The choice of airspeed and altitude have a far more critical effect on economy of operation throughout the flight of turbo-jet aircraft than with the piston-engined type. The method of operating piston-engined aircraft for maximum range is well known, though it is normally only used on very long stages when fuel is critical; generally constant-power or constant-airspeed cruising, which result in a small penalty of fuel or payload, are preferred. The choice of operating height is also fairly wide within certain obvious limits. With jet aircraft, however, airspeed and operating height become critical and variations from economic operation reduce the payload or range considerably. The preparation of the flight plan, involving the choice of cruising altitude and speed, therefore becomes of major importance to the jet operator and navigator. The guidance of the aircraft's flight path in the vertical plane, in other words, takes on a new significance and affects the navigator's task both in planning and in flight.