Environmental implications of allocating delay to aircraft at high vs. low altitude

An analysis of the impact of taking delay at high vs. low-altitude was conducted. This analysis was focused on the incongruity between a substantial volume of recent work that suggests significant fuel savings by taking delay at higher altitude and two counter-arguments: that the calculations that indicate such fuel savings are dependent on particular operational procedures; and that taking delay at high altitude may have a greater negative effect on the environment. A synthesis of this work suggests that delay at high cruise altitudes, including above 30,000 feet in the northern hemisphere, should be avoided, and that delay at somewhat lower cruise altitudes should be planned to be taken closer to the descent point and at lower speed than typically used for cruise flight. Moreover, some work has pointed to the possibility that supersonic aircraft may be re-introduced; if so, these aircraft would be affected by these recommendations to a greater degree due to the higher altitude at which they cruise.