compatible coatings and finishes an integral part of its PMC development effort. Finding 4-3. The adequacy of the materials supplier base could become a critical issue when industry considers whether to make an HSCT program launch decision. Factors that interfere with establishing the necessary supplier base include restrictions on the involvement of foreign industry and NASA's ownership of a major structural material (PETI-5). These factors lower the incentive for large suppliers of aerospace materials to develop materials on their own. Recommendation 4-3. HSR Program managers and airframe industry executive managers should meet with material suppliers to solicit financial and technical commitments to participate in the overall effort to develop materials needed for an HSCT. The HSR Program should also ensure the ability of foreign industry to reduce risk in critical areas is adequately considered, either through the independent actions of industry participants in the HSR Program or through direct action by the HSR Program (after obtaining necessary exemptions to NASA policy restrictions on the involvement of foreign industry). This is especially important for the cost-effective development of required sealants. 4    Finding 4-1 and Recommendation 4-1 further explain the committee's conclusions regarding efforts by the HSR Program to develop PMCs.

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U.S. Supersonic Commercial Aircraft: Assessing NASA's High Speed Research Program Finding 4-4. The HSR Program's materials specimen and element testing is well planned. The testing schedule, however, extends beyond the end of the current program and is generally limited to thermomechanical testing of the PMC material being developed by the HSR Program (PETI-5) and thermoset materials that are commercially available. Risk associated with the selection of structural concepts is increased by the need to rely on (1) predicted end-of-lifetime material properties based on less than one lifetime of real-time test data and (2) accelerated durability test methods that have not been validated by adequate real-life tests. An improved scientific understanding of individual composite degradation mechanisms is needed to reliably predict lifetime performance and to design accelerated tests. Recommendation 4-4. During Phase II, the HSR Program should conduct a focused, critical, detailed, technical assessment of alternatives for including new PMC materials in the durability testing of specimens and elements. This assessment should include personnel from inside and outside the HSR Program, and it should use the findings and recommendations of Accelerated Aging of Materials and Structures (NRC, 1996) as a guide. The assessment should include the results of ongoing real-time testing, and it should be oriented toward understanding aging characteristics and accelerated testing techniques suitable for a broad class of materials applicable to HSCT development. Finding 4-5. In the areas of manufacturing, processing, and producibility, the HSR Program is focused on developing processing methods for the fabrication of small numbers of subscale and full-scale components to support the materials testing program. However, this will not resolve issues associated with how to affordably manufacture components in production quantities. In addition, manufacturing processes for PMCs required for a Mach 2.4 HSCT, such as PETI-5, will be complex and costly because of handling characteristics, high volatile content, and high temperatures and pressures (up to 700°F and 200 psi). Recommendation 4-5. The NASA and industry participants in the HSR Program should jointly place greater emphasis on the development of manufacturing technology and producibility demonstrations so the HSR Program can properly support the HSCT product launch decision. NASA and industry should develop an integrated manufacturing technology plan that enables the HSR Program's materials technology development efforts, which currently seem to be focused on near-term component fabrication, to adequately consider overall, long-term manufacturing issues. Development of this plan should be closely coordinated with the wing and fuselage design teams. The HSR Program should also review and incorporate the integrated design and manufacturing approach described by Marx et al. (1996), which is based on a combined performance and economic perspective.

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U.S. Supersonic Commercial Aircraft: Assessing NASA's High Speed Research Program Implementation of this recommendation will require changes to the Phase II program and approval of the recommended technology maturation phase. Finding 4-6. The performance of innovative structural concepts depends on successful development of the materials upon which they are based. It is virtually impossible to separate the structures design effort from the materials and manufacturing development effort. The materials development program and the structural concept development program are both well planned. However, the current schedule of the HSR Program does not facilitate a sequential approach that would reduce overall risk by validating the performance of proposed new materials before they are incorporated into new structural concepts. Finding 4-7. The use of multiple weight estimation methods has confounded weight tracking and obscured HSR Program successes in using innovative structural concepts to reduce vehicle weight. There is no longer a clear relationship between structure areal weights and MTOW, which are both top-level audit metrics used by the HSR Program. Finding 4-8. Structural design and analysis tools vary from company to company. Improvements are needed in these tools, including life prediction tools, weight estimation tools, thermal stress analysis tools, and rapid preliminary sizing tools. These improvements would give the U.S. aerospace industry a distinct competitive advantage. Recommendation 4-6. During Phase II, the HSR Program should concentrate more resources on developing structural design tools tailored for HSCT applications. These tools should include validated materials databases, rapid preliminary sizing tools, validated thermal stress analysis tools, and validated analytical life prediction tools. Resources could be reallocated to this task from full-scale component tests. 5 Finding 4-9. The large amount of work needed to carry forward and analyze the many design concepts still under consideration increases the risk of not meeting the program schedule. Finding 4-10. Full-scale testing of large components, because of the cost and time involved, is more appropriate to the final structural validation of a specific vehicle point design. Large component tests, as currently planned, would not address critical structural issues for the full-scale vehicle or major structural joints. Nor would they validate that fabrication methods used for component tests would be representative of the manufacturing methods that will be used during production. 5    See Finding 4-10 and Recommendation 4-7.

