3
Research and Technology Challenges

The highest priority R&T Challenges for each R&T Area are listed and discussed below. The section for each Area includes a table showing the results of the quality function deployment (QFD) evaluation of R&T Challenges for that area. Each section also discusses general characteristics of high- and low-priority challenges in the relevant Area, R&T Thrusts that encompass multiple Challenges from a given area, and specific Challenges that rank high in national priority, but low in NASA priority. More detailed information for each Challenge appears in Appendixes A to E.

AERODYNAMICS AND AEROACOUSTICS

Introduction

Aerodynamics and aeroacoustics research is required to support development of advanced aeronautical systems. The scope of this R&T Area includes a wide range of fundamental fluid dynamic research ranging from low-speed, low-Reynolds-number flows to hypersonic, chemically reacting flows to aerodynamic issues associated with flight in alternative atmospheres. It does not include aerodynamic issues associated with ground transportation systems or fluid dynamic issues associated with hydrodynamic flows or the space environment.

The QFD process described in Chapter 2 was used to prioritize 19 R&T Challenges related to aerodynamics and aeroacoustics. Table 3-1 and Figure 3-1 show the results. The text that follows describes the 11 R&T Challenges that ranked highest in terms of NASA priority, the general characteristics of high- and low-priority Challenges, and the R&T Thrusts in this Area.1 Further details on all the challenges, including the rationale for scoring, are found in Appendix A.

In terms of national priority, challenges A1, A2, A3, A6, and A7b all fall within a narrow range. Taking account of the weighting factors and scores, and noting that small changes in many of those elements can produce important changes in the final order, it should be concluded that these challenges are of roughly equal importance.

Top 11 R&T Challenges

A1 Integrated system performance through novel propulsion–airframe integration

Research into improved techniques for propulsion– airframe integration is required to enable greater aircraft flexibility and improve performance, especially as aircraft speeds increase. Improvements in the accuracy of predictions for three-dimensional (3-D) steady and unsteady interactions between external and internal aerodynamics and aeroacoustics would enable the design of advanced aeronautical systems, especially with systems of unconventional design. These interactions include the effects of steady and dynamic distortion on engine operations and the effects of hot, reacting exhaust flows on vehicle aerodynamics. These interactions are particularly important in the design of vertical and short takeoff and landing (V/STOL), extremely short takeoff and landing (ESTOL), supersonic, and hypersonic airplanes.2

1

This chapter describes the top 10 R&T Challenges in each Area, except for the aeronautics and aeroacoustics Area. As shown in Figure 3-1, the NASA priority scores for Challenges A4a through A11 were relatively close, and there was a large difference in the scores for A11 and A12, so in this Area, unlike the remaining four, the top 11 R&T Challenges are described in the aeronautics and aeroacoustics Area.

2

VTOL airplanes can take off and land vertically. This includes tilt-rotors, the AV-8 Harrier, and the Joint Strike Fighter (JSF), for example. VTOL airplanes do not routinely take off or land vertically because of the range-payload penalty associated with the weight limitations of purely vertical operations. Rather, they use any available field length to develop some forward motion and wing lift during takeoff to increase the useful load (fuel plus payload). They tend to land vertically only at the end of the mission, when they are lighter, after burning fuel and/or dropping weapons.STOL airplanes use high-lift systems to take off in less distance than



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Decadal Survey of Civil Aeronautics: Foundation for the Future 3 Research and Technology Challenges The highest priority R&T Challenges for each R&T Area are listed and discussed below. The section for each Area includes a table showing the results of the quality function deployment (QFD) evaluation of R&T Challenges for that area. Each section also discusses general characteristics of high- and low-priority challenges in the relevant Area, R&T Thrusts that encompass multiple Challenges from a given area, and specific Challenges that rank high in national priority, but low in NASA priority. More detailed information for each Challenge appears in Appendixes A to E. AERODYNAMICS AND AEROACOUSTICS Introduction Aerodynamics and aeroacoustics research is required to support development of advanced aeronautical systems. The scope of this R&T Area includes a wide range of fundamental fluid dynamic research ranging from low-speed, low-Reynolds-number flows to hypersonic, chemically reacting flows to aerodynamic issues associated with flight in alternative atmospheres. It does not include aerodynamic issues associated with ground transportation systems or fluid dynamic issues associated with hydrodynamic flows or the space environment. The QFD process described in Chapter 2 was used to prioritize 19 R&T Challenges related to aerodynamics and aeroacoustics. Table 3-1 and Figure 3-1 show the results. The text that follows describes the 11 R&T Challenges that ranked highest in terms of NASA priority, the general characteristics of high- and low-priority Challenges, and the R&T Thrusts in this Area.1 Further details on all the challenges, including the rationale for scoring, are found in Appendix A. In terms of national priority, challenges A1, A2, A3, A6, and A7b all fall within a narrow range. Taking account of the weighting factors and scores, and noting that small changes in many of those elements can produce important changes in the final order, it should be concluded that these challenges are of roughly equal importance. Top 11 R&T Challenges A1 Integrated system performance through novel propulsion–airframe integration Research into improved techniques for propulsion– airframe integration is required to enable greater aircraft flexibility and improve performance, especially as aircraft speeds increase. Improvements in the accuracy of predictions for three-dimensional (3-D) steady and unsteady interactions between external and internal aerodynamics and aeroacoustics would enable the design of advanced aeronautical systems, especially with systems of unconventional design. These interactions include the effects of steady and dynamic distortion on engine operations and the effects of hot, reacting exhaust flows on vehicle aerodynamics. These interactions are particularly important in the design of vertical and short takeoff and landing (V/STOL), extremely short takeoff and landing (ESTOL), supersonic, and hypersonic airplanes.2 1 This chapter describes the top 10 R&T Challenges in each Area, except for the aeronautics and aeroacoustics Area. As shown in Figure 3-1, the NASA priority scores for Challenges A4a through A11 were relatively close, and there was a large difference in the scores for A11 and A12, so in this Area, unlike the remaining four, the top 11 R&T Challenges are described in the aeronautics and aeroacoustics Area. 2 VTOL airplanes can take off and land vertically. This includes tilt-rotors, the AV-8 Harrier, and the Joint Strike Fighter (JSF), for example. VTOL airplanes do not routinely take off or land vertically because of the range-payload penalty associated with the weight limitations of purely vertical operations. Rather, they use any available field length to develop some forward motion and wing lift during takeoff to increase the useful load (fuel plus payload). They tend to land vertically only at the end of the mission, when they are lighter, after burning fuel and/or dropping weapons.STOL airplanes use high-lift systems to take off in less distance than

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Decadal Survey of Civil Aeronautics: Foundation for the Future TABLE 3-1 Prioritization of R&T Challenges for Area A: Aerodynamics and Aerocoustics       Strategic Objective National Priority Why NASA? Why NASA Composite Score NASA Priority Score       Capacity Safety and Reliability Efficiency and Performance Energy and the Environment Synergies with Security Support to Space Supporting Infrastructure Mission Alignment Lack of Alternative Sponsors Appropriate Level of Risk R&T Challenge Weight 5 3 1 ¼ each A1 Integrated system performance through novel propulsion–airframe integration 9 3 9 9 9 9 132 3 9 3 9 6.0 792 A2 Aerodynamic performance improvement through transition, boundary layer, and separation control 9 3 9 9 3 3 120 3 9 3 9 6.0 720 A3 Novel aerodynamic configurations that enable high performance and/or flexible multimission aircraft 9 3 9 9 3 1 118 3 9 3 9 6.0 708 A4a Aerodynamic designs and flow control schemes to reduce aircraft and rotor noise 9 1 3 9 3 1 90 3 9 3 9 6.0 540 A4b Accuracy of prediction of aerodynamic performance of complex 3-D configurations, including improved boundary layer transition and turbulence models and associated design tools 3 3 9 3 3 3 72 9 9 3 9 7.5 540 A6 Aerodynamics robust to atmospheric disturbances and adverse weather conditions, including icing 9 9 3 1 9 1 112 3 9 3 3 4.5 504 A7a Aerodynamic configurations to leverage advantages of formation flying 3 1 9 9 3 1 78 3 9 9 3 6.0 468 A7b Accuracy of wake vortex prediction, and vortex detection and mitigation techniques 9 9 3 1 1 1 104 3 9 3 3 4.5 468 A9 Aerodynamic performance for V/STOL and ESTOL, including adequate control power 9 3 3 1 3 1 76 3 9 3 9 6.0 456 A10 Techniques for reducing/mitigating sonic boom through novel aircraft shaping 3 1 3 9 3 1 60 9 9 3 9 7.5 450 A11 Robust and efficient multidisciplinary design tools 3 3 9 9 3 3 90 3 9 3 3 4.5 405 A12 Accurate predictions of thermal balance and techniques for the reduction of heat transfer to hypersonic vehicles 1 1 3 1 9 9 40 9 9 3 9 7.5 300 A13 Low-speed takeoff and landing flight characteristics for access-to-space vehicles 1 3 1 1 3 9 38 3 9 9 9 7.5 285 A14 Efficient control authority of advanced configurations to permit robust operations at hypersonic speeds and for access-to-space vehicles 1 1 3 1 9 9 40 3 9 3 9 6.0 240 A15 Decelerator technology for planetary entry 1 1 1 1 3 9 28 3 9 9 9 7.5 210 A16 Low-Reynolds-number and unsteady aerodynamics for small UAVs 1 1 3 1 9 3 34 3 9 3 9 6.0 204 A17 Low-drag airship designs to enable long-duration stratospheric flight 1 3 1 3 9 1 42 3 3 3 9 4.5 189 A18 Prediction of communication capability through reentry trajectory and techniques to mitigate impact of communication blackouts 1 1 1 1 9 9 34 3 9 3 3 4.5 153 A19 Aircraft protective countermeasures based on a range of small deployed air vehicles 1 3 1 1 9 1 36 3 3 3 3 3.0 108

