Fuel takes an important part in preflight planning. An airplane needs enough fuel to reach its destination, and this fuel contributes to the weight carried by the airplane. A propulsion system generates thrust in order to move an airplane through the air. The amount of thrust is an important factor, but the amount of fuel used to generate that thrust is more important because the airplane has to carry the fuel throughout the flight. Jet aircraft efficiencies are improving. There was a 55% overall fuel efficiency gain between 1960 and 2000. Most of the improvements in efficiency were gained in the first decade when jet craft first came into widespread commercial use.