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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