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Decadal Survey of Civil Aeronautics: Foundation for the Future FIGURE 3-1 NASA and national priorities for Area A: aerodynamics and aeroacoustics. Minimization of drag by propulsion–airframe integration will reduce fuel burn and CO2 emissions. A2 Aerodynamic performance improvement through transition, boundary layer, and separation control Viscous drag at subsonic, supersonic, or hypersonic speeds may be reduced by controlling the onset of boundary layer transition using active control or passive 3-D design concepts. Direct reduction of skin friction drag is possible with extensive laminar flow, which can be achieved with a combination of vehicle shaping and flow control concepts. One example is natural laminar flow using reduced sweep and control of crossflow pressure gradients through shape optimization. A second example is boundary layer manipulation through suction, blowing, or distributed effectors. Related concepts may also be used to reduce separation at high lift and other conditions (e.g., buffet), which improves performance at high-lift conditions. In some conditions of flight, particularly at high lift, a turbulent boundary layer is needed. Active flow control techniques are emerging, including piezoelectric, voice-coil, dielectric barrier discharges, and surface electrical discharges. The potential advantages are clear, but implementation has been hampered by the lack of accurate and efficient methods for prediction (see Challenge A4b) and design and by the difficulty of conducting experiments that require high Reynolds numbers and are sensitive to disturbances such as free-stream turbulence and noise. Work on this Challenge should identify the most promising application domains, control approaches, and actuator concepts and develop efficient methods for design and experimental validation. A3 Novel aerodynamic configurations that enable high performance and/or flexible multimission aircraft Most classes of aircraft configuration have remained constant for many years (e.g., the tube and wing of a subsonic transport and the main rotor plus tail rotor of a helicopter). Novel aerodynamic configurations provide substantial opportunities for long-term breakthroughs in aircraft capabili- conventional aircraft (typically a few thousand feet). Very few STOL aircraft can safely take off on runways shorter than 3,000 ft and none on runways less than 2,000 feet. (This class does not include ultralight aircraft, kit planes, etc. that can operate out of short fields due to their small size but do not have high-lift systems.)ESTOL airplanes would be able to safely take off on runways of 2,000 ft. They would have high-lift systems and thrust-to-weight ratios that are higher than conventional aircraft but not as high as VTOL aircraft. ESTOL aircraft have not yet been developed for commercial or military operations.V/STOL refers to both VTOL and STOL airplanes that convert to fixed-wing flight after takeoff; it does not include helicopters.

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Decadal Survey of Civil Aeronautics: Foundation for the Future ties. A number of innovative concepts have been proposed in the past and pursued to differing levels. Examples include the blended wing body, canard rotor wing, oblique flying wing, and strut-braced wing. A sustained research program should be promoted to develop novel aircraft configurations, including further development of existing concepts where appropriate, with emphasis on achieving breakthroughs related to the high-priority Strategic Objectives. Other R&T Challenges would also contribute to enabling novel aerodynamic configurations. Advances in flight mechanics and propulsion–airframe integration (R&T Challenge A1) are required to make advanced concept airplanes viable and robust. Flow control (R&T Challenge A2) could significantly enhance the capability of novel configurations, since it could be assumed a priori in the design process rather than added as an improvement to an existing airplane. Research related to the Common Theme of physics-based analysis tools is needed to move beyond empirical design tools.3 In addition, flight testing is a critical element of a successful research program in novel configurations. A4a Aerodynamic designs and flow control schemes to reduce aircraft and rotor noise Many of today’s airports now limit operations because of the noise emitted to the surrounding community. Future passenger growth at many airports will be limited if the noise levels emitted by the newer aircraft are not reduced further, thus adversely affecting capacity. Off-loading the main runway of regional jets by using ESTOL aircraft and rotorcraft, thus reducing congestion for larger passenger aircraft on the main runway, will dramatically increase capacity by allowing more takeoffs and landings at existing airports without increasing demand for runway usage (NRC, 2003; FAA, 2000). However, it will only be possible if these ESTOL aircraft and rotorcraft are quiet. Aerodynamic noise research should be pursued to (1) improve understanding of the underlying flow physics, (2) develop novel technologies, and (3) create improved and validated acoustic prediction and design tools. This research should include a balance of physics modeling, tool development, and experiments. Important physical phenomena that require research include cavity flows, unsteady flow–solid surface interactions, flow separation, rotor dynamic stall, and wake vortex dynamics. Novel needs include quiet, high-lift devices; technologies to enable steep, quiet, slow-approach trajectories; technologies to reduce the strength of vortices shed from the rotor blades and/or vortex/blade position control; integrated advanced control schemes for active rotorcraft noise reduction; and technologies to reduce rotor response to vortex-induced disturbances. Physics-based source noise prediction methods and improved computational aeroacoustic tools are key requirements. Design tools are needed both at the technology level and at the aircraft system level, with particular attention to integrated solutions for aerodynamic and operational issues. A4b Accuracy of prediction of aerodynamic performance of complex 3-D configurations, including improved boundary layer transition and turbulence models and associated design tools The aerospace industry lacks computational analysis and design tools that can rapidly and accurately predict complex flow behavior driven by boundary layer transition, flow separation, novel configurations, off-design operations, and multidisciplinary interactions. To meet this need, physics-based design tools must be developed and systematically validated in representative environments. Ideally, these tools should have the following attributes: Adaptive and intelligent self-generating grids that are easily implemented using simple computer-aided design surface instructions, minimal boundary condition definition, and desktop operation. Seamless applicability over the continuum of fluid flows (speed regimes, phase, periodicity) and reference frames. Ability to accurately predict transitional and separated flows, validated through experimentation. Ability to fully describe the state of the fluid at any point in the solution domain, with useful information on the surfaces. Inverse design capability. The benefit of technologies developed by this Challenge would be enhanced by parallel development of multidisciplinary design tools to address complex nonlinear interactions, and parameter uncertainties and models, while still being computationally efficient (see Challenge A11). A6 Aerodynamics robust to atmospheric disturbances and adverse weather conditions, including icing Adverse weather conditions, including storms and icing conditions, significantly reduce the capacity and reliability of the air transportation system. Adverse weather also degrades system safety. This issue is of importance to both civil and military aviation. Research is needed to improve the ability to predict and monitor environmental conditions and develop aerodynamic designs and techniques that are robust to adverse conditions. At present, wind-shear warning systems are built into commercial aircraft, icing hazards are handled by regulatory constraints on flight operations, and prediction techniques are largely empirical. Low-cost techniques to mea- 3 “Physics-based” refers to the general use of scientific principles in the place of empirical data. It includes the use of principles from chemistry, biology, material science, etc.

