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For Greener Skies: Reducing Environmental Impacts of Aviation 1 Contemporary Realities of Aviation, the Economy, and the Environment INTRODUCTION Flight through the air—by insects, birds, or airplanes— requires sufficient power to overcome the forces of gravity and drag. Since that first flight at Kitty Hawk in 1903, aviation has advanced at an astonishing rate to become a key component of developed economies and societies. Because of the success of aviation, aircraft operations consume increasing amounts of fuel and produce more emissions and noise. Today, the environmental impacts of aircraft, mainly engine noise and emissions, are a small but significant fraction of the total consequences of fossil fuel consumption. In the future, expected growth in the aviation sector, as well as the larger impact of some emissions when they are released at higher altitudes, will make aviation noise and emissions increasingly significant here and in other countries. The list of contemporary and future environmental issues that aviation must address includes the following: takeoff and approach noise (which present different technological problems for subsonic and supersonic aircraft) flyover noise from cruise altitudes in very quiet areas sonic booms and hyperbooms (i.e., the thermospherically refracted and very low intensity remains of sonic booms) taxi and engine run-up noise fuel venting and fuel dumping emission of CO, hydrocarbons, and NOx in the airport area (below 3,000 feet) contrail formation emissions of CO2 emissions in the upper troposphere and stratosphere (from both subsonic and supersonic aircraft) of water vapor, NOx, sulfur particles, and carbon particles potential for greenhouse effects and depletion of stratospheric ozone As discussed in Box 1-1, federal responsibilities for controlling the environmental effects of aviation reside primarily with the Environmental Protection Agency (EPA), the Federal Aviation Administration (FAA), and the National Aeronautics and Space Administration (NASA). The roar of a single jet transport taking off or passing close overhead seems to generate more complaints than some other sources of noise that are just as loud but more familiar. Objections to noise are preventing the expansion of some airports and are constraining operations, and noise is most frequently cited by officials at the nation’s 50 busiest airports as their major environmental concern (see Figure 1-1).1 Aviation provides significant national benefits to the United States— as an engine of commerce and social interaction, transporting people and goods rapidly and safely on diverse missions all over the world as a vigorous sector of the economy that provides direct economic benefits by generating jobs and exports in the design and manufacture of engines, airframes, and avionics used by airlines, airports, and associated industries It is clearly in the best interests of the United States and other nations that their aviation industries grow and prosper at the same time that aviation’s impacts on the environment are reduced. The importance of federal action to maintain the vitality of the aviation enterprise while reducing adverse environmental impacts was recognized by the National Science and Technology Council in a 1995 report, which 1 As indicated by Figure 1-1, water quality and land use issues are also important at some airports. However, this study is focused on environmental issues that are most directly associated with aircraft technologies (i.e., noise and emissions).
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For Greener Skies: Reducing Environmental Impacts of Aviation BOX 1-1 Federal Responsibilities In general, the EPA is responsible for the establishment and enforcement of U.S. environmental protection standards consistent with national environmental goals. For aircraft, however, the FAA is responsible for enforcing EPA clean air standards—by issuing Federal Aviation Regulations (FARs) that define how clean air standards will be applied to specific aircraft (Clean Air Act of 1970, as amended, 42 U.S.C. §7571). With regard to standards for noise and sonic boom, the EPA must submit proposed aircraft noise control regulations to the FAA. The FAA then seeks public comment and either issues new regulations or publishes a notice explaining why new regulations are not appropriate. In making such a decision, the FAA must consider whether the standard or regulation proposed by the EPA is “consistent with the highest degree of safety in air transportation . . . [and] economically reasonable, technologically practicable, and appropriate for the applicable aircraft” (Noise Control Act of 1972, as amended, 42 U.S.C. §4902; and 49 U.S.C. §44715). The role of NASA is to increase the range of options that are technologically feasible. NASA is charged with conducting aeronautical research and development, including long-range studies of potential problems and benefits, to preserve “the role of the United States as a leader in aeronautical . . . technology.” NASA is also expected to “carry out a comprehensive program of research, technology, and monitoring of the phenomena of the upper atmosphere so as to provide for an understanding of and to maintain the chemical and physical integrity of the Earth’s upper atmosphere.” Industry and academia are expected to participate in this research, and the results are to be given to appropriate regulatory agencies to assist them in generating new standards and regulations (National Aeronautics and Space Act of 1958, as amended, 42 U.S.C. §2451). warned that “environmental issues are likely to impose the fundamental limitation on air transportation growth in the 21st century” (NSTC, 1995). In response, federal agencies have identified noise and emissions targets for the next few decades and are pursuing a research agenda intended to achieve the linked goals of supporting the growth of aviation and reducing environmental impacts. The present report offers recommendations intended to increase the effectiveness of that agenda and the associated research efforts. ENERGY CONSUMPTION AND ITS CONSEQUENCES The adverse environmental effects of jet aircraft are primarily a consequence of the combustion of petroleum. Jet fuel is largely carbon and hydrogen, and so combustion releases carbon dioxide (CO2). Other gases, including oxides of nitrogen (NOx), are also produced by chemical interactions with the air flowing through the engine. Water vapor emitted by the engine combines with water vapor already FIGURE 1-1 Environmental issues that most concern officials at the 50 busiest U.S. airports. SOURCE: GAO, 2000.
