The future operation of all aircraft classes will be constrained by requirements to reduce their environmental impact. These constraints include a complex system of restrictions on both noise and ozone-depleting or ozone-generating emissions. It is a proper role for the National Aeronautics and Space Administration (NASA) to attack these issues because of the effects that noise, smog, ozone depletion, and sonic booms have on the quality of life around airports and major cities. It should also be noted that many of the aircraft that are being designed today will still be in production 20 years from now and may still be in operation as much as 30 years from now. Without significant effort to make U.S.-built aircraft less intrusive, substantial opportunity for market growth may be forfeited. Throughout this report the Committee has stressed the need for NASA to be more involved in helping industry meet this type of challenge. Thus, NASA, the U.S. aircraft industry, and the Federal Aviation Administration (FAA) as well, must find a way to work together to address these national and international constraints to ensure that U.S. aircraft remain competitive into the next century, and to avoid increasing the adverse effects that aircraft have on the quality of life of the citizens of the United States and the world.
This chapter discusses the issues involved in reducing the impact of aircraft, and offers a number of recommendations to focus NASA's ongoing noise and atmospheric research programs to address these problems. The boxed material summarizes the primary recommendations that appear throughout the chapter, with specific recommendations given in order of priority, and the benefits that can be gained through research and development aimed at the environmental compatibility of aircraft.
Much has been said and written about the atmospheric effects of fossil fuels. However, the lack of sufficiently accurate analytical models does not permit detailed definitions of acceptable emissions. Rather, the best that can be hoped for is setting of reasonable goals for
NASA, the U.S. aircraft industry, and the FAA must work together to address national and international environmental concerns both to help the United States gain a competitive edge and to avoid increasing the adverse environmental effects of aircraft on the ground and in flight.
reducing emissions that can be worked toward until research into the basic chemistry and dynamics of the atmosphere can yield a more definitive answer. Thus, the Committee believes that a two-pronged attack should be mounted: current and proposed research programs sponsored by NASA, as well as the National Science Foundation, Environmental Protection Agency, National Oceanic and Atmospheric Administration, and the Department of Energy, should be continued to enhance understanding of the impact of engine emissions on the atmosphere. While that research is being performed, aircraft designers should be aggressive in their efforts to reduce emissions from all categories of aircraft. In particular, NASA is strongly encouraged to investigate worst-case scenarios for stratospheric ozone depletion from high-speed civil transport (HSCT) to establish a basis for reasonable regulation of aircraft emissions, and to begin developing engineering solutions. For example, an emission
Benefits of Research and Technology Development in Aircraft Environmental Compatibility
Enhanced understanding of atmospheric effects of emissions
Realistic regulations for emissions
Better analytical modeling of atmospheric effects
Reduced airport/community noise
Possibility of acceptable sonic boom for HSCT
index of 3–5 g of nitrogen oxide (NOx) per kg of fuel burned for a commercial HSCT fleet operating in a cruise mode in the stratosphere (15–30 km) would represent a reduction by a factor of 10 over existing engine combustion technology. This magnitude of reduction can be used as a guide for designers as researchers work toward more precise definition of the limits.
Much is known about ozone depletion. It is clear that an HSCT, which will fly higher in the stratosphere and thus closer to the ozone layer, will have a more profound effect on ozone depletion than lower-flying aircraft. Furthermore, an HSCT will burn more fuel per seat-mile than a corresponding subsonic aircraft. These problems can be alleviated somewhat by improving (lowering) the NOx emission index of HSCT engines or by flying at lower altitudes at correspondingly lower Mach numbers. To enable a successful commercial HSCT fleet, NASA must continue or accelerate aggressive programs in advanced emission reduction technology related to the chemistry and dynamics of the stratosphere.
The corresponding problem for aircraft that fly in lower levels of the atmosphere is generation of noxious emissions that contribute to ozone depletion, but also produce smog, affect atmospheric oxidation (cleansing of the atmosphere), and contribute to global warming. Finally, at both takeoff and landing, emissions of NOx can increase smog and ozone near the ground. Thus, it is imperative that improved modeling, data collection, and verification of models of the chemistry and dynamics of the troposphere and tropopause also be included in NASA's long-term subsonic aircraft program.
Development of future aircraft of all classes must consider both U.S. and international noise standards. Hence, there are two interrelated challenges. The FAA, NASA, and other federal agencies must effectively represent the interests of the U.S. public, as well as the U.S. aircraft and airline industries, in the development of international noise standards.
Also, NASA must maintain a very aggressive noise reduction program to ensure the existence of technology that will enable U.S. aircraft to meet international noise standards while operating on a sound fiscal basis.
Noise standards (FAR-36)1 exist for current short-haul, general aviation, and subsonic aircraft, but not specifically for the HSCT. Standards for subsonic aircraft are divided into three classifications depending on the date of the certification application, with Stage 3 designating the most quiet aircraft. Aircraft that meet Stage 3 noise limits have provided substantial noise reductions relative to older aircraft. Nevertheless, many airports impose additional noise restrictions that penalize payload or range by requiring operations at reduced takeoff weight or prohibit night operations. It is clear that pressure to further reduce noise will continue to increase.