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Decadal Survey of Civil Aeronautics: Foundation for the Future sure upstream environmental conditions should be developed. Examples of promising techniques include microwave, lidar, and laser-acoustic measurement techniques. Efforts to miniaturize and reduce the cost of the measurement equipment should be supported. Techniques to predict and mitigate the impact of adverse environmental conditions on the aircraft operation should be improved. Required improvements include the development of models to predict the impact of multiphase, nonequilibrium situations encountered under icing conditions; validation of icing prediction capabilities to enable a reduction in the high cost of aircraft and helicopter icing certification; and models for the complex-flow, time-dependent, 3-D interactions encountered during wind shear or ambient turbulence on the aircraft flowfield. A7a Aerodynamic configurations to leverage advantages of formation flying Formation flight is currently used by military airplanes for a variety of operational reasons, although rarely for drag reduction. Recent breakthroughs in accurate navigation and control make possible extended precision formation flight at cruise and permit exploitation of favorable interference for vortex drag reduction. Although this phenomenon is well known, the magnitude of the potential savings is not widely appreciated. Three airplanes flying in formation and designed to best exploit these effects could reduce vortex drag by more than 50 percent in cruise, a greater reduction than that obtainable by extensive laminar flow control on the wing. This would mean roughly a 20 percent reduction in total drag under identical operating conditions. However, with less induced drag the optimum altitude increases, reducing viscous drag as well. The net result is almost a 30 percent reduction in total drag. Unlike the tight formations required for military applications, drag savings are possible even with longitudinal separations of several miles (Spalart, 1998), reducing safety concerns associated with formation flight. Initial NASA work on autonomous formation flight has identified some of the technology requirements for achieving these savings, but considerable research remains in both control methodology and aerodynamic design to take most advantage of the concept. Applications to cargo airplanes, rotorcraft, and even supersonic flight are possible but have not been studied extensively. Aerodynamic challenges include vortex location prediction, sensing and control, and wing design for efficient high-lift cruise. Suggested work in this area would result in improved methods for predicting wake vortex evolution; design tools for evaluation and optimization of multiple interacting airplanes; and experimental validation, including flight testing (which is especially important for evaluating real atmospheric effects). The aerodynamic aspects of formation flying are related to R&T Challenges D1 and E2. A7b Accuracy of wake vortex prediction, and vortex detection and mitigation techniques Wingtip vortices produced by airplanes present a danger to following aircraft, so airplane designs and techniques that mitigate the strength of these vortices, techniques to locate and determine their strength, and techniques to predict their propagation and decay are important factors in minimizing aircraft separation and enhancing safety.4 (Since aircraft lift is intimately tied to the production of circulation, these vortices cannot be completely eliminated.) Currently, aircraft separation standards are set by conservative estimates of the wake vortex trajectory (generally a sinking trajectory, but also affected by local weather conditions) and decay rate. Techniques to measure the characteristics of upstream wake vortices include lidar and laser-acoustic techniques, but these technologies are currently expensive (limiting their use to larger aircraft) and are less reliable than desired. Research into techniques to predict the formation, trajectory, and decay of vortices needs to be performed. This includes development and validation of numerical methods to accurately predict the trajectory and dissipation of vortices, integration of local weather prediction techniques into existing larger-scale weather models, demonstration of low-cost techniques for locating and measuring the strength of wake vortices for both ground-based and aircraft-based applications, and investigation of airplane designs that mitigate the strength of wake vortices. A9 Aerodynamic performance for V/STOL and ESTOL, including adequate control power The development of ESTOL regional jets able to operate from 2,000 ft runways and taxiways and to cruise in existing air traffic corridors will significantly reduce congestion problems on the main runways of hub airports. V/STOL aircraft will be able to operate from taxiways and other paved areas at major airports, further relieving congestion. In responding to natural disasters and carrying out military operations, low-cost VTOL tactical transports would be able to operate from short, austere landing fields near the focus of attention (e.g., the location of injured civilians or troops, battle areas, and landslides). Development of an efficient high-lift system is not the most important enabling technology for ESTOL airplanes. Conventional aerodynamic control surfaces become ineffective at the low landing and takeoff speeds of ESTOL airplanes (on the order of 65 knots). The challenge is to generate the forces needed for pitch trim and to control the aircraft at these slow speeds. It is also important to develop a thrust vectoring and reversing nozzle technology that not only provides the required lift but can also be integrated into a low- 4 The scope of this Challenge does not include and would not directly apply to helicopter blade wakes.

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Decadal Survey of Civil Aeronautics: Foundation for the Future drag configuration. (ESTOL airplanes require much more thrust than conventional or STOL airplanes, but not as much as VTOL airplanes.) In addition, wing design and fuselage shaping are needed to reduce cruise drag in the transonic regime for ESTOL regional jets. An important task for research related to rotorcraft and VTOL airplanes is to improve hovering and cruise efficiency. Reductions in downward forces in near-hovering flight dramatically improve the payload capability of tilt-rotor and powered-lift aircraft. Active control of large separation regions on these aircraft through blowing, zero-mass effectors, and integrated mechanical devices are promising methods for reducing download. Active twist control of the rotor also allows the rotorcraft to be designed to better match the hover and cruise design conditions, thereby improving efficiency. Active control of separation regions and smart design guided by high-fidelity codes will decrease cruise drag and improve the performance of VTOL airplanes. A10 Techniques for reducing/mitigating sonic boom through novel aircraft shaping Safe, efficient, cost-effective, environmentally acceptable supersonic flight over land remains elusive nearly 60 years after airplanes broke the sound barrier. The principal remaining problems are sonic boom mitigation, public acceptance, and sustained supersonic flight performance. Today, federal regulations prohibit civil supersonic flight over land. If this regulatory barrier can be overcome, it will probably stimulate investment that would overcome the other barriers and help usher in a new era of time-critical air travel. Building on the recent in-flight validation of NASA’s theory of shaped sonic boom persistence, a robust and comprehensive plan of research for technology maturation and tool development should be pursued to determine if practical supersonic airplanes can be developed whose sonic boom is acceptable to the public (Pawlowski et al., 2005). Such a plan should comprise public sonic boom acceptability determination; community exposure testing; aircraft shaping techniques that result in a low-amplitude, acceptable acoustic signature with minimal performance impact; critical propulsion-airframe integration technologies commensurate with low-boom design; aircraft and acoustic scaling methodologies; sensitivities to off-design conditions under a variety of atmospheric conditions; rapid and inverse computational design tools that address multiple design constraints; systematic validation through ground and flight tests; and metrics to assess progress and guide continuation according to plan. This Challenge is closely tied to Challenge B8. A11 Robust and efficient multidisciplinary design tools Multidisciplinary design tools are pervasive in aeronautics. More effective multidisciplinary tools would likely shorten the design cycle time for conventional aircraft and facilitate the discovery of new highly integrated aircraft designs with better performance than conventional designs. The development of physics-based models for this design environment is addressed in R&T Challenge A4b. This Challenge is associated with the research required to efficiently and effectively integrate multidisciplinary design tools of varying fidelity and numerical complexity into a seamless design environment. Research is also needed on automated techniques for handling and propagating parameter uncertainties throughout the design to allow development of robust aircraft designs. High-Priority R&T Challenges and Their Associated Thrusts Some of the high-priority R&T Challenges significantly impact multiple Strategic Objectives; others are high priority because NASA possesses unique capabilities to address them. In particular, R&T Challenges that significantly improve capacity or safety and reliability scored high due to the relevant weightings. The principal factors affecting an increase in capacity relate to expanding the operational capabilities near airports, expanding flight capabilities under adverse weather conditions, and enabling an expansion of operation from smaller airports. The expansion of operations near airports will require research into noise reduction and aircraft wake physics. Expansion of operations under adverse weather conditions will require research associated with techniques to monitor and then mitigate adverse environmental conditions, including icing, wind shear, and free-stream turbulence. Expansion of operations from smaller airports involves research on shortened takeoffs and landings and the associated noise reduction. The development of improved physical models and design tools for aerodynamic and aeroacoustic phenomena and techniques aimed at understanding and providing the option of controlling these phenomena rank high in the R&T Challenge prioritization. Mastery of these Challenges will enable significant advances in the performance and operability of aircraft through development of improved and possibly revolutionary designs and reduction of design margins associated with uncertainties. The following four R&T Thrusts describe threads of commonality among the R&T Challenges within the aerodynamics and aeroacoustics Area. Improved understanding and control of the fundamental physics of aerodynamic and aeroacoustic phenomena Complex fluid dynamic processes often present barriers to improved aircraft performance, so a better understanding of these phenomena is required. These processes can occur across significant spatial and temporal scales and involve interactions with processes that come under the purview of other disciplines. With a deeper knowledge of the funda-

