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Reliability Issues for DoD Systems: Report of a Workshop
The Crow Paper
A second system that also uses early reliability assessments to improve system design and development is the Integrated Reliability Growth Strategy (IRGS), currently in use at General Dynamics Advanced Technology Systems and several other institutions. IRGS, which was outlined at the workshop by its primary developer, Larry Crow, is a process that generates early and substantial reliability growth through continuous testing and assessment to determine which of a system’s components have a mature or immature design. On this basis, IRGS directs design modifications of the immature components. The result is a reliability growth program, iterating between design modification and testing, that tends to reduce substantially the time needed to achieve reliability goals, for example, to attain a required reliability level before entering operational test.
Permitting a system to enter late-stage developmental test with a substantial number of reliability flaws places too heavy a burden on developmental and operational test to discover the remaining problems. This is also an expensive way of discovering defects since it is likely that the system will experience difficulties in operational test, and it may have to undergo design modifications and later repeat some operational test events. Today, it is not uncommon for some DoD systems to enter into late-stage developmental test when their reliability is at 30 percent of the ultimate goal, whereas the goal for industrial applications is for a system to be at 75 percent of its eventual reliability before entering into formal testing. The latter is accomplished by identifying design flaws in earlier stages of the development process, thereby producing a mature system design much earlier. Again, the overall change in strategy is based on modifying the function of reliability assessment from that of a statistic used to support promotion decisions to that of an early and continuing objective measurement (combining a wide variety of types of information) that is used to support system development by helping to identify components in need of redesign or maturation.
IRGS takes as input a system design that supports prototypes with approximately 25–30 percent of the final required reliability. The complete system undergoes a requirements review, including performance requirements and requirements involving the environment, reliability, safety, maintainability, and support. IRGS then categorizes failure modes for complex systems into type A and type B failure modes. Type A modes correspond to components that have mature designs and are unlikely to be