|Fig. 1: Feedback loop for Fly-By-Wire Systems. (Source: Wikimedia Commons)|
According to the Bureau of Transportation, approximately one in every three Americans surveyed traveled by air at least once in the 12 months prior to the survey.  As air travel becomes more common for the average American, the need for more efficient airplanes will increase.
The amount of fuel burned during flight is approximately proportional to the amount of drag on the aircraft. Additionally, as weight of an airplane increases, more lift is required to keep the plane in the air, which in turn increases the drag. When thinking about increasing airplane efficiency, engineers often look to reduce drag and weight. 
Aircraft manufacturers have constantly been updating the basic materials that go into the plane. By using new composites aircraft weight can be significantly reduced. Additionally, the invention of the fly-by-wire system has allowed for serious reductions in weight. With fly-by-wire planes are electronically controlled, rather than controlled directly - ensuring a more stable flight. Thus in turn, natural stabilizing components (such as a tail wing) can be reduced.  An outline of the feedback loop for the fly-by-wire system can be seen in Fig. 1. Note that the negative feedback loop minimizes the impact of fluctuations.
In addition to the mentioned fly-by-wire system, one of the largest areas for airplane efficiency improvement is with friction reduction. New coatings are being tested for the outside of aircraft to reduce the coefficient of friction, reducing the drag associated with high speed travel. 
© Reinier Eenkema van Dijk. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 "Airline Passenger Travel," US Bureau of Transportation Statistics, September 2003.
 "Beginner's Guide to Aviation Efficiency," Air Transport Action Group, November 2010.
 "ICAO Environmental Report: Aviation and Climate Change," International Civil Aviation Organization, (2010), Ch. 2.