Aircraft have been equipped with Auxiliary Power Units (APUs) to provide compressed air and shaft power. The Hamilton Standard APU for the Lockheed TriStar airplane represents a new and different design concept that will result in major improvements in operating economy and reliability to the airline operators. Design and development stages of the APU are presented in this paper. The free-turbine engine design offers excellent full- or part-time economy and efficiency while operating at high engine-cycle pressure ratios. This APU also offers the concept of automatic paralleling of electrical equipment.