An aggressive noise reduction research program for advanced subsonic aircraft should be aimed at achieving Stage 3 minus 10 dB. Increased bypass ratio engines with advanced fan noise reduction are critical, and full achievement will require airframe noise reduction as well. The accepted noise goal for future HSCT aircraft is to be certified to a standard equivalent to the Stage 3 limitations that subsonic aircraft are subject to. This will require HSCT engines to be 10–20 dB quieter than unsuppressed low bypass ratio turbofan or turbojet engines. Although in recent years there has been considerable progress toward the HSCT goal with advanced suppressor designs and advanced engine cycle selection, a considerable challenge remains. The level of jet noise suppression achievable is the critical element, and considerable development is necessary.
Required future reductions in advanced subsonic and HSCT system noise will require significant reductions in engine and airframe noise. Aircraft operations can have an impact on noise footprint by trading between airport noise and community noise. Advances in the air traffic management (ATM) system, flight operations, and smart aircraft controls are enhancing elements in producing the trade-offs.
Noise levels inside current subsonic and supersonic aircraft that passengers and crew are exposed to are not as comfortable as they should be. This is an area in which U.S. aircraft could gain a clear competitive advantage over foreign competitors.
Concerted and continuing efforts are required to significantly reduce noise. NASA should lead the basic research to substantially improve the sophistication and technical strength of noise analysis and design computational tools. It should also lead in the development of new noise reduction concepts. NASA must maintain an aggressive research and development program in aircraft noise reduction that includes the following elements:
Conduct and sponsor fundamental research to understand and reduce high-speed jet and turbomachinery noise.
Conduct and sponsor fundamental research to understand human response to noise and vibration in aircraft cabins, flight decks, and crew rest areas.
Develop technology for predicting and measuring long-range ground-to-ground and flight-to-ground sound propagation for airport and en route noise assessment.
Develop technology for prediction and control of vibroacoustic responses of aircraft structure and materials.
Develop experimental techniques and facilities to permit acquisition of accurate acoustic, aerodynamic/structural dynamic, and psychoacoustic data in support of aircraft acoustic technology development.
Develop signal processing technology to improve analysis and utilization of data from acoustic wind tunnel testing.
Unless the United States mounts concerted efforts to augment the development of advanced computational and experimental capabilities, it will not be possible to achieve technology parity with similar European efforts. In the absence of these acoustic research facility capabilities in the United States, our industry and government research and development efforts typically utilize foreign facilities. In order to compete, it is necessary for the United States to support the development of appropriate facilities.
Sonic boom disturbance produced by a conventional HSCT will make routine supersonic flight over land impossible. Thus, although a large potential market may exist for an aircraft that cruises supersonically over water and subsonically over land, the capability to fly supersonically over land would expand the market considerably and represent an important competitive edge. As is the case with emissions and noise standards, a U.S. response to sonic boom standards must enable the U.S. aircraft industry to remain competitive in international markets. Hence, the U.S. response must include an integrated approach, with NASA, industry, and FAA participation.
Reduction in the loudness of a sonic boom requires a reduction in maximum overpressure. Current configurations for large supersonic aircraft typically produce sonic booms having a maximum overpressure of 2–3 pounds per square foot (psf). Although alternate configurations show promise for overpressure levels below 2.0 psf, much research is still required.
The duration of the sonic boom also impacts loudness, but very little benefit is obtained by modification of aircraft configuration that influences sonic boom duration. In addition, modifications to the configuration that are great enough to have a significant impact on duration usually lead to other design penalties. Reduction of the size and shape of the initial shock (shock front) will also reduce the loudness of the sonic boom.
The Committee strongly encourages NASA to continue its intensive and aggressive research and development program in sonic boom reduction. This program should include the following components:
development of sonic boom exposure criteria needed to assess the potential of acceptable supersonic flight over land;
sonic boom propagation studies to better predict environmental (temperature, typography) influence on sonic boom impact; and
development of analysis and design methodologies required to produce aircraft configurations that generate sonic boom having minimized impact.
The NASA program should reflect a balance between the pursuit of overland supersonic flight and optimized designs for mixed subsonic/supersonic operations. Given the various international parameters, it is in the best interest of the U.S. aircraft industry for NASA to pursue optimized designs for mixed operations, while developing technology for supersonic flight over land at a lower priority.
Boeing Commercial Airplane Group. 1990. High-Speed Civil Transport Study—Special Factors. NASA Contractor Report 181881, Contract NAS1-18377.
Douglass, A.R., M.A. Carrol, W.B. DeMore, J.R. Holton, I.S.A. Isaksen, H.S. Johnston, and M.K.W. Ko. 1991. The Atmospheric Effects of Stratospheric Aircraft: A Current Consensus. NASA Reference Publication 1251. Washington, D.C.: National Aeronautics and Space Administration, Office of Management, Scientific and Technical Information Division.
Johnston, H.S., M.J. Prather, and R.T. Watson. 1991. The Atmospheric Effects of Stratospheric Aircraft: A Topical Review. NASA Reference Publication 1250 . Washington, D.C.: National Aeronautics and Space Administration, Office of Management, Scientific and Technical Information Division.
Rowberg, R. E., K. Hancock, and C. T. Hill. 1989. Commercial High-Speed Aircraft Opportunities and Issues. CRS Report for Congress, CRS 89-163 SPR. Washington, D.C.: Congressional Research Service.
U.S. Geological Survey, Committee on Earth and Environmental Sciences. 1990. Our Changing Planet: The FY 1991 Research Plan of the U.S. Global Change Research Program. Reston, Va.
World Meteorological Organization. 1990. Scientific Assessment of Stratospheric Ozone: 1989. Global Ozone Research and Monitoring Project, Report No. 20. Scientific Assessment of Stratospheric Ozone: 1989. Geneva.