Since the early days of jet aircraft and commercial travel, the technique of gradually climbing in cruise altitude as fuel burns off and the aircraft becomes lighter has been widely used by pilots. The altitude that provides the most fuel-efficient cruise (at the desired speed) at the start of a long flight, when the aircraft is fully loaded with fuel, is not the same as the altitude that provides the best efficiency at the end of the flight when most of the fuel aboard has been burned. This latter altitude is usually significantly higher than the former.