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.