Technology
General science and technology subjects, particularly in the area of energy, have normally been considered "on-topic" on this board.
Airline engine manufacturers pump enormous amounts of money into efficiency and weight in jet engines. The engines used by modern planes are light-years ahead of the ones on the last generation of jet airliners.
There has been research into hydrogen for airliners, but the fuel storage (LH2 is the only practical means) is a real problem.