WASHINGTON — The auto industry isn't the only one trying to boost mileage. Airplane fuel-efficiency is improving too, according to the United States Office of Technology Assessment.Since 1970, airlines have doubled their efficiency, measured in seat-miles per gallon, by raising load factors, using more-efficient engines, and improved aerodynamics, the OTA notes in a report on "Energy Technology Choices," released in August. Technology continues to improve, the report notes. "Ultrahigh-bypass" turbofan engines send as little as 15 percent of the air entering the engine shroud through the combustor. The rest is accelerated by fans driven by the turbine engine. Advanced materials and designs promise to further improve engine efficiency, and to reduce aircraft frame weight by as much as 30 percent.