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For Greener Skies: Reducing Environmental Impacts of Aviation TABLE 1-1 U.S. Energy Consumption Fractions, 1999 Energy Source 1015 Btu Percent of Total Petroleum for jet fuel 3.2 3.4 Renewable energy 7.4 7.6 Coal 21.7 22.5 Natural gas 22.1 22.9 Petroleum total 37.7 39.0 Fossil fuel total 81.6 84.4 Total energy 96.6 100.0 SOURCE: EIA, 2002a. present in the atmosphere and sometimes freezes to form condensation trails (contrails) behind the aircraft. Under some atmospheric conditions, ice crystals in the contrails grow and disperse, increasing the amount and intensity of regional cirrus clouds and modifying the atmospheric radiation budget that controls average global temperatures. With respect to aviation noise, the internal noises of the turbine engines combine with the noise generated by the jet exhaust and the rush of air over the airframe itself. The major forms and amounts of energy used by the U.S. economy and trends for consumption are given in Table 1-1 and Figure 1-2. Between 1989 and 1999, the United States increased its consumption of natural gas (14 percent), petroleum (10 percent), and coal (15 percent). The consumption of jet petroleum increased by 10 percent, and the consumption of petroleum products by the entire transportation industry increased by 14 percent. In 1999, all transportation sectors combined accounted for approximately 22 percent of energy consumption. Jet fuel accounted for approximately 13 percent of the transportation total, with automotive gasoline accounting for 66 percent and diesel fuel oil accounting for 20 percent. Jet petroleum represented 3 percent of total U.S. energy consumption, indicating that the environmental effects of aviation must be small compared with those caused by other users of fossil fuels. The demand for jet petroleum is increasing steadily, consistent with the overall growth in demand for energy. Finding 1-1. Increasing Rate of Fuel Consumption. Fuel consumption is a key indicator for assessing trends in emissions. The aviation industry is growing and the use of aviation fuel is increasing at a rate comparable to that of other uses of fossil fuels. Recognizing that reductions in fuel consumption rates are advantageous from both economic and environmental perspectives, industry has increased the efficiency of aircraft engines and aircraft. Indeed, the amount of fuel consumed per revenue-passenger-kilometer has been considerably re- FIGURE 1-2 Sources of energy in the United States. SOURCE: EIA, 2002b.
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For Greener Skies: Reducing Environmental Impacts of Aviation FIGURE 1-3 Decadal trends for demand, efficiency, and fuel usage. Dashed portions of curves are projections. SOURCE: Lukachko and Waitz, 2001. duced since the advent of commercial jet transports, albeit at a slowing rate of improvement in recent years. As indicated in Figures 1-3 and 1-4, however, the growth in demand for commercial air transportation has consistently exceeded increases in fuel efficiency.2 The problem is becoming more serious as efficiency improvements become technologically more difficult to achieve. Continued improvements remain essential, however. U.S. demand for air transportation tripled between 1977 and 1996 and is expected to double in the next 15 to 20 years. To maintain the status quo in terms of environmental impact, fuel consumption per passenger-kilometer must be cut in half, but government and industry have done only a little better than that in the entire 40-year history of commercial jet aviation. Dramatic new improvements are essential, but they are likely to be achieved only with a vigorous research and technology program that yields advances not yet foreseen. Globally, in 1976 military aviation consumed about 55 percent of all aviation fuel. As shown in Figure 1-3, military demand has been dropping as commercial demand has increased, and in 1996 military demand for aviation fuel was only 16 percent of the global total. General aviation consumes a much smaller fraction of aviation fuel, globally and nationally. This study focuses on commercial aviation. The situation is similar with respect to aircraft noise: aircraft performance has improved, but not as fast as demand has increased. Part 36 of the Federal Aviation Regulations covers noise requirements that aircraft must meet for FAA certification. Since it was issued in 1969, Part 36 has been amended more than 20 times and now covers virtually all types of aircraft. Several of these amendments have instituted more stringent noise requirements. On two occasions, amendments required large numbers of aircraft that could not meet new noise restrictions to be phased out of operation even though they were still flightworthy. New, stricter regulatory requirements are expected in the future as a result of ongoing action by the International Civil Aviation Organization (ICAO), which takes the lead in setting international 2 Future demand for air transportation depends upon a wide variety of factors, including the general state of the economy. Figure 1-4 depicts expected outcomes for two different demand scenarios. Between 2000 and 2015, Scenario 1 assumes that growth in passenger air travel will average about 5 percent per year, and Scenario 2 assumes a growth of about 3 percent per year. The FAA predicts that domestic air travel on U.S. airlines will increase about 4 percent per year between 2000 and 2012, down from the 5 percent growth per year experienced between 1995 and 2000 (FAA, 2001). Limitations on growth associated with environmental concerns and airport and airway capacity, however, may prevent the air transportation system from meeting future demand.
