In a report addressing the effects of aviation on the global atmosphere, the link between emissions and the technological status of aircraft now and in the future is clearly a central issue. The subject is complex. Our approach here, therefore, has been to identify a number of key questions, the answers to which provide an assessment of technical issues, problems, and the prospects of solving them.

The questions and their corresponding answers are as follows:

• Question-What are the principal technological factors that determine the nature and scale of emissions from aircraft at altitude?

Answer-The overriding technological consideration in the design of aircraft today is safety. Given that prerequisite, aircraft are designed to provide an efficient and environmentally acceptable system of transport from ground level to the demanding conditions associated with high-speed flight at high altitudes. To achieve high efficiency, fuel consumption must be minimized by reducing the weight and drag of the aircraft. This requirement also ensures that there is a constant drive toward the highest levels of energy conversion efficiency from the engine. Together, these factors ensure that carbon dioxide CO2) and water outputs are minimized.

The most fuel-efficient engines for today's aircraft are high bypass, high pressure ratio gas turbine engines. No known alternatives are in sight. These engines have high combustion pressures and temperatures; although these features are consistent with fuel efficiency, they increase NOx formation rates-especially at high power take-off and at altitude cruise conditions.

Current low-sulfur fuels minimize SOxO emissions. Small amounts of fuel-bound sulfur (400-600 ppm) and associated organic acids provide important lubricity properties for critical fuel system components. Processing to remove all traces of sulfur would remove important organic acids, so sulfur-free fuels are unlikely to be adopted in the short term. Sulfur removal would also result in a small net rise in CO2.

At present there is only limited knowledge about the formation and behavior of minor, trace species and aerosols found in the exhaust plumes of engines. Even less is known about how they are influenced by engine features and characteristics. . Question-What progress has been made to date in reducing emissions, and how may new advances in aircraft and engine technology help reduce them further in the future?

Answer-In the past 40 years, aircraft fuel efficiency has improved by 70% through improvements in airframe design, engine technology, and rising load factors. More than half of this improvement has come from advances in engine technology. These trends are expected to continue, with airframe improvements expected to play a larger role through improvements in aerodynamic efficiency, new materials, and advances in control and handling systems. New, larger aircraft with, for example, a blended-wing body or double-deck cabin offer prospects of further benefits by relaxing some of the design constraints attached to today's large conventional aircraft. Because of the very long total lifetimes of today's aircraft (up to 50 years), however, replacement rates are low, and the fuel efficiency of the whole fleet will improve slowly. Rising market demand will ensure that this trend is maintained, however.

The intrinsic link between lower CO and rising levels of NOx is being successfully countered with relatively simple strategies in state-of-the-art combustors. These combustors have achieved 20-40% reductions in NOx. Consolidation of these improvements to broaden their applicability to newer, even more fuel-efficient engines demands further improvements in combustor technology. Major research programs are underway to do so.

Although the use of hydrogen as a fuel offers a way to eliminate CO2 and further reduce NOx from aircraft, widespread use of hydrogen fuel presents major design problems for aircraft and would entail global changes in supply, ground handling, and storage. Hydrogen would also substantially increase water vapor emissions from aircraft. Thus, kerosene-type fuels are considered to be the only viable option for aircraft within the next 50 years (to 2050).

• Question-What data exist about actual emissions from aircraft? What is being done and what needs to be done to improve our understanding of and our ability to predict the scale and nature of these emissions?

  Answer-The International Civil Aviation Organization (ICAO) engine emissions databank is a substantial and growing source of reliable information that is now being used to develop aircraft emissions inventories and to analyze specific emissions. The databank-which includes information on smoke, hydrocarbons (HC), carbon moNOxide (CO), and NOx emitted during a defined landing and take-off cycle-is collated with prescribed correction procedures to guarantee consistency and comparability. There is no comparable source of data relating to sulfur compounds (SOxO) and minor trace species or aerosols from engines. Such data are emerging from individual research programs, but much more must be done to increase the breadth and depth of knowledge about the formation, nature, and scale of these potentially important aircraft emissions.

  Important progress has been made in the measurement and observation of the transient behavior of minor and trace species in engines. Again, further work is needed before conclusions can be drawn that might offer clues concerning their control and reduction in future engines.

• Question-How are emissions from aircraft currently regulated, and how do these regulations influence emissions at altitude?

  Answer-Present aircraft emissions regulations apply only to the landing and take-off cycle up to an altitude of 900 m. However, these regulations exert a controlling influence on emissions from aircraft at cruise altitudes because design changes to achieve lower NOx at take-off are equally beneficial at medium-power cruise conditions. Methods have been developed to use the ICAO databank to predict aircraft emissions at altitude cruise conditions. These predictions are accurate to within 5-10% for a modern, high bypass ratio engine.

• Question-What performance might we expect from new aircraft entering the fleets in 2015 and 2050 (the dates of the scenarios discussed in Chapter 9)?

  Answer-The emerging effects of research and technology programs on airframes and engines will influence future fleets of subsonic aircraft. A group of aerospace industry experts has developed some technology projections relating to fuel efficiency and NOx emissions of aircraft by the years 2015 and 2050. According to these scenarios, average fuel efficiency of new production aircraft in the scheduled commercial fleet may improve by 20% between 1997 and 2015. The corresponding scenarios for improvement between 1997 and 2050 involved two different technology scenarios to take account of tradeoffs between fuel efficiency and low NOx in aircraft designs. In the first case, with fuel efficiency taking priority, a 40-50% improvement in the fuel efficiency of new production aircraft was projected. In the second case, where NOx reductions took priority, a 30-40% improvement in fuel efficiency was envisaged.

  New commercial supersonic transport aircraft, operating at speeds of Mach 2 to 2.4, have been proposed for introduction into service, though not before 2015. It now seems unlikely that any commercial supersonic transports will exceed flight speeds of Mach 2.5 within the next 50 years as a result of engineering problems, materials limits, fuel efficiency, and other economic considerations. Supersonic aircraft are intrinsically less fuel efficient than subsonic aircraft. They consume about twice as much fuel, on a passenger-kilometer basis, as subsonic aircraft of the same size and range. To minimize stratospheric ozone depletion, the major design criteria for supersonic aircraft focus on flight altitude and low NOx output. Water vapor emissions may become more important than NOx emissions. If so, control of H2O emissions will depend solely on the achievement of greater fuel efficiency. Sulfur aerosols originating in the fuel are an emerging concern in the altitude bands used by supersonic aircraft. As in the subsonic aircraft case, more data are needed to determine their true impact.

• Question-What are the likely effects of small aircraft and military aircraft on the environment?

  Answer-Small aircraft, including commuter aircraft and general aviation, pose little environmental threat because they consume a very small fraction of the total of aviation fuel. Similarly, military aircraft, which consume less than 20% of the total aviation fuel supply today and, as civil aviation grows, perhaps less than 5% in the next 50 years, are seen as having potentially small environmental impacts.

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