A functional reliability and safety approach for analyzing complex, aerospace systems

The approach described in this paper replaces the Reliability Block Diagrams (RBD) with a mission event functional architecture that is used to identify and mitigate mission or safety critical functions. The functional architecture is linked to functional test points that are used to demonstrate a reliable and safe product. A product is reliable and safe when the likelihood of mission and safety critical functions are within acceptable levels. This paper describes an approach that promotes both systems thinking and supports data driven decisions based on system performance, in the form of test data assessed against functional limits. The approach is a functional based assessment which is intended to meet the definition of Reliability, which is `The probability that an item can perform its intended function, for a specified interval, under stated conditions, without failure'. The approach decomposes a system into a set of functions and expected behaviors under specified interval and stated conditions. The approach links functions and expected behaviors to requirements in order to use requirement verification results to demonstrate a safe a reliable product. The approach does not rely on physical architecture decomposition, which enables the approach to be applied early in the design development process. The approach can be started as early as concept development to support trade studies or system functional reviews. When verification results become available, the approach uses bootstrapping techniques to determine the probability of performing the assessed function without failure, which is referred to as functional reliability herein. The resulting functional reliability from this approach is used to monitor and isolate adverse statistical behavior to maintain a safe and reliable product.