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For Greener Skies: Reducing Environmental Impacts of Aviation FIGURE 1-4 Effects of demand (in terms of revenue-passenger-kilometers) on the production of NOx and CO2 and on fuel consumption. The curves show the boundaries created by scenarios with the least and greatest demand projections. SOURCE: Prather and Sausen, 1999, and Henderson and Wickrama, 1999. standards that many nations, including the United States, subsequently adopt as national standards. Because of advanced technology, the perceived noise level produced by new commercial jet aircraft of a given size has been reduced by about 10 dB since the 1960s, which is equivalent to reducing annoyance by roughly a factor of 2 (FAA, 1997). These improvements have resulted primarily from technological advances that were incorporated into more economical aircraft and propulsion systems. Despite these improvements, noise is becoming more of a problem for several reasons: The amount of air traffic is growing. The number of very large aircraft is increasing (for a given level of technology, one large aircraft generally produces a higher noise level than several smaller aircraft with the same total passenger capacity). The hub-and-spoke routing system used by most airlines concentrates a lot of traffic and noise at a relatively small number of airports. Public acceptance of noise is diminishing. Community concerns about noise and other environmental effects result in airport curfews, flight path restrictions, and delayed or canceled airport expansions. Three-quarters of the delays experienced by expansion projects at the 50 busiest U.S. airports are primarily because of environmental issues, and 12 of the 50 busiest airports have had at least one expansion project canceled or indefinitely postponed because of environmental issues (GAO, 2000). The results are congestion, flight delays, and, on occasion, diversions and cancellations when aircraft are delayed so long that they would arrive at their destination airport after a curfew. The current situation is expected to deteriorate; during the next 6 years alone, the number of large U.S. airports operating at or above capacity is expected to more than double (see Table 1-2). TABLE 1-2 Estimated Time for the 50 Busiest U.S. Airports to Reach Capacity Estimated Time Number of Airports Already at capacity 13 1 to 2 years 4 3 to 4 years 7 5 to 6 years 8 7 to 9 years 2 10 or more years 11 Other 5 SOURCE: GAO, 2000.