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Decadal Survey of Civil Aeronautics: Foundation for the Future mental physical phenomena, effective techniques will likely evolve to control these processes, enabling improved aircraft performance. Accurate and robust multidisciplinary design tools Aeronautics is fundamentally multidisciplinary, so many aspects of aerodynamics and aeroacoustics are impacted by cross-discipline factors. Multidisciplinary aerodynamic and aeroacoustic design tools are needed that are accurate and robust yet cost-effective in terms of computing time and computational resources. Sensing and responding to the external environment Development of aircraft systems that respond dynamically to the local environment could significantly improve capacity and safety. With measurement techniques to sense the local environment ahead of an aircraft and allow it to respond accordingly, aircraft spacing can be reduced and operations in adverse weather can be expanded, with no degradation of safety. Revolutionary aerodynamic configurations Even though the basic design of civil aircraft has remained remarkably stable for many decades, it is not clear that the configuration has already been optimized. The steering committee believes that improved understanding and control of fluid dynamic phenomena will result in novel aircraft designs offering revolutionary advances in performance and operability in all mission areas. Low-Priority R&T Challenges No attempt was made to compile and assess all possible aerodynamic and aeroacoustic issues. All of the Challenges described above are relevant to fundamental aeronautics of civil aircraft. The aerodynamic Challenges that ranked low in the prioritization were largely research areas that support national or homeland security or the NASA space mission but minimally impact Strategic Objectives directly related to the performance of the air transportation system. Examples of these Challenges include hypersonic vehicle technologies, small UAVs, and stratospheric airships. These Challenges could play a vital role in NASA’s space exploration mission and in matters of national and homeland security; however, they ranked low in terms of both national and NASA priority for this report, where the focus is on civil aeronautics. Hypersonic technologies appear in Challenges throughout the prioritization list. Challenges associated with the development of a more complete understanding of hypersonic issues, such as transition, turbulence, and separation phenomena and the development of techniques to control these phenomena, are included in high-priority R&T Challenges that encompass multiple speed regimes. Challenges specific to hypersonic vehicles, such as low-speed handling characteristics, are rated much lower. PROPULSION AND POWER Introduction This section describes key R&T Challenges and Thrusts associated with aircraft propulsion and electrical power generation that should be addressed via basic and applied research to advance national civil aeronautics capabilities. These advances will permit the U.S. aeronautics enterprise to bring highly competitive products to market and improve the national capacity to move people and goods quickly and affordably with minimal energy usage and environmental impact. Historically, paradigm shifts in propulsion capability have enabled significant advances in aircraft performance. The replacement of water-cooled piston engines with radial, air-cooled engines enabled the great airframe advances of the first half of the 20th century, while those in the second half were greatly expedited by the gas turbine engine. The gas turbine will very likely continue to be the dominant means of propulsion for both civilian and military aircraft for the next half century. With oil prices at historic highs and increasingly stringent noise and emissions regulations, gas turbine designers face formidable obstacles to create more fuel efficient, cleaner, and quieter engines. Opportunities abound for significant advances, with current gas turbine performance still well below theoretical limits. For example, improvements in overall efficiency and, concomitantly, fuel economy, of more than 30 percent appear attainable (Koff, 2004) but will only occur with significant advances in high-temperature materials and rotating machinery aerodynamics. With advances in information technology, sensor miniaturization, and modeling, intelligent engines capable of self-diagnosis and adaptation, similar to those in the automotive realm, are in the offing. Advances in information technology are also driving electrical power demands for both flight systems and passenger needs—that is, entertainment and productivity. The desire for rapid yet affordable transcontinental and intercontinental travel will continue to motivate research into supersonic flight engines; it is difficult to imagine commercial aviation being restricted to subsonic flight regimes 50 years from now. Airbreathing engine technology also has the potential to contribute significantly to the development of reusable higher payload fraction, access-to-space vehicles. Technical progress will be greatly expedited by the use of validated, physics-based computational simulation tools, which will permit designers to optimize designs and greatly minimize the number of design cycles typical of empirical design-build-test-redesign approaches.

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Decadal Survey of Civil Aeronautics: Foundation for the Future The QFD process described in Chapter 2 was used to prioritize 16 R&T Challenges related to the Area of propulsion and power. Table 3-2 and Figure 3-2 show the results. The text that follows describes the 10 R&T Challenges that ranked highest in terms of NASA priority, the general characteristics of high- and low-priority Challenges, and the R&T Thrusts in this Area. Further details on all Challenges, including the rationale for scoring, are found in Appendix B. Top 10 R&T Challenges B1a Quiet propulsion systems Public concerns over the environmental impact of aircraft and airport operations—primarily noise and emissions— have prompted increasingly strict legal and regulatory requirements, which can severely constrain the ability of civil aviation to meet national and global needs for mobility, increased market access, and sustained economic growth. Aircraft noise concerns include takeoff and landing noise; taxi and engine run-up noise; flyovers at cruise altitude over very quiet areas; and sonic booms associated with supersonic flight. Figure 3-3 shows how the impact of aviation noise on people living around airports has declined in the United States. It contrasts the growth of air travel with the reduction in the number of people exposed to 65-decibel (dB) day-night average sound level (DNL), which is what the federal government has defined as the “significant noise level.” Since 1975, the number of persons exposed to significant noise levels has greatly declined, with the transition of commercial aircraft to quieter models even as air travel has grown dramatically. The availability of low-noise technologies, such as high-bypass-ratio engines, contributed significantly to this transition. Assuming the industry’s continued recovery, and given the goal of doubling capacity over the next 10 to 35 years, the dramatic improvements in noise exposure in the last two decades are unlikely to persist. The environmental impact of aircraft noise is projected to remain roughly constant in the United States for the next several years and then increase as air travel growth outpaces expected technological and operational advancements (Waitz et al., 2004). Furthermore, the public currently reports considerable annoyance even when DNLs are below 65 dB. Regulatory actions to limit or reduce noise exposure will likely lead to even more stringent limits. Future abatement efforts may need to reduce allowable noise levels to as low as 55 dB DNL in both the United States (NASA, 2003) and Europe (ACARE, 2001). Meeting future noise targets will be extremely challenging and will require continued fundamental research in noise phenomena and advanced propulsion technologies. The development of validated, physics-based noise prediction tools by NASA will greatly aid the development of quieter engines. Research is needed to reduce the noise of engine systems, including fan noise, jet noise, and core noise. Research should also encompass systems analysis; advanced concepts, such as adaptable chevrons; the community impact of aircraft noise; and improved metrics to quantify and mitigate these impacts. B1b Ultraclean gas turbine combustors to reduce gaseous and particulate emissions in all flight segments Emissions from aircraft constrain the growth of aviation due to their environmental impacts and potential human health consequences. For example, airports located in air quality nonattainment or maintenance areas increasingly find that air emissions add to the complexity, length, and uncertainty of the environmental review and approval of expansion projects (Akin et al., 2003). Key pollutants of concern include oxides of nitrogen and sulfur (NOx and SOx), carbon monoxide (CO), unburned hydrocarbons (UHCs), hazardous air pollutants, and particulate matter (PM). In addition, emissions of CO2 and water vapor (H2O) in the upper troposphere and stratosphere are of concern because of their potential impact on Earth’s climate (IPCC, 1999). Both CO2 and H2O are inherent combustion products of hydrocarbon fuels, and their emissions can only be reduced through improvements in overall cycle efficiency (see R&T Challenge B4)—or a change in fuels. Emissions of NOx, CO, UHC, and PM from the combustor can be reduced through the development of ultraclean combustion approaches, a critical step to mitigate the environmental impacts of aviation. Low NOx emissions can be achieved with both rich- and lean-burning combustor designs. Lean combustion concepts have received substantial market penetration through their widespread implementation in land-based gas turbine applications over the last two decades. The key technical issues associated with these combustors concern unsteady combustion phenomena, including combustion instability, flame blow-off, flashback, and autoignition. Although combustors run lean overall, the majority of commercial aircraft engines run rich in the front end. The key issues associated with them are PM emissions and quench zone mixing (Lefebvre, 1999). B3 Intelligent engines and mechanical power systems capable of self-diagnosis and reconfiguration between shop visits In the future, advances in sensing, control, and information technology will lead to engines that are more sophisticated and more intelligent. Research thrusts should investigate how more intelligent systems can (1) improve engine health diagnostics and remedial actions in flight, (2) optimize the mission, and (3) use flight data to improve maintenance on the ground. For current engines, the focus will be very much on diagnostics. Better physics-based modeling will be essential. Development of better computational fluid dynamics (CFD) tools, life-prediction tools, and steady-state

