Auxiliary Power Unit Evolution - Meeting Tomorrow's Challenges 932541

In a little over thirty-five years the Auxiliary Power Unit (APU) has evolved from a simple onboard source of starting energy to a flight-essential component for today's commercial transports engaged in Extended Range Twin Engine Operations (ETOPS). In the course of this transition, the APU has progressed from a simple turbomachine to one employing comparable technologies to similarly sized propulsion engines. Reliability, troubleshooting, maintainability, and similar characteristics critical to economical airline operation have necessitated progressively increased sophistication in the design and testing of APUs. Peculiar requirements to APUs, such as totally dependable starting at 40,000 feet and above when cold soaked to -65F exceed the challenges of most propulsion engine systems.

This paper addresses the issues facing the APU designer, with particular reference to reliability and maintainability characteristics, major drivers for current APU designs. The paper will highlight some of the special design considerations imposed by the unique environmental and installation demands applicable to today's APUs.