TRENDS IN ENERGY USE
Fuel efficiency gains due to technological and operational change can mitigate the influence of growth on total emissions. Increased demand has historically outpaced these gains, resulting in an overall increase in emissions over the history of commercial aviation. The figure of merit relative to total energy use and emissions in aviation is the energy intensity (EI). When discussing energy intensity, the most convenient unit of technology is the system represented by a complete aircraft. In this section, trends in energy use and EI are elaborated. In the following section, the discussion focuses on the relation of EI to the technological and operational characteristics of an aircraft.
Reviews of trends in technology and aircraft operations undertaken by Lee et al. and Babikian et al. indicate that continuation of historical precedents would result in a future decline in EI for the large commercial aircraft fleet of 1.2-2.2%/year when averaged over the next 25 years, and perhaps an increase in EI for regional aircraft, because regional jets use larger engines and replace turboprops in the regional fleet. When compared with trends in traffic growth, expected improvements in aircraft technologies and operational measures alone are not likely to offset more than one-third of total emissions growth. Therefore, effects on the global atmosphere are expected to increase in the future in the absence of additional measures. Industry and government projections, which are based on more sophisticated technology and operations forecasting, are in general agreement with the historical trend. Compared with the early 1990s, global aviation fuel consumption and subsequent CO2 emissions could increase three - to sevenfold by 2050, equivalent to a 1.8-3.2% annual rate of change. In addition to the different demand growth projections entailed in such forecasts, variability in projected emissions also originates from different assumptions about aircraft technology, fleet mix, and operational evolution in air traffic management and scheduling.
Figure 2 shows historical trends in EI for the U. S. large commercial and regional fleets. Year-to-year variations in EI for each aircraft type, due to different operating conditions, such as load factor, flight speed, altitude, and routing, controlled by different operators, can be +30%, as represented by the vertical extent of the data symbols (Fig. 2A). For large commercial aircraft, a combination of technological and operational improvements led to a reduction in EI of the entire U. S. fleet of more than 60% between 1971 and 1998, averaging about 3.3%/year. In contrast, total RPK has grown by 330%, or 5.5%/year over the same period. Long - range aircraft are ~5% more fuel efficient than are short-range aircraft because they carry more passengers over a flight spent primarily at the cruise condition. Regional aircraft are 40-60% less fuel efficient than are their larger narrow - and wide-body counterparts, and regional jets are 10-60% less fuel efficient compared to turboprops. Importantly, fuel efficiency differences between large and regional aircraft can be explained mostly by differences in aircraft operations, not technology.
Reductions in EI do not always directly imply lower environmental impact. For example, the prevalence of contrails is enhanced by greater engine efficiency. NOx emissions also become increasingly difficult to limit as engine temperatures and pressures are increased—a common method for improving engine efficiency. These conflicting influences make it difficult to translate the expected changes in overall system performance into air quality impacts. Historical trends suggest that fleet-averaged NOx emissions per unit thrust during landing and takeoff (LTO) cycles have seen little improvement, and total NOx emissions have slightly increased. However, HC and CO emissions have been reduced drastically since the 1950s.