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For Greener Skies: Reducing Environmental Impacts of Aviation REGULATORY GOALS ICAO tries to harmonize international regulatory standards for aircraft noise and emissions by recommending appropriate standards that regulatory bodies around the world can adopt. Intervention by local and regional groups, however, has made this process ineffective in providing a common set of rules. Airports and airlines face sometimes-differing restrictions imposed by local and national governments and multinational bodies such as the European Union. The operational impact of such restrictions, as well as the rate at which they are changing and the extent to which they are aligned with ICAO standards, varies widely around the world. Often, several different sets of regulations apply to a flight in a single airport area, with different standard-setting bodies having jurisdiction over different environmental impacts, but with little or no coordination among them. As a consequence of differing standards, conflicts arise. One locality may emphasize low-noise takeoffs and landings, but the lower noise flight path and lower engine power settings may result in aircraft spending more time at low altitude, which increases the effects of aircraft emissions on ground-level air quality. To address these conflicts, the federal government should carry out its responsibilities for mitigating the environmental effects of aircraft noise and emissions with a balanced approach that includes commitment and leadership at the highest level and cooperation among federal, state, and local governments; industry; and public and private research organizations. To ensure that regulations protect the public without unnecessarily constraining the availability of air transportation services, the federal government must also support research to develop a comprehensive perspective on the environmental effects of aviation and how they can be mitigated or accommodated. INDUSTRY RESPONSES AND RESPONSIBILITIES Regulatory and economic incentives are too small—and the technical challenges too large—for industry acting alone to eliminate the environmental effects of growth in air travel. The development of environmental protection technologies that reduce noise and emissions from aircraft is what economists term “externalities” in air travel. Manufacturers have little or no motivation to pay for developing aircraft that are quieter or produce less emissions than required by regulation unless they can recoup the additional expense by selling cleaner, quieter airplanes at a higher price (or by selling more of them). Airlines, however, have little or no motivation to pay more for quieter, cleaner aircraft if their customers—the traveling public—do not consider noise and emissions important in selecting an airline. Even if air travelers would fly preferentially on quieter or lower-emission aircraft, information is not available to allow them to make informed decisions. Like manufacturers and airlines (and most other successful businesses), airports focus on meeting the demands of their paying customers (i.e., airlines and passengers). As a result, environmental compatibility tends to fall to the bottom of everyone’s list of priorities—except for people with a special interest in the environment, especially those who live close to an airport (see Table 1-3 for the committee’s perception of priorities). Although all agree that the environment is important, in the highly competitive air transportation industry environmental performance is not as critical as safety or economics (as long as aircraft are clean and quiet enough to meet regulatory standards). Industry by nature responds to economic and regulatory incentives created by governments and the public. Thus, it falls to governments and regulatory bodies, acting on behalf of the public, to ensure that aviation growth is as environmentally compatible as possible. TABLE 1-3 Perceived Priorities of Consumers and Industry Passenger Priorities When Purchasing Airline Ticketsa Airline Priorities for Meeting Passenger Preferences Airport Priorities for Meeting Airline and Passenger Preferences Manufacturer Priorities for Meeting Airline Preferences 1. Safety and security 1 Safety and security 1. Safety and security 1. Safety 2. Ticket price 2. Reliability 2. Adequate capacity and dispatch reliability 2. Reliability 3. Frequent flyer benefits 3. Economics a. Runways 3. Durability (airframe fatigue) 4. Schedule and trip length a Aircraft compatible with airlines’ route system b. Air traffic control 4. Economics 5. Comfort c. Gates and terminals a. Passenger ticket cost 6. On-time performance b. Cost per seat-kilometer (durability, cost of maintenance, fuel burn, etc.) d. Ice and snow removal (depending on climate) b. Airline profitability c. Manufacturer profitability 7. Environmental impact e. Parking 5. Passenger comfort 4. Passenger convenience f. Ground access 6. Environmental impactb a. In-flight entertainment 3. Economics b. Seat comfort 4. Environmental impactb c. Cabin noise d. Other 5. Environmental impactb aPriorities can vary among different groups of passengers. For example, schedule and trip length are often a higher priority than ticket price among business travelers. bEnvironmental impact is a low priority unless mandated by regulation; then it goes up on the list.
