The CoDeF structure: A way to evaluate Ai including failures caused by multiple minor degradations

Recent developments in the Logistic Engineering field are expanding the range of analysis for Operational Availability (Ao) from purely statistical models to include prognostic models. The prognostic method allows to obtain data on otherwise costly to test failure mechanics, while also providing information on the component degradation physics and their performance alteration. Failure in a system is commonly considered to be dependent on the detected malfunctioning of one or more items, as per the standard FMECA approach. A system failure though may also be caused by the concurrent degradation of multiple items performances which brings the system performance below a critical level. The performance degradation of those items is insufficient to trigger an alarm, which leads to cases of system failures with all components nominally in a working state. By using the degradation data obtained by the new prognostic models this paper introduces an analysis (named CoDeF) alternative to the RBD (Reliability Block Diagram). This approach is potentially able to take into consideration this latter type of system failure in the determination of the system Ai.