runway throughput. Minimizing the ground footprint of aircraft relative to their capacity also allows for more efficient use of limited airport space.

Although aircraft performance is important, systemwide performance is the overriding concern. Without a systemwide perspective, research and development runs the risk of suboptimization—for example, by improving the performance of a vehicle system in a way that degrades overall performance of the air transportation system. The above discussion of aircraft performance should, therefore, be understood in the larger context of air transportation system performance.

In the discussion that follows, improvements in aircraft performance will be discussed in terms of (1) environmental considerations, (2) advanced airframe concepts, (3) advanced propulsion concepts, and (4) the potential of a cross-cutting technology of particular interest: nanotechnology.

ENVIRONMENTAL CONSIDERATIONS

The air transportation system already expends considerable resources to deal with public concerns and government regulations related to the effects of aviation on local and regional air quality, climate change, and community noise. All of these environmental problems will be aggravated by growth in air traffic. Problems related to emissions are abated by propulsion and airframe concepts and technologies that improve aircraft efficiency. However, the rapid growth of demand for air transportation and the growth in capacity have exceeded the rate of improvement of specific fuel consumption, so that over time aviation consumes larger amounts of fuel. The amount of carbon dioxide (CO2) released in the atmosphere is roughly proportional to fuel consumption, so more CO2 is being released. The amount of other emissions, such as oxides of nitrogen (NOx) and particulates, is also increasing even though engines are becoming more efficient and cleaner, producing fewer emissions per pound of fuel burned. Higher engine combustion temperatures tend to improve the efficiency of the propulsion system, but higher temperatures also increase NOx emissions. The production of specific emissions can be minimized by changes to the combustion cycle and other aspects of engine design, although changes in engine design to reduce one emission might increase the production of other emissions.

Noise can also be reduced by improvements in the design of the integrated aircraft as well as specific changes to the engine and propulsion system. In some cases, noise reduction technologies reduce overall aircraft efficiency because, for example, they increase aircraft weight.

Breakthroughs could be achieved through use of an alternative fuel such as liquid hydrogen or revolutionary technologies such as fuel cell-electric propulsion. However, breakthrough technologies such as these are likely to take several decades, at least, to become operational. Accordingly, research in environmental technologies should focus on conventional jet propulsion systems, while continuing to explore promising longer-term technologies. Environmental considerations are discussed in more detail below and in a recent report by the National Research Council (NRC, 2002a).

Local and Regional Air Quality

The principal concerns regarding local air quality are high levels of NOx and particulate matter. At a regional level, NOx and unburned hydrocarbon emissions from aircraft engines also contribute to the formation of ozone and are currently regulated in accordance with standards established by the International Civil Aviation Organization. A standard for measuring particulate matter from aircraft engines is currently being developed. The contribution of aircraft to regional emissions of NOx and particulate matter is currently on the order of 1 percent of all anthropogenic emissions. The aircraft contribution is increasing, however, as air traffic increases, while emissions from other sources are decreasing as a result of more stringent emissions standards and improved emissions reduction technologies.

Limits on total NOx emissions established by local authorities are already threatening to limit capacity at some airports in Europe, while the imposition of landing fees proportional to NOx emissions by each aircraft have been implemented at other European airports. Stringent emissions standards and the threat of emissions caps have led to modest emissions reductions through optimization of current gas turbine emissions technology. However, these reductions have been largely offset by higher engine pressure ratios (for improved fuel efficiency), which tend to increase emissions of NOx and particulate matter. Emissions of NOx by aircraft have not been reduced as much as emissions by surface sources because alternative fuels and exhaust gas cleanup technologies used in other transportation and industrial sources require large, heavy devices that are not practical in aircraft applications. Design improvements that reduce aircraft weight or improve aircraft and engine aerodynamics tend to reduce NOx emissions because less fuel is consumed.

Better dispersion models will lead to a better understanding of the impact of aircraft emissions and will be a better guide to technology development. Modeling the movement of emissions released by aircraft in flight is significantly more difficult than modeling emissions from a static point source, such as an industrial facility. Another emerging need is the development of a standardized method for measuring emissions of particulate matter; current data on aircraft emissions of particulate matter are sparse and of questionable quality.

Research is needed to develop combustor technologies to reduce emissions of NOx and particulate matter in engines that operate at high pressure ratios with current jet fuels. If



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