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Decadal Survey of Civil Aeronautics: Foundation for the Future and dynamic performance checks will be keys to success. Reducing in-flight shutdowns by a factor of 3 and unscheduled engines removals and delays and cancellations by a factor of 5 should be achievable and would reduce maintenance costs by 50 percent. Requirements include (1) smaller sensors with better response and higher operating temperatures and (2) better materials with narrower property tolerances. This should increase disk and airfoil life by 50 percent. Intelligent engine development will include active control of many engine components: combustor control to permit operation with leaner burners, leading to lower NOx emissions; compressor active stall control to allow operation at higher pressure ratios, leading to higher fuel efficiency; and closed-loop clearance control to increase turbine efficiencies and extend on-wing life by 3 years. B4 Improved propulsion system fuel economy The fuel economy of gas turbine propulsion systems is a function of engine efficiency, propulsion-induced drag, and propulsion weight. Overall engine efficiency is the product of the efficiency of creating hot, high-pressure gases (thermal or cycle efficiency), the efficiency of transferring energy from the hot high-pressure gases to a more desirable form (transfer efficiency), and the efficiency of creating thrust from the engine fan and core flows (propulsion efficiency). The thermal efficiency for a gas turbine (Brayton cycle) is primarily a function of overall engine pressure ratio. That is, as long as the turbine can tolerate the inlet temperature corresponding to a given pressure ratio, the overall pressure ratio sets the efficiency of the cycle. Figure 3-4 illustrates very clearly that state-of-the-art gas turbines have not reached the theoretical limits of thermal efficiency. The technologies identified in the figure have the potential to improve the thermal efficiency of gas turbines, to significantly increase fuel economy, and to decrease the environmental impact of the air transportation system. Transfer efficiency is determined by the component efficiencies of the fan and low-pressure turbine and the losses of the shaft bearings. High-efficiency, low-pressure turbines need high rotor speeds, but highly efficient fans require low rotor speeds. Therefore, engines with high transfer efficiency must have reduction gearboxes or other technologies that permit different rotor speeds for the fan and low-pressure turbine. Propulsion efficiency is a function of the difference between the velocity of engine exhaust and the forward velocity of the aircraft. Increasing the mass flow of air through the system at slower speed improves propulsion efficiency and decreases noise. However, this increases the diameter of the engine, which increases friction and flow blockage. Since larger engines will also be heavier, the use of composites or other lightweight materials for construction of the large structural pieces of the turbofan will also be necessary. As shown in Figure 3-4, improving thermal efficiency by 15 percent requires advances in several technologies: 3D aerodynamics, active flow control, cooled cooling air and a thermal management system, multiwalled cooling, and ceramic matrix composites (CMCs) and intermetallics. Over the long term, advances in all three efficiencies (thermal, transfer, and propulsion) should be able to improve fuel economy by 30 percent relative to the GE-90 for large commercial engines and 30 percent relative to T700/CT7 for small engines. B5 Propulsion systems for short takeoff and vertical lift The utilization of V/STOL airplanes and increased use of helicopters could greatly increase the capacity of civil aviation by allowing more takeoffs and landings at existing airports without increasing demand for runway usage (NRC, 2003). V/STOL airplanes include tilt-wing aircraft, tilt-rotor aircraft, vertical-lift fan aircraft, and blown-wing aircraft. Currently, the fuel economy of V/STOL propulsion systems is not on par with that of fixed-wing commercial airplanes. Propulsion systems for all new aircraft must also demonstrate extremely high levels of reliability. Propulsion systems for V/STOL aircraft are in an early state of development or do not exist for civil aircraft. In addition, engine-out strategies need to be developed and verified for certification. This Challenge should support development of V/STOL and helicopter propulsion systems with fuel economy comparable to future small commercial aircraft—namely, 20 percent better than the CT7 family of engines that are currently in production for small conventional aircraft. Many of the same technologies that apply to large and small engines for conventional aircraft also apply to V/STOL propulsions systems. However, additional technologies such as high-efficiency, angled gearboxes; high-efficiency reduction gear-boxes; large-bleed systems; thrust vectoring systems; noise reduction both inside and outside the aircraft; fan-tip-driven turbines; and high-power clutch systems will be required to put V/STOL airplanes into affordable, large-scale commercial service with minimal environmental impact. There are three major technology efforts to be undertaken in support of V/STOL airplanes for civil aviation. The first and most important is to demonstrate an engine sized for most helicopters or for UAVs (roughly 3,000-shaft horsepower) that meets the fuel economy goals. The important characteristics of this demonstration engine are to achieve overall pressure ratios of 25:1 or 30:1 and turbine inlet temperatures of 2800°F. This will require some combination of the following technologies: (1) new compressor disk materials, (2) greatly improved turbine cooling configurations, (3) new turbine blade alloys and coatings, (4) component aerodynamics designed with the latest computational models, and (5) highly effective, low-pressure-drop dirt separation. Such an engine would benefit helicopters as well. Second, the powertrain system of most V/STOL airplanes (as well as helicopters) will consist of shafting with speed reduction gearboxes, angled gearboxes, and perhaps

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Decadal Survey of Civil Aeronautics: Foundation for the Future TABLE 3-2 Prioritization of R&T Challenges for Area B: Propulsion and Power       Strategic Objective National Priority Why NASA? NASA Priority Score     Capacity Safety and Reliability Efficiency and Performance Energy and the Environment Synergies with Security Support to Space Supporting Infrastructure Mission Alignment Lack of Alternative Sponsors Appropriate Level of Risk Why NASA Composite Score R&T Challenge Weight 5 3 1 ¼ each B1a Quiet propulsion systems 9 1 3 9 3 1 90 3 9 3 9 6.0 540 B1b Ultraclean gas turbine combustors to reduce gaseous and particulate emissions in all flight segments 9 1 3 9 3 1 90 3 9 3 9 6.0 540 B3 Intelligent engines and mechanical power systems capable of self-diagnosis and reconfiguration between shop visits 3 9 3 3 3 1 82 3 9 3 9 6.0 492 B4 Improved propulsion system fuel economy 3 1 9 9 3 1 78 3 9 3 9 6.0 468 B5 Propulsion systems for short takeoff and vertical lift 9 1 3 3 3 1 72 3 9 3 9 6.0 432 B6a Variable-cycle engines to expand the operating envelope 3 1 9 3 3 9 68 3 9 3 9 6.0 408 B6b Integrated power and thermal management systems 3 1 9 3 3 9 68 3 9 3 9 6.0 408 B8 Propulsion systems for supersonic flight 3 1 3 1 9 9 50 9 9 3 9 7.5 375 B9 High-reliability, high-performance, and high-power-density aircraft electric power systems 1 3 9 3 3 3 62 1 9 3 9 5.5 341 B10 Combined-cycle hypersonic propulsion systems with mode transition 1 1 3 1 9 9 40 9 9 3 9 7.5 300 B11 Alternative fuels and additives for propulsion that could broaden fuel sources and/or lessen environmental impact 3 1 3 9 3 1 60 3 3 3 9 4.5 270 B12 Hypersonic hydrocarbon-fueled scramjet 1 1 3 1 9 9 40 9 3 3 9 6.0 240 B13 Improved propulsion system tolerance to weather, inlet distortion, wake ingestion, bird strike, and foreign object damage 3 9 3 1 3 1 76 3 3 3 3 3.0 228 B14 Propulsion approaches employing specific planetary atmospheres in thrust-producing chemical reactions 1 1 1 1 1 9 26 3 9 9 9 7.5 195 B15 Environmentally benign propulsion systems, structural components, and chemicals 1 1 1 9 3 1 44 3 3 3 3 3.0 132 B16 Reduced engine manufacturing and maintenance costs 3 3 3 3 3 1 52 3 1 1 3 2.0 104

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Decadal Survey of Civil Aeronautics: Foundation for the Future FIGURE 3-2 NASA and national priorities for Area B: propulsion and power. FIGURE 3-3 Actual and predicted exposure to significant noise (65-dB day-night average sound level) and enplanement trends for the United States, 1975-2005. SOURCE: C. Burleson, FAA, “Aviation environmental challenges,” Presentation to Panel B, December 13, 2005.

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Decadal Survey of Civil Aeronautics: Foundation for the Future Equivalent visual operations, which will allow pilots and controllers to see the same picture, enabling controllers to delegate some tasks to pilots. Super-density operations, which will use advanced capabilities, including detection and avoidance of hazardous wake vortices, to enable closely spaced and converging approaches in the air as well as more efficient airport ground operations. This study did not assess and does not necessarily endorse the above set of capabilities. However, many of the capabilities would be supported by R&T Challenges in this Area. These Challenges also encompass the basic and applied research necessary to establish a proper balance between automated and human-centric system configurations and operational concepts in the air transportation system. The scope of research in this Area includes new NASA R&T Thrusts as well as the expansion of existing technology programs. One Area of particular near-term interest is the incorporation of autonomous and semiautonomous aircraft into the national (and global) air transportation system. The QFD process described in Chapter 2 was used to prioritize 20 R&T Challenges related to intelligent and autonomous systems, operations and decision making, human integrated systems, and networking and communications. Table 3-5 and Figure 3-7 show the results. The text that follows describes the 10 R&T Challenges that ranked highest in terms of NASA priority, the general characteristics of high-and low-priority Challenges, and the R&T Thrusts in this Area. Further details on all Challenges, including the rationale for scoring, are found in Appendix E. As shown in Table 3-5, many of the Challenges in this Area ranked high because they would enhance the performance of the air transportation system as a whole, bringing about noteworthy improvements related to many of the air transportation system strategic objectives (capacity, safety and reliability, etc.). As shown Figure 3-7, the top 10 Challenges fall into three groups: R&T Challenge E1 stands alone, with a NASA priority score of 936. R&T Challenges E2 and E3 stand together with scores of 780 and 744. R&T Challenges E4 to E8c stand together with scores of 624 to 576. The difference in scores and rankings of the R&T Challenges in each of the last two groups is not significant. FIGURE 3-7 NASA and national priorities for Area E: intelligent and autonomous systems, operations and decision making, human integrated systems, and networking and communications

