Air transport cruise altitude restrictions to minimize contrail formation

Abstract

While quantification of the effects of NO _x and water vapor is still at an early stage there is evidence that contrail formation could make a significant contribution to global warming. This paper builds on previous research that analyzed a policy of restricting air transport cruise altitudes to eliminate contrail formation. Our previous work [Transport. Res. D 7(6) (2002) 451], examined altitude restrictions in European airspace and concluded that this could be a beneficial policy for reducing climate change impacts from aviation. Since most of the flights in European airspace are short-haul flights, this paper evaluates the trade-offs between altitude restrictions, fuel burn and journey times for longer haul flights of up to 6000 nm. Our focus is on the North Atlantic and US airspace and we examine potential contrail fraction to determine optimal cruise altitudes for reducing contrail formation. Changes in fuel burn and travel times associated with flight levels of 18,000 and 31,000 ft for different aircraft types are analyzed. We find that, in most cases, CO₂ emission increases would be unlikely to entirely counteract the benefit of possible reductions in contrail formation. For some aircraft types, the percentage increase in emitted CO₂ was found to be strongly dependent on journey length. In general, journey times appear not to be a major issue except for some aircraft types. Our results suggest that reducing aircraft cruise altitudes could be a beneficial policy for mitigating climate change impacts from the aviation sector. This is clearly dependent on aircraft type and the distances traveled, but more importantly on ambient atmospheric conditions which can vary significantly between regions and due to daily variation. This suggests that real time flight planning to minimize contrail formation should be investigated as a possible climate mitigation policy.