An Auxiliary Power System includes an APU controller and an aircraft load controller which include an APU load capacity algorithm which provides an uncomplicated and robust method of APU load control. The APU load capacity algorithm defines three load zones: a continuous APU load capacity zone (zone A); a time controlled zone (zone B); and a fault zone (zone C). Each zone is related to altitude to the service ceiling of the APU to provides consistent APU load capacity that addresses APU performance variation and deterioration effects to an end of the APU useful life.