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Decadal Survey of Civil Aeronautics: Foundation for the Future TABLE 3-5 Prioritization of R&T Challenges for Area E: Intelligent and Autonomous Systems, Operations and Decision Making, Human Integrated Systems, and Networking and Communications       Strategic Objective National Priority Why NASA? NASA Priority Score       Capacity Safety and Reliability Efficiency and Performance Energy and the Environment Synergies with Security Support to Space Supporting Infrastructure Mission Alignment Lack of Alternative Sponsors Appropriate Level of Risk Why NASA Composite Score R&T Challenge Weight 5 3 1 1/4 each E1 Methodologies, tools, and simulation and modeling capabilities to design and evaluate complex interactive systems 9 9 9 9 9 3 156 3 9 3 9 6.0 936 E2 New concepts and methods of separating, spacing, and sequencing aircraft 9 9 9 3 3 1 130 3 9 3 9 6.0 780 E3 Appropriate roles of humans and automated systems for separation assurance, including the feasibility and merits of highly automated separation assurance systems 9 9 9 1 3 1 124 3 9 3 9 6.0 744 E4 Affordable new sensors, system technologies, and procedures to improve the prediction and measurement of wake turbulence 9 9 3 1 1 1 104 3 9 3 9 6.0 624 E5 Interfaces that ensure effective information sharing and coordination among ground-based and airborne human and machine agents 3 9 9 1 9 3 102 3 9 3 9 6.0 612 E6 Vulnerability analysis as an integral element in the architecture design and simulations of the air transportation system 3 9 9 1 9 1 100 3 9 3 9 6.0 600 E7 Adaptive ATM techniques to minimize the impact of weather by taking better advantage of improved probabilistic forecasts 9 3 9 3 1 1 98 3 9 3 9 6.0 588 E8a Transparent and collaborative decision support systems 3 9 9 1 3 3 96 3 9 3 9 6.0 576 E8b Using operational and maintenance data to assess leading indicators of safety 3 9 9 1 3 3 96 3 9 3 9 6.0 576 E8c Interfaces and procedures that support human operators in effective task and attention management 3 9 9 1 3 3 96 3 9 3 9 6.0 576 E11 Automated systems and dynamic strategies to facilitate allocation of airspace and airport resources 9 3 9 3 3 1 100 3 9 1 9 5.5 550 E12 Autonomous flight monitoring of manned and unmanned aircraft 3 9 3 1 9 1 82 3 9 3 9 6.0 492 E13 Feasibility of deploying an affordable broad-area, precision-navigation capability compatible with international standards 9 9 3 1 3 1 106 3 3 3 9 4.5 477 E14 Advanced spacecraft weather imagery and aircraft data for more accurate forecasts 3 3 9 3 1 1 68 3 9 3 9 6.0 408 E15 Technologies to enable refuse-to-crash and emergency autoland systems 1 9 1 1 3 1 60 3 9 3 9 6.0 360 E16 Appropriate metrics to facilitate analysis and design of the current and future air transportation system and operating concepts 3 3 9 3 3 1 70 3 9 3 3 4.5 315 E17 Change management techniques applicable to the U.S. air transportation system 9 9 9 1 3 1 124 1 3 3 3 2.5 310 E18 Certifiable information-sharing protocols that enable exchange of contextual information and coordination of intent and activity among automated systems 3 1 9 1 9 1 60 3 9 3 3 4.5 270 E19 Provably correct protocols for fault-tolerant aviation communications systems 3 9 3 1 3 1 76 3 3 1 1 2.0 152 E20 Comprehensive models and standards for designing and certifying aviation networking and communications systems 3 9 3 1 1 1 74 3 3 1 1 2.0 148

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Decadal Survey of Civil Aeronautics: Foundation for the Future Top 10 R&T Challenges E1 Methodologies, tools, and simulation and modeling capabilities to design and evaluate complex interactive systems The U.S. air transportation system is a complex interactive system whose behavior is difficult to simulate with currently available models. Methodologies, tools, and simulation and modeling capabilities suited for the design and integration of complex interactive systems are needed to understand the air transportation system as an integrated, adaptive, distributed system that includes aircraft, ATM facilities, and airports, each with its own complex systems, all of which interact with one another, the environment, and human operators. Simulations and models for complex interactive systems are needed to accurately estimate system performance, to properly allocate resources, and to select appropriate design parameters. Additionally, the large number of possible future system designs requires models that can be reconfigured to model a wide range of design parameters. E2 New concepts and methods of separating, spacing, and sequencing aircraft Expected growth in the demand for air transportation will require efficient, denser en route and terminal area operations. This necessitates procedures that reduce minimum spacing requirements during all phases of flight and in all weather conditions, through an integrated approach that leverages a suite of emerging technologies such as required navigation performance and automatic dependent surveillance broadcast (ADS-B). The objective of this Challenge is to efficiently accommodate a large number and wide range of aircraft, including UAVs, through spacing and sequencing based on aircraft type and equipment rather than a common worst-case standard. Several concepts of operation should be systematically compared in terms of their technological, business, and human factors issues as well as their impact on capacity, safety, and the environment. This Challenge will study reduced separation operations within the context of existing ATM protocols and revolutionary paradigms that could significantly increase capacity, although the latter would involve a much more complicated transition process. Integration of UAVs into the air transportation system will require procedures that can safely manage aircraft with diverse performance characteristics and highly automated onboard flight management systems (Sabatini, 2006). Safe, high-capacity operations in a complex future airspace environment will require fundamental research into alternative ATM paradigms such as simultaneous noninterfering operations (Xue and Atkins, 2006) in which general aviation, rotorcraft, and UAV traffic are threaded through airspace unused by commercial air traffic. As onboard automation and cooperative control algorithms are matured (McLain and Beard, 2005), UAV traffic might also be efficiently managed using formations of UAVs that are coordinated locally but treated as a single entity by air traffic controllers and pilots of nearby aircraft. E3 Appropriate roles of humans and automated systems for separation assurance, including the feasibility and merits of highly automated separation assurance systems Air traffic control is currently a labor-intensive process. FAA controllers—aided by radar, weather displays, and procedures—maintain traffic flow and assure separation by communicating instructions to aircraft in their sector of responsibility. Limitations to this traditional paradigm are, in some areas, constraining the capacity of the air transportation system. For example, the FAA required airlines serving the Chicago O’Hare airport to reduce some of their flights during 2005 because of congestion-related delays. A recent study of en route sector congestion suggested that capacity could be increased by a factor of two or more while maintaining existing spacing, by developing new systems that merge human and computer decision making and automate time-critical separation assurance tasks (Andrews et al., 2005). Initiatives to reduce aircraft separation by providing automated advisories to air traffic controllers and flight crews have not lived up to expectations, because of controller workload concerns, institutional resistance, and other factors. The advent of UAVs has caused additional concern because it may not be feasible for UAVs with human-in-the-loop collision avoidance schemes to act in time to prevent midair collisions. This has led to interest in determining whether automating aircraft separation, whereby the controller is neither in the loop nor responsible for separation, is feasible and desirable. However, changing the role of the controller from tactical separation to traffic flow management and trusting automated systems to manage the tactical separation of aircraft would require resolution of major human factors, safety, and institutional issues (Wickens et al., 1998; Woods and Hollnagel, 2006). Collisions could occur if a UAV fails to respond or the automated traffic separation system fails and if human intervention is not effective. This Challenge would determine the appropriate roles of humans and automated systems to assure separation in high-density airspace during nominal and off-nominal operations. As part of this challenge, NASA should assess the feasibility and merits of highly automated separation assurance systems. E4 Affordable new sensors, system technologies, and procedures to improve the prediction and measurement of wake turbulence Existing wake vortex separation standards reduce system capacity during takeoff and landing operations and instru-

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Decadal Survey of Civil Aeronautics: Foundation for the Future ment approaches. Encounters with a wake vortex are also a growing concern in en route Reduced Vertical Separation Minima (RVSM) airspace (Reynolds and Hansman, 2001).12 Current research by the FAA and NASA is focused on procedural enhancements that take advantage of wake transport by winds (Mundra, 2001). For example, the capacity of San Francisco International Airport is expected to improve by using this approach to enable arrivals on both closely spaced parallel runways during low-visibility weather. However, the relaxation of in-trail wake separation standards awaits improved measurement and prediction of wake behavior. Existing sensors and models do not adequately characterize wake decay phenomena, especially at typical final approach altitudes. Improved sensors, including coherent pulsed lidars, capable of directly measuring wake rotational momentum, are needed to support phenomenological studies and enable more accurate predictions of wake magnitude and decay in various atmospheric conditions. Those predictions, combined with models of aircraft upset risk, should allow reduced wake separation standards without degrading safety. R&T Challenge A10 will conduct research to improve techniques for predicting and measuring the formation, trajectory, and decay of vortices, including methods to accurately predict wingtip vortex formation and define changes in aircraft design to mitigate the strength of the vortices. This Challenge would complement that work by developing affordable new sensors, system technologies, and procedures to improve prediction and measurement of wake strength, location, motion, and aircraft upset risk in terminal and en route airspace. Together, Challenges A10 and E4 will enable safe flight with reduced in-trail wake separation. E5 Interfaces that ensure effective information sharing and coordination among ground-based and airborne human and machine agents The potential for sharing a wide range of information within the air transportation system raises additional questions about how multiple agents (pilots, controllers, other system users, and automated system elements) can coordinate and share information given their disparate viewpoints and contexts. For information sharing to be effective, information must be provided to the right agents, at the right time, and in a fashion that facilitates accurate interpretation regardless of the source of the information. Some of the shared information may be factual (e.g., aircraft position, speed, heading, altitude, and flight plan), while some of it may be less tangible (e.g., potential responses to disruptions). The information elements will also likely vary in their timeliness and accuracy, and access to some information will be restricted for security and business reasons. Developing appropriate interfaces (in terms of information-sharing protocols, as well as display and visualization technology) is a nontrivial challenge, because agents can be easily overwhelmed by too much information or by the need to translate and analyze the information relative to their own situation and goals (Woods et al., 2002). Interfaces for human agents, in particular, will need to include methods for visualizing and interpreting operational situations to facilitate effective judgments and decisions. In addition, information sharing and decision-making processes will often be conducted collaboratively by multiple agents. Therefore, they will require knowledge of both individual human cognition and of collaborative work among agents with potentially conflicting goals and different representations of the immediate situation (Brennan, 1998; Olson et al., 2001). Information-sharing protocols become exceptionally critical during crises, such as 9/11, when control of the national airspace was transferred to the military. Communications and decision-making protocols were fragmented. Research related to this Challenge must be coordinated with DoD and DHS to avoid a recurrence of such problems. The Challenge should also capitalize on technologies pioneered in the telecommunications industry that would facilitate the transfer of diverse information through dynamically reconfigured networks using thousands of disparate nodes. E6 Vulnerability analysis as an integral element in the architecture design and simulations of the air transportation system More than three-fourths of air transportation system delays are weather related (Meyer, 2005). Snow or thunder-storms at major hub airports often significantly reduce overall system capacity and efficiency. Abnormal en route winds cause unexpected peaking and depeaking at arrival gateways. En route convective weather causes disruptive and unpredictable rerouting, precipitating en route delays and reducing capacity and efficiency. Disruptions can also be caused by natural disasters (such as volcanoes, hurricanes, tornadoes, and wildfires), electronic attacks (such as power outages, hurricanes, GPS spoofing, spurious communication messages, and hacking into navigation aids), and physical attacks (such as destruction of control facilities and radars). The effects of these disruptions may be local, regional, or national. In all cases, system capacity and efficiency are directly affected, and, more important, the safety of the air transportation system may be compromised by an inadequate response. Airlines use a variety of techniques to respond to such disruptions. Some reduce schedule to preposition aircraft for the recovery, when the weather abates; others try to fly their full schedule, hoping that the recovery will take care of itself. 12 Reduced Vertical Separation Minima apply to the airspace from flight levels 290 to 410 (which is equivalent to altitudes of approximately 29,000 feet to 41,000 feet) and create twice as many usable flight levels, decreasing the vertical separation between aircraft from 2,000 feet to 1,000 feet. While increasing capacity, this also could exacerbate the effects of wake turbulence.