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U.S. Supersonic Commercial Aircraft: Assessing NASA's High Speed Research Program Thus, full-scale testing of large components during Phase II would probably add little value to the technology development process. Finding 4-11. Using surrogate materials and fabrication processes for components in large-scale tests could significantly reduce the ability of those tests to objectively assess the economic viability of the proposed design approaches. Correlation of component test results with analytical predictions would be complicated by inherent differences between the ''as-designed'' and the "as-tested" materials and fabrication processes. Finding 4-12. Structural issues should be resolved in a cost-effective manner, which means using the smallest and simplest tests that can provide the required information. Addressing materials and structures issues using tests at the coupon, element, and subcomponent levels during Phase II may offer a higher payoff than testing full-scale components. Recommendation 4-7. Testing of full-scale components should be deferred to the recommended technology maturation phase. Funds allocated for testing full-scale components during Phase II should be reallocated to achieve higher levels of technology readiness in the critical enabling materials and structures technologies, including the following: materials characterization and life prediction methodologies rapid, efficient design and analysis tools robust structural concepts technical criteria related to dynamic interactions among the airframe, propulsion, and flight control systems (APSE effects) and the relationship of these criteria to structural concepts Finding 4.13. The current estimate of drag for the TCA is reasonable. However, there remains some uncertainty (on the order of 3 to 5 percent) about the actual drag of an HSCT in flight. Finding 4-14. The projected 10 percent improvement in L/D (relative to the L/D of the current TCA design) is optimistic. Design optimization can be expected to yield about 5 to 6 percent. Some of the other projected improvements may be offset by drag increments on the real production aircraft. Finding 4-15. SLFC has the potential to improve L/D by 10 to 15 percent, which could offset shortfalls in the attainment of predicted L/D, specific fuel consumption, or structural weight fraction, and would provide a margin for attaining performance goals in terms of aircraft range.

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U.S. Supersonic Commercial Aircraft: Assessing NASA's High Speed Research Program Recommendation 4-8. NASA should continue to conduct SLFC research as part of a long-term commitment to HSR technology. This research could also have significant payoffs for other aeronautical projects. Long-term planning by the HSR Program should provide for the possible incorporation of SLFC in future configurations, including the full-scale technology demonstrator the committee recommends flight testing during the advanced technology demonstration phase. CHAPTER 5 INTEGRATED AIRCRAFT Finding 5-1. An HSCT similar to the TCA will experience complex dynamic interactions involving the pilot and the APSE system, which includes the airframe (e.g., aerodynamic effects and elastic properties of the airframe structure), the propulsion system, and the flight control system (including the XVS). It is not yet clear how to design an HSCT that overcomes these effects and provides safe flying and handling qualities. Furthermore, the current HSR Program does not adequately address this problem. Finding 5-2. The impact of APSE effects on flight dynamics and handling qualities may require changes in the aerodynamic and/or structural design of the TCA that would significantly reduce aerodynamic efficiency and/or increase structural weight, thereby reducing maximum range. Finding 5-3. It is unlikely that the technical risk associated with APSE effects can be adequately addressed without building and flight testing an aircraft like the FAST (full-scale advanced supersonic technology) demonstrator. Recommendation 5-1. The development of design tools and techniques for synthesizing and validating a highly integrated flight and propulsion control system and for properly addressing APSE effects on flight control and flight management systems should be established as a top-level research issue within the HSR Program during Phase II and the subsequent phases proposed by the committee. How to address APSE effects early in the aircraft-design cycle, before detailed structural models are developed, requires special attention. Recommendation 5-2. The HSR Program should reevaluate the current TCA configuration in light of APSE effects. Because structural mode control will almost surely be required to achieve adequate flight dynamics and handling qualities, the optimum vehicle configuration may include additional and/or nontraditional aerodynamic surfaces. If necessary, such "control-configured" concepts should be included in future evaluations of aircraft design configurations. Recommendation 5-3. An interdisciplinary team should be formed to fully address relevant aspects of the APSE problem, and the organizational distance between

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U.S. Supersonic Commercial Aircraft: Assessing NASA's High Speed Research Program the groups responsible for (1) guidance and control systems and (2) control laws should be reduced or eliminated. Finding 5-4. Validating the performance of the XVS and other key flight deck technologies being developed by the HSR Program is crucial to the public acceptance and economic viability of an HSCT. Flight testing XVS technology using displays with lower resolution than the resolution needed for an operational HSCT increases the risk that test results will be unsatisfactory, which could reduce public acceptance of the XVS design concept. Recommendation 5-4. To address flight deck system risk adequately, the preliminary flight tests planned during Phase II of the HSR Program should be supplemented by additional flight tests during the proposed technology maturation and advanced technology demonstration phases. These additional flight tests should use displays with resolution equal to the resolution needed for an operational HSCT. Finding 5-5. HSCTs must be able to meet applicable regulatory noise standards. However, the HSR Program has established appropriate noise goals and is using an effective approach to achieve them. Recommendation 5-5. The FAA and NASA (i.e., the HSR Program) should develop and periodically update a master certification plan that supports timely resolution of key HSCT certification issues and shows how the activities needed to resolve these issues are related to other HSR Program efforts. Finding 5-6. The design and operating characteristics of an HSCT will require some changes in airline, airport, and air traffic control system facilities and procedures. However, making changes to airport facilities (including taxiways) and air traffic control procedures, although technologically simple, may be difficult from a funding and environmental/community acceptance perspective. Failure to anticipate problems and make required changes could reduce HSCT market demand below current estimates. Recommendation 5-6. During the recommended advanced technology demonstration phase, industry and the HSR Program should identify and evaluate changes in airline, airport, and air traffic control system facilities and procedures that may be required to accommodate an HSCT. In particular, industry should conduct a detailed study of infrastructure issues at key airports worldwide that an HSCT is expected to service. As a first step, these issues should be included in the recommended effort to validate HSCT market size (see Chapter 2).