A Prescriptive Maintenance Policy for Degrading Units in a Civil Aircraft Context

The goal of this paper is to propose a maintenance policy for a deteriorating system which integrates concepts of modern prescriptive maintenance. In particular, the policy focuses on systems where inspections cannot be performed at all times and repairs/replacements incur high costs. This is the case, for example, of some aviation applications where inspections can be performed when the aircraft is grounded, and interventions such as repairs or replacements may necessitate specialized equipment or incur significant delays. In this paper, we propose a maintenance policy where it is assumed that the system under study must work, with prespecified performances, for a fixed time horizon, at the end of which it is systematically replaced, regardless of its state, for a cheap/negligible price. Within this time horizon, inspections are regularly planned which return the true degradation level of the unit. If a failure is detected at an inspection time, corrective replacements can be carried out immediately, albeit incurring a much higher cost than the preplanned replacement at the end of the mission. Another possible action that the policy can take is to derate the system. This action entails voluntarily reducing the performances of the system (thereby incurring a cost) with the aim of decelerating its degradation process and mitigating the risk of failure. The optimal maintenance policy is defined by optimizing an economic performance criterion. The lifetime of the unit is defined by using a failure threshold model.