OPTIMUM ALTITUDE is defined as the pressure altitude for the best possible fuel efficiency in cruise, at which a given thrust setting results in the corresponding maximum range speed at a given weight and speed. As a result, if flown at a higher or lower pressure altitude than the optimum altitude will decrease the aircraft range. The optimum altitude is not constant and changes over the period of a long flight as atmospheric conditions and the weight of the aircraft change.

STEP CLIMB means that the airplane climbs to about 2000 ft above the optimum altitude and levels off. As fuel is used and weight falls, the optimum altitude will increase to a point where it is again 2000 ft above the airplane’s current level. And thereafter the climb process can be repeated. This is the most common way of operating jets in real life.

• The step climb process can be repeated throughout the cruise and it helps to explain why the cruise altitudes at the end of the flight are higher than at the start. Remaining within 2000 ft of the optimum altitude ensures the range is 99% of the maximum specific range.