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Decadal Survey of Civil Aeronautics: Foundation for the Future Assessing vulnerabilities and risks should be the first step in reducing the likelihood and consequences of unplanned system disruptions (Volpe, 2003, p. 4). System safety impacts of disruptions should be evaluated early in the development cycle of new ATM system architectures, operating concepts, and system components. An agile ATM design should include provisions to counter or recover from system disruptions, and the design of the overall air transportation system should be evaluated by research and simulation to develop both system design concepts and/or operational procedures. In addition, quantitative analyses should be used to assess the safety impact of system architecture options. This Challenge would introduce vulnerability analyses as an integral element in the architecture design and simulations of the air transportation system to reduce the likelihood that the system will experience major system disruptions, to mitigate the severity of specific system disruptions, and to facilitate recovery from system disruptions. The result would be an air transportation system that is self-diagnosing and self-healing. E7 Adaptive ATM techniques to minimize the impact of weather by taking better advantage of improved probabilistic forecasts Adaptive traffic flow management methods are needed to take advantage of recent improvements in automated aviation weather forecasts. About 70 percent of aviation delay is due to operationally significant weather, including thunder-storms, low ceilings and visibilities, high winds, and turbulence. Exploitation of weather data collected from ground sensors and satellites using advanced image processing and machine intelligence has enabled significant improvements in aviation weather forecasts. One- to two-hour storm motion products are now being routinely displayed in key airport and en route air traffic facilities and in airline dispatch centers. Included are automatically updated estimates of the forecast accuracy, expressed as a probability (Robinson et al., 2004). This information is beginning to be used by air traffic managers and dispatchers, but only manually (Wolfson et al., 2004). Algorithms are needed that automatically translate the weather forecasts into actionable traffic flow recommendations, with the goal of fully incorporating the weather data into air traffic automation designs. A few examples of automation that translate probabilistic weather forecasts into traffic flow recommendations have been developed, and FAA air traffic managers have shown they can reduce delays. For example, the LaGuardia Airport traffic flow managers are using storm motion forecast tools, such as the Route Advisory Planning Tool, to automatically identify safe departure routes (Evans, 2006). However, many automation systems are not incorporating the new weather information into their designs. This Challenge would demonstrate the use of automated weather forecasts in making traffic flow decisions and determine where this capability is cost beneficial. E8a Transparent and collaborative decision support systems Air traffic operations are enhanced by effective decision support systems that assist pilots, controllers, traffic flow managers, and airline personnel in tasks such as routing, flight planning, scheduling, and traffic separation. These decision support systems contribute to safe and efficient operations by using technology to enhance human capabilities and collaborate with the operator, as opposed to fully automated systems, which use technology rather than an operator to perform tasks. Collaborative decision support systems are most effective when the operators understand the basis for and limitations in the system’s reasoning process and can judge the appropriateness of system-generated recommendations. Similarly, the system’s recommendations should take into account operators’ knowledge and intentions as well as the context in which they operate. Support for reciprocal information sharing and mutual understanding of intentions and actions—a process called grounding—is critical to avoid breakdowns in human–machine collaboration and overall system performance (Sorkin et al., 1988; Lee and Moray, 1994; Smith et al., 2001; McGuirl and Sarter, 2006). This Challenge will identify the type of information to be shared between human operators and automated decision support systems and develop candidate designs for these systems. E8b Using operational and maintenance data to assess leading indicators of safety Safety analysis is often a reactive, ad hoc process made difficult, in part, by the very high level of safety required of air transportation in the United States. Few unambiguous data points (accidents) are available for analysis, the number of data points continues to decrease because of the success of ongoing safety efforts, and accidents that do occur are increasingly the result of a complex chain of unlikely circumstances, each of them benign (Leiden et al., 2001). While human error is often cited as a major safety concern, successful human performance is also a major (and under-reported) contributor to system safety. Thus, a particular concern for safety analysis is the human contribution to safety, especially when predicting the safety impact of dramatic changes to the role of human operators and increased reliance on automation. Likewise, safety analysis must consider individual aircraft as well as systemwide safety, which involves complex interactions among many agents. Using a common set of safety metrics (see R&T Challenge E16), this Challenge would develop methods both for monitoring the current system through ongoing analysis of operational and

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Decadal Survey of Civil Aeronautics: Foundation for the Future maintenance data and for predicting potential safety problems associated with proposed changes to the air transportation system. E8c Interfaces and procedures that support human operators in effective task and attention management The expected growth in air transportation demand will likely require operators to perform a wider range of tasks and to collaborate more closely with one another and with modern technologies. Pilots may begin to play a more active role in traffic separation or spacing and will need to coordinate their activities and intentions with other pilots and controllers. They will need to interact and exchange information, often interrupting each other and creating new tasks for one another. In general, more information will need to be distributed in a timely manner, task sets will increase, interruptions will become more likely, and the tolerance for delayed action or intervention will probably be reduced. It will be critical to ensure that operators are supported in properly scheduling and prioritizing their tasks, to improve attention management and avoid errors caused by unnecessary task switching, unnecessary interruptions, or inappropriate dismissals of demands (i.e., the failure to switch attention when appropriate and necessary) (Woods, 1995; McFarlane and Latorella, 2002; Ho et al., 2004). High-Priority R&T Challenges and Their Associated Thrusts Only one R&T Challenge in this Area (E1) had major relevance and impact on the energy and environment Strategic Objective. The other nine high-priority R&T Challenges in this Area each had major relevance and impact on at least two of the other three highly weighted strategic objectives (capacity, safety and reliability, and efficiency and performance). In some cases, the R&T Challenges in this Area would have direct operational impact, for example, by developing new concepts and methods of separating, spacing, and sequencing aircraft to increase capacity and safety in all flight conditions (E2). In other cases, the benefits would be less direct, for example, by developing more capable methodologies, tools, and simulation and modeling capabilities to design and evaluate complex interactive systems (E1) and determining appropriate roles of humans and automated systems for separation assurance in high-density airspace during nominal and off-nominal operations (E3). Although the results of research in these areas would take longer to produce operational benefits, the research is essential, it is appropriate for NASA to include the research in a 10-year plan, and NASA involvement in the research is necessary to ensure that this research moves forward and can be readily applied to the air transportation system. The ultimate objective of NASA’s aeronautics research as it relates to intelligent and autonomous systems, operations and decision making, human integrated systems, and networking and communications is to provide the fundamental capabilities required for an adaptive and robust air transportation system that meets the nation’s goals for economic growth, public well-being, and national security. Because the air transportation system is a complex interactive system, the linkages among its component systems are just as important as the component systems themselves. The committee identified four R&T Thrusts that describe threads of commonality among the Challenges in the intelligent and autonomous systems, operations and decision making, human integrated systems, and networking and communications Area: Decision making, negotiation, collaboration, information sharing, and allocation of airspace resources. Aircraft separation, spacing, and sequencing. Simulation, modeling, and analysis of complex, adaptive distributed systems. Wake and weather sensing, modeling and prediction, and other enabling air transportation technologies. Each of these thrusts is discussed below. As shown in Figure 3-8, the first two Thrusts would lead directly to improvements in air transportation system operations, and the last two Thrusts would provide enabling technologies and capabilities that support the first two. Decision making, negotiation, collaboration, information sharing, and allocation of airspace resources Mechanisms must be constructed to facilitate and structure the interactions of all air transportation system components—businesses, organizations, individual humans, technologies—such that the emergent system performance is adequate. To do so requires foundational research into several topics, including the appropriate roles of automation; methods of supporting effective decision making and task management by individual humans, by automated systems, and by humans and automation working together; and information sharing, negotiation, and coordination within and between organizations. One outcome of these functions of particular importance to the design of the Next Generation Air Transportation System is allocation of airspace and airport resources, insofar as the method used to allocate these resources can determine how demand relates to capacity and whether airports and flights experience outcomes such as delays or denial of service. The following R&T Challenges are most closely related to this Thrust: E5, Interfaces that ensure effective information sharing and coordination among ground-based and airborne human and machine agents.

