Cost Based Risk Analysis to Identify Inspection and Restoration Intervals of Hidden Failures Subject to Aging

This paper develops a cost rate function (CRF) to identify the optimum interval and frequency of inspection and restoration of aircraft's repairable components which are undergoing aging, and whose failures are hidden, i.e. are detectable by inspection or upon demand. The paper considers two prevalent strategies, namely Failure Finding Inspection (FFI), and a combination of FFI with restoration actions (FFI+Res), for both the “non-safety effect,” and the “safety effect” categories of hidden failures. As-bad-as-old (ABAO) inspection effectiveness, and as-good-as-new (AGAN) restoration effectiveness are considered. In case of repair due to findings by inspection, as-bad-as-old repair effectiveness is considered. The proposed method considers inspection and repair times, and takes into account the costs associated with inspection, repair, and restoration; and the potential losses due to the inability to use the aircraft (maintenance downtime). It also considers the cost associated with accidents caused by the occurrence of multiple failure. The approach used in this study for risk constraint optimization is based on the mean fraction of time during which the unit is not functioning within inspection intervals (MFDT), and the average interval unavailability behavior within the restoration period. In the case of an operational limit, when it is not possible to remove the unit for restoration, or one needs to use the unit longer than the expected operating time, the paper introduces an approach to analyzing the possibility of and conditions for providing an extension to the restoration interval that satisfies the risk constraints and the business requirements at the same time.