Aircraft Design: Trading Cost and Climate Impact

Abstract

Commercial aircraft operations impact the atmosphere through emissions of greenhouse gases and greenhouse gas precursors and through the formation of contrails and cirrus clouds. Aircraft are attributed with emitting approximately 2% of all anthropogenic carbon dioxide (CO2), and the total radiative forcing caused by aircraft emissions is a factor of two to four times greater than radiative forcing due to aircraft CO2 emissions alone [IPCC 1999]. Non-CO2 radiative forcing results primarily from emissions of oxides of nitrogen (NOx) and the formation of contrails and cirrus clouds. The magnitudes of climate impacts due to both of these phenomena depend strongly on cruise altitude. Additionally, aircraft fuel burn and NOx emissions rate depend on ight altitude. This study combines altitude variation of fuel consumption, emissions rate, and emissions eects to examine the relative climate impacts of dierent aircraft designs, as measured by the global mean temperature change resulting from 100 years of sustained aircraft operation. A simple time- and altitudevarying climate model is presented and integrated with an aircraft conceptual design code. Optimization studies are performed to compare economic and environmental performance of aircraft congurations. First, we optimize for a minimum-cost conguration, which is used as a reference for comparing performance of a variety of green designs. By optimizing an aircraft for minimum fuel burn or minimum NOx emissions, savings of up to 16% in fuel burn and 50% in NOx emissions are possible compared with a minimum-cost design. However, these aircraft achieve small reductions in global warming impacts of at most 7%. More signicant reductions in global warming impacts can be attained by designing aircraft to y at lower altitudes. A 30% reduction in global warming impacts can be reached by designing aircraft to cruise at about 28,000 ft and Mach 0.75, which increases operating costs by 1 to 2% relative to a minimum cost aircraft. A 50% reduction in global warming impacts can be achieved by aircraft designed to cruise at 25,000 ft and Mach 0.72, with a 1.5 to 3% increase in total operating costs. Up to a 75% reduction in global warming impacts can be attained by designing aircraft to cruise as low as 18,000 ft. Unfortunately, there is considerable uncertainty in estimating the global warming eects of NO x emissions and cloud formation. To address this, studies are performed to examine the sensitivity of results to variation in the magnitude of radiative forcing due to NOx emissions and cloud formation. Regardless of whether these forcings are much greater or less than current best estimates, there is still a potential net global warming impact savings of 20 to 55% possible by designing aircraft to cruise 10,000 ft lower and at a 12% lower Mach number than the minimum-cost conguration.