In the design of a civil aircraft the condition of steady level cruise is of prime importance because improved fuel economy in this flight regime makes a direct and valuable contribution to the reduction of operating costs. Performance in the climb is often less important, but it cannot be ignored because a climb is always needed to reach the required cruising height after take-off and Air Traffic Control may also require the aircraft to change height during the cruise. For military aircraft, performance in the climb may be a primary design requirement because there is often a need to reach a specified height and speed in the shortest possible time, either from take-off or from some other prescribed initial conditions of height and speed.

The quantities that are of most interest in calculations of climbing performance are the rate of climb VC = V sin γ and the time required and fuel used in climbing from one specified height to another. In many cases there is a change of speed during the climb, so that the aircraft is accelerating, but it will be shown that a correction can easily be made for the effect of the acceleration on the rate of climb. The angle of climb is also of some interest, although it is important mainly at low altitudes where there may be obstacles to be cleared or where a large angle of climb may be required for reasons of noise abatement.