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For Greener Skies: Reducing Environmental Impacts of Aviation TABLE 1-4 NASA’s Goals for Reducing the Environmental Effects of Future Aircraft (percentage reductions compared with levels that existed in 1997) Reduction in By 2007 By 2022 Noise 50% 75% NOx emissions 70% 80% CO2 emissions 25% 50% SOURCE: NASA, 2002. There are interesting contrasts to the industry attitudes toward safety and environmental requirements. All involved know that meeting expected levels of safety and security are absolutely top priorities and that bearing the necessary costs is essential to staying in business. As societal demands for reducing adverse environmental impacts become more strident, the priority attached to environmental compatibility will likely rise. But there are significant differences between safety and environmental engineering. Safety engineering in aviation is a well-established field, and the length of time between identification of a safety need and implementation of required changes is typically on the order of a few years—except in the case of urgent needs. On the other hand, achieving the ambitious goals set by NASA for environmental compatibility (see Table 1-4) is likely to take decades and require large investments in new, high-risk technologies with uncertain payoffs. Individual technology development programs may require on the order of $100 million before it is possible to learn whether the results justify further investment in flight demonstration hardware. This kind of high-risk, extremely long-term research and technology investment is incompatible with normal corporate research practices, which are typically aimed at commercial payoffs within a few years. The fact that individual companies suffer no short-term adverse consequences for not investing in environmental compatibility research also tends to discourage them from doing so. SUPERSONIC AIRCRAFT A large commercial supersonic aircraft with a cruise speed of Mach 2 to 2.4 has about twice the drag and burns more fuel per passenger-kilometer than a large subsonic aircraft with an equivalent level of technology. Also, the most efficient cruise altitude for aircraft becomes higher as cruise speed increases. Commercial supersonic aircraft with cruise speeds of about Mach 2 or higher will likely cruise in the stratosphere, where the effects of aircraft emissions on the environment can be much greater than at lower altitudes (in the troposphere) frequented by subsonic aircraft. Even water vapor, which is benign in the troposphere (unless it forms a contrail), may contribute to ozone depletion and global warming when exhausted into the stratosphere. From a fleetwide, climate-change perspective, this would be a problem if a large number of commercial supersonic aircraft were built. However, that is not likely in the next 25 years (NRC, 2001). In addition, the uncertainty of current atmospheric models is still substantial; carefully researched estimates of the impact of stratospheric water vapor on climate vary by a factor of about 3. The next step in the development of commercial supersonic aircraft may be the development of a supersonic business jet that would be much smaller, consume much less fuel, and operate in smaller numbers than would the fleet of large supersonic aircraft postulated in previous studies. Another alternative would be to develop a large commercial aircraft with a cruise speed close enough to Mach 1 that the aircraft could be designed to (1) incur a significantly smaller drag penalty than a Mach 2 aircraft and (2) avoid creating a sonic boom that would propagate to the ground. Boeing is currently conducting design studies of such an aircraft. When future supersonic aircraft enter service they may need to meet the same community noise standards as subsonic aircraft. Also, Federal Aviation Regulations prohibit commercial supersonic aircraft from producing a sonic boom over land. Those regulations are unlikely to be revised except, perhaps, to allow sonic booms at such low intensities that they do not create a public nuisance. The ability to fly at supersonic speeds over land would greatly improve the utility of supersonic aircraft, but research is needed both to determine what level of sonic boom might be acceptable and to develop a practical technological approach for achieving it. For the foreseeable future the vast majority of commercial air travel will be via subsonic aircraft, and the environmental impact of aviation will be determined by the noise and emissions produced by these aircraft. Therefore, this study focuses on subsonic aircraft. Additional information related to supersonic aircraft, including findings and recommendations, appears in reports published by the National Research Council (NRC, 1997, 1998, 1999, 2001) and the Intergovernmental Panel on Climate Change (IPCC, 1999). RESEARCH STRATEGIES The U.S. air transportation system is a critical industry and an invaluable national resource now caught between two powerful but conflicting expectations: the first for more services, the second for decreased environmental impact. The two demands can be reconciled only through a systematic approach that provides the following: a better understanding of the scientific issues involved realistic goals that avoid raising false expectations a comprehensive research strategy that provides advanced technologies for dramatically improving engine and airframe performance enhancements to the other portions of the air transpor-
