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A glossary of basic terms and definitions useful for working in reliability, maintainability, and sustainability (RMS). The terms used in most engineering technologies tend to be physical characteristics such as speed, rate of turn, and fuel consumption. While they may require very careful definition and control of the way in which they are measured, the terms themselves are not subject to different interpretations. Reliability, maintainability, and sustainability (RMS), however, use terms that are defined in a variety of ways with multiple interpretations. The variety of definitions given to a single term creates problems when trying to compare the performance of one system to another. To eliminate the confusion, a literature search that listed current and past RMS terms and definitions was conducted. The literature search included input from the U.S. military, UK military, NATO, SAE, IEEE, NASA, ISO, university research, and other publications.

Future tactical aircraft will have increased capabilities that will place greater demands on their secondary power systems. Added capabilities such as low observability or internal weapons storage are being planned for without significantly increasing the aircraft's size and weight. The power system must therefore have reduced volume, weight, and complexity, while also being more reliable and maintainable. The auxiliary power unit (APU) is a critical component that must be improved to upgrade the capabilities of the power system. Increasing the APU's power density is one important way for reducing the power system's size and weight. Increased power density, however, will require a power unit operating with higher gas generator temperatures, so this condition will be the major challenge for new APU designs.

Future fighters will require more compact, lighter weight, small gas turbine auxiliary power units (APUs) capable of faster starting, and operation, up to altitudes of 50,000 ft. The US Air Force is currently supporting an Advanced Components Auxiliary Power Unit (ACAPU) research program to demonstrate the technologies that will be required to accomplish projected secondary power requirements for these advanced fighters. The requirements of the ACAPU Program represent a challenging task requiring significant technical advancements over the current state-of-the-art, prominent among which are: Small high heat release high altitude airbreathing combustors. High temperature monolithic ceramic and metallic small turbines. Capability to operate, and transition from non-airbreathing to airbreathing modes. This paper discusses these challenging requirements and establishes technology paths to match and exceed the required goals.

AIR5317 establishes the foundation for developing a successful APU health management capability for any commercial or military operator, flying fixed wing aircraft or rotorcraft. This AIR provides guidance for demonstrating business value through improved dispatch reliability, fewer service interruptions, and lower maintenance costs and for satisfying Extended Operations (ETOPS) availability and compliance requirements.