 Originally, a simple "cruise climb" was used by pilots.This amounted to a simple, continuous, very gradual climb from an initial cruise altitude to a final cruise altitude, and made the most efficient use of fuel.However, with increasing air traffic and the assignment of distinct flight levels to specific flights, airways, and directions of flight, it is no longer safe to climb continuously in this way, and so most flights compromise by climbing in distinct steps—a "step climb"—with ATC approval, in order to ensure that the aircraft is always at an appropriate altitude for traffic control.