8.3.2. Ambient Factors

In the past decade, discussions of daylight flying restrictions and on the potential sensitivity to aviation emissions of specific geographical areas and flight levels have resulted in research into consequences for the aviation transportation system of avoiding these areas (Fransen and Peper, 1993; ICAO, 1995; Sausen et al., 1996). Chapters 2, 3, and 6 of this report describe evolving knowledge on the possible atmospheric effects of these issues. In general, this scientific assessment is unable to make clear-cut recommendations regarding possible reductions in adverse environmental impacts that might be achieved with such flight restrictions. The description in this section is confined to consequences for the aviation transportation system.

Figure 8-2: Aviation share of world transport CO2 emissions.

Figure 8-3: CO2 emissions by transportation mode: United States - 1996.

Aircraft operate most efficiently at specific cruise altitude; generally, less fuel is consumed at higher altitudes, but more engine thrust and equipment may be required to reach those altitudes. Any requirement to stay, for example, below the tropopause would undoubtedly increase fuel consumption. In that case, flights travelling close to the North Pole during the winter would have to descend to levels that could be 2,500-5,000 m below the most fuel efficient level. Consequently, fuel consumption would increase for many flights (ICAO, 1995). A study by Fransen and Peper (1993) showed that total fuel burned by aviation in the North Atlantic corridor would increase by 4-5% using flight level 310 (approximately 9,500 m) as the maximum allowed level; the increase for an individual flight could be as much as 20% (Lecht, 1994). This restriction would lead to payload and range limitations because some current aircraft operate at or close to maximum range. Such limitations, in turn, could lead to requirements for intermediate stops-resulting in less direct routes, increased flight times, and increased fuel burn. Another effect would be concentration of flights, hence increased congestion, which would contribute to additional fuel burn.

Figure 8-4: CO2 intensity of passenger transport (TEST, 1991; Whitelegg, 1993; Faiz et al., 1996; Centre for Energy Conservation and Environmental Technology, 1997a; OECD, 1997a).

Limiting flights to specific parts of the day is practically impossible for a number of reasons, not least because long-range commercial flights can last for up to 14 hours and during that time can cross a number of time zones. Confining travel to the hours of darkness would require intermediate stops, which would result in an increase in the number of landing and take-offs and an associated increase in the amount of fuel used. An additional problem concerning night flights is public pressure to reduce these flights for noise reasons.

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