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For Greener Skies: Reducing Environmental Impacts of Aviation tation system (e.g., the air traffic control system) to improve operational efficiency Urgent action is essential for several reasons: Environmental issues are already interfering with the efficient operation of the U.S. air transportation system. Air traffic is increasing. Environmental standards are becoming more stringent. Research and technology development takes a long time to change the face of commercial aviation. New technology may take 10 years or more to be proven commercially acceptable and certified by the FAA for use on commercial aircraft. In addition, the production run on a successful aircraft may last for 15 to 20 years, and individual aircraft may have service lives of 25 to 35 years. As a result, it can take decades for a major technological improvement to show up in a majority of the commercial fleet unless it can be retrofitted into existing aircraft at reasonable cost. Given the current funding available, the Committee on Aeronautics Research and Technology for Environmental Compatibility has concluded that federal research programs in noise reduction technology are focused appropriately. However, much remains to be done, and uncertainties persist in many areas. In collaboration with other stakeholders (such as manufacturers, airlines, airport authorities, local governments, nongovernmental organizations, and foreign regulatory bodies and researchers), NASA and the FAA should support research to resolve uncertainties in the following areas: long-term atmospheric effects of aircraft emissions locally, regionally, and globally reliable goals for noise and emissions reductions for each phase of flight the optimum long-term strategy for improving the understanding of the many specific issues, including economic factors, associated with aircraft noise and emissions Economic analyses must form a key element in much of the research in the above areas because economic incentives for providers and users of air transportation equipment and services are likely to be a key component of a successful long-term strategy. NASA and other agencies should sustain promising research long enough to ensure that new technology developed by federal research programs is mature enough to warrant commercial development. This will require a balanced allocation of federal funds devoted to mitigating the environmental effects of aviation. In particular, federal expenditures to reduce noise should be balanced between abatement of noise at specific airports (e.g., through soundproofing of privately owned buildings located outside the airport perimeter) and the development of advanced aircraft technologies that will ultimately reduce aircraft noise globally. Finding 1-2. Vigorous Action Required. Environmental concerns will increasingly limit the growth of air transportation in the 21st century unless vigorous action is taken to augment current research and technology related to the environmental impacts of aviation. REFERENCES EIA (Energy Information Administration). 2002a. Energy Consumption by Source, 1949-2000 . Available online at <http://www.eia.doe.gov/emeu/aer/txt/tab0511.htm>. EIA. 2002b. Petroleum Products Supplied by Type, 1949-2000. Available online at <http://www.eia.doe.gov/emeu/aer/txt/tab0103.htm>. FAA (Federal Aviation Administration). 1997. FAA Advisory Circular 36-1G. Noise Levels for U.S. Certificated and Foreign Aircraft. AEE-110. Washington, D.C.: Federal Aviation Administration. Available online at <http://www.airweb.faa.gov/Regulatory_and_Guidance_Library/rgAdvisoryCircular.nsf/MainFrame?OpenFrameSet>. FAA. 2001. FAA Aerospace Forecasts Fiscal Years 2001-2012. March. Table I-2R. Washington, D.C.: FAA Office of Aviation Policy and Plans. Available online at <http://api.hq.faa.gov/foreca01/Table%20I-2R.pdf>. GAO (General Accounting Office). 2000. Aviation and the Environment— Results from a Survey of the Nation’s 50 Busiest Commercial Service Airports. August. Washington, D.C.: General Accounting Office. Henderson, Stephen C., and Upali K. Wickrama. 1999. Aircraft emissions: Current inventories and future scenarios. Table 9-10: Traffic projections and 5-year average growth rates from FESG [Forecasting and Economic Analysis Sub-Group]. IPCC Special Report on Aviation and the Global Atmosphere. Cambridge, U.K.: Cambridge University Press. Available online at <http://www.grida.no/climate/ipcc/aviation/138.htm>. IPCC (Intergovernmental Panel on Climate Change). 1999. Aviation and the Global Atmosphere. Cambridge, England: Cambridge University Press. Lukachko, S., and I. Waitz. 2001. Environmental compatibility of aviation graphs. Cambridge, Mass.: Massachusetts Institute of Technology Gas Turbine Laboratory. NASA (National Aeronautics and Space Administration). 2002. Technology Goals and Objectives. Office of Aerospace Technology. Available online at <http://www.aerospace.nasa.gov/goals/index.htm>. NRC (National Research Council). 1997. U.S. Supersonic Commercial Aircraft: Assessing NASA’s High Speed Research Program. Aeronautics and Space Engineering Board. Washington, D.C.: National Academy Press. NRC. 1998. The Atmospheric Effects of Stratospheric Aircraft Project: An Interim Review of Science and Progress. Board on Atmospheric Sciences and Climate. Washington, D.C.: National Academy Press. NRC. 1999. A Review of NASA’s Atmospheric Effects of Stratospheric Aircraft Project. Board on Atmospheric Sciences and Climate. Washington, D.C.: National Academy Press. NRC. 2001. Commercial Supersonic Technology: The Way Ahead. Aeronautics and Space Engineering Board. Washington, D.C.: National Academy Press. NSTC (National Science and Technology Council). 1995. Goals for a National Partnership in Aeronautics Research and Technology. Washington, D.C.: White House Office of Science and Technology Policy. Available online at <http://www.ostp.gov/html/aero/cv-ind.html>. Prather, Michael, and Robert Sausen. 1999. Potential climate change from aviation. Table 6-2: Emissions, atmospheric concentrations, radiative forcing, and climate change (global mean surface temperature) projected for the years 1990, 2000, 2015, 2025, and 2050 using IPCC’s [Intergovernmental Panel on Climate Change’s] IS92a and the aviation scenarios from Tables 6-1 and 6-3. IPCC Special Report on Aviation and the Global Atmosphere. Cambridge, U.K.: Cambridge University Press. Available online at <http://www.grida.no/climate/ipcc/aviation/068.htm#tab61>.
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