This report assesses how the fuel efficiency of commercial aircraft has developed since their introduction in the 1930s. Existing estimates, such as the oft-cited 70% improvement from the IPCC Special Report on Aviation and the Global Atmosphere, ignore the record of the pre-jet era. Based on bottom-up (micro) and top-down (macro) analyses of aircraft fuel efficiency, it can be concluded that the last piston-powered aircraft were as fuel-efficient as the current average jet. This result was obtained by comparing several large piston-engined aircraft with both old and new jet airliners and was confirmed by the macro analysis, which reveals a sharp increase in fuel consumption per seat-kilometre as piston-engined aircraft were replaced by jet- engined. The last piston-powered airliners were at least twice as fuel-efficient as the first jet-powered aircraft.
Aircraft fuel efficiency is just one of the design parameters of interest to aircraft designers and the market. The common practice of defining future cuts in energy consumption per seat-kilometre in terms of a constant annual percentage reduction is therefore not very accurate. It ignores the fact that current aircraft configurations can never achieve zero fuel consumption. Nor does it take into account that the annual reduction rate is not a constant, but is itself also falling, as clearly demonstrated by both macro and micro analysis. This means that many studies on predicted future efficiency gains are rather optimistic.