Any technological change may involve a balance among a range of environmental impacts. The majority of this gain has been achieved through engine improvements and the remainder from airframe design improvement. The 2050 scenarios developed for this report already incorporate these fuel efficiency gains when estimating fuel use and emissions. These aspects have not been adequately characterized or quantified in this report. Assuming that the goals can be achieved, the transfer of this technology to significant numbers of newly produced aircraft will take longer typically a decade.
Research programs addressing NO x emissions from supersonic aircraft are also in progress.
Fuel Options There would not appear to be any practical alternatives to kerosene-based fuels for commercial jet aircraft for the next several decades. Reducing sulfur content of kerosene will reduce SO x O emissions and sulfate particle formation. Jet aircraft require fuel with a high energy density, especially for long-haul flights. Other fuel options, such as hydrogen, may be viable in the long term, but would require new aircraft designs and new infrastructure for supply. Hydrogen fuel would eliminate emissions of carbon dioxide from aircraft, but would increase those of water vapor.