Reliability and availability estimation for complex systems: a simple concept and tool

This paper describes a simple modeling concept and an associated computerized tool for estimating complex repairable availability. The method is intended as a first step in computing dependability metrics for making architecture choices during conceptual system design. The modeling approach is based on classifying element failures as permanent, intermittent and transient and treats both hardware and software failures. It comprises a combination of the top-down reliability block diagram to represent system hierarchy, a simplified Markov chain representation of equipment state transitions, and conditional probability definitions to represent dependence between equipment levels. The key concept is the partitioning of the failure rates in correspondence to the failure types. This permits simpler computation and as a result the method is implemented as a computerized analysis tool in the form of a customized Microsoft Excel menu