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Decadal Survey of Civil Aeronautics: Foundation for the Future FIGURE 3-8 R&T Thrusts related to Area E: intelligent and autonomous systems, operations and decision making, human integrated systems, and networking and communications. E8a, Transparent and collaborative decision support systems. E8c, Interfaces and procedures that support human operators in effective task and attention management. E11, Automated systems and dynamic strategies to facilitate allocation of airspace and airport resources. E18, Certifiable information-sharing protocols that enable exchange of contextual information and coordination of intent and activity among automated systems. Aircraft separation, spacing, and sequencing The high-density airspace of the future will require effective management of aircraft separation, spacing, and sequencing in all flight conditions. The future air transportation system must develop improved models and novel operational concepts to support capacity growth and accommodate scheduled and unscheduled operations in airspace shared by manned and unmanned aircraft without compromising safety. Of particular importance to this research are separation assurance methods and understanding the appropriate roles of humans and automation in high-capacity air-space. Research in several R&T Challenges is needed to provide critical inputs, including accurate wake and weather forecasting as well as flight monitoring capabilities. The following Challenges are most closely related to this Thrust: E2, New concepts and methods of separating, spacing, and sequencing aircraft. E3, Appropriate roles of humans and automated systems for separation assurance, including the feasibility and merits of highly automated separation assurance systems. Simulation, modeling, and analysis of complex, adaptive distributed systems Design of the Next Generation Air Transportation System is a tremendous engineering challenge. This network of safety-critical, complex interactive systems will be vast in scope and involve multiple disparate organizations with separate objectives and capabilities. Examining the challenges facing this development, the committee found that individual technologies and systems will contribute to system performance only indirectly through their influence on how the larger system is collectively operated by the many user organizations; thus, system operations must also be a focus of research and development. Understanding the complexities of these operations requires new design tools and methodologies. Metrics are also important, because system performance metrics have a direct impact on the design of the system: Parameters that are not measured—or are measured incorrectly or incompletely—will not be fully considered or accounted for in the final design. Thus, it is critically important that the appropriate metrics be identified and incorporated into system analysis and design tools and processes. However, there is no comprehensive, widely held set of metrics to analyze and design the current and future air transportation system. Because many issues (e.g., economic, efficiency, safety, environment) must be addressed simultaneously, the problem cannot be decomposed into isolated examinations of capacity, safety, technology, human factors, etc. New simulation tools are needed with extensive predictive capabilities. Likewise, new analysis methods are required to integrate safety and vulnerability assessments into

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Decadal Survey of Civil Aeronautics: Foundation for the Future the design process. Coordinating all of these insights requires understanding of complex, adaptive distributed systems with the unique characteristics of air transportation, including the need to simulate and predict the behavior of radically different system configurations. The following R&T Challenges are most closely related to this Thrust: E1, Methodologies, tools, and simulation and modeling capabilities to design and evaluate complex interactive systems. E6, Vulnerability analysis as an integral element in the architecture design and simulations of the air transportation system. E8b, Using operational and maintenance data to assess leading indicators of safety. E16, Appropriate metrics to facilitate analysis and design of the current and future air transportation system and operating concepts. E17, Change management techniques applicable to the U.S. air transportation system. E20, Comprehensive models and standards for designing and certifying aviation networking and communications systems. Wake and weather sensing, modeling and prediction, and other enabling air transportation technologies Several critical technologies warrant fundamental research by NASA for their likely value as enablers of many possible operational concepts, and because some are still in a nascent state. Of particular importance are the sensing, modeling, and prediction of aircraft wakes and hazardous weather. Other enabling technologies center on the creation of enhanced automation capabilities for safety (e.g., “refuse-to-crash”) and for capacity (e.g., agents for negotiating resource allocation) beyond those that the research community currently knows how to develop and certify as robust in a wide range of conditions. Additional technologies focus on further enhancements to the communication and navigation needs of air traffic management functions. The following R&T Challenges are most closely related to this Thrust: E4, Affordable new sensors, system technologies, and procedures to improve the prediction and measurement of wake turbulence. E7, Adaptive ATM techniques to minimize the impact of weather by taking better advantage of improved probabilistic forecasts. E12, Autonomous flight monitoring of manned and unmanned aircraft. E13, Feasibility of deploying an affordable broad-area, precision-navigation capability compatible with international standards. E14, Advanced spacecraft weather imagery and aircraft data for more accurate forecasts. E15, Technologies to enable refuse-to-crash and emergency autoland systems. E19, Provably correct protocols for fault-tolerant aviation communications systems. Low-Priority R&T Challenges Seven of the 10 R&T Challenges that did not rank in the top 10 by NASA priority also ranked low in national priority. Which is to say, they had substantial impact on only one of the highly weighted strategic objectives (capacity, safety and reliability, efficiency and performance, or energy and the environment). The other three R&T Challenges (E11, E13, and E17) would have substantial impact on two of the highly weighted strategic objectives, but they ranked low in terms of NASA priority because of low Why NASA? scores. Because these Challenges rank high in national priority, it is important that some part of the national civil aeronautics R&T effort (by NASA, other government agencies, industry, or academia) support work to overcome them. E17 Change management techniques applicable to the U.S. air transportation system The current ATM airspace architecture and associated procedures are antiquated and so reliant on interim fixes that there is significant resistance to additional changes. A novel and consistent approach is needed to manage changes to the ATM system and to overcome barriers and organizational inertia within the FAA and other stakeholders. The FAA should support research related to this Challenge, though it may take external pressure (e.g., from the Office of Management and Budget, the Government Accountability Office, and the White House) to prompt action. It may be appropriate for NASA to also conduct research related to this Challenge, but NASA would first need to increase its relevant capabilities. E11 Automated systems and dynamic strategies to facilitate allocation of airspace and airport resources The current allocation of airspace and airport resources (e.g., airport departure and arrival slots) at major airports is heavily biased toward airline operations. The competition for airspace and airport resources will be exacerbated by growth in commercial and private air travel, including the introduction of very light jets. Future air transportation systems would benefit from built-in automatic response systems driven by software agents that quickly negotiate and make decisions regarding, for example, real-time allocation of airspace and landing slots among aircraft with diverse size and performance characteristics, while considering the needs of all stakeholders, air transportation system efficiency, and energy conservation (Cramton et al., 2002). This Challenge ranked low in NASA priority because the FAA already is

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Decadal Survey of Civil Aeronautics: Foundation for the Future funding some related research and is best suited to address this Challenge. E13 Feasibility of deploying an affordable broad-area, precision-navigation capability compatible with international standards Many small airports cannot operate when visibility is restricted because they do not have the equipment (e.g., a Category I, Category II, or Category III instrument landing system) necessary for a safe approach and landing.13 The number of ILS frequencies available in large metropolitan areas, where multiple runways require precision approach and landing capabilities, is limited. Increased access to these airports and runways would increase efficiency significantly, particularly for some segments of the aviation industry (e.g., feeder airlines, business aircraft, and air taxis). While satellite navigation systems are currently deployed by the United States and others, they do not provide sufficient coverage, accuracy, and reliability for aviation requirements to replace or substitute for ground-based aids, particularly as regards precision landing guidance provided by ILS (Shively and Hsaio, 2005). The objective here is to design, develop, and deploy a space-based navigation system augmentation that complements GPS, Galileo, and the Global Navigation Satellite System (GLONASS), so that guidance equivalent to Category IIIc ILS is universally available and no additional ground-based capability (e.g., pseudolites) needs to be installed. Deployment of this capability would open up any airport or temporary landing area for all-weather operations with no need for expensive and time-consuming construction; facilitate reconfiguration of approach paths; and improve safety. It would allow many existing runways and small airports not equipped with ILS to operate in low visibility conditions and would support emergency operations and homeland security. It would have the added benefit of allowing the FAA to remove thousands of existing en route and terminal navigation aids, such as very high frequency (VHF) omnidirectional range (VOR) equipment, distance measurement equipment (DME), tactical air navigation (TACAN) systems, ILSs, and nondirectional beacons (NDB). Decommissioning these systems would eliminate associated maintenance and operating costs. This Challenge ranked low in NASA priority primarily because the feasibility of deploying an affordable, broad-area precision navigation capability will be determined by technical, economic, and regulatory issues. 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