  Motivated by original equipment manufacturer (OEM) service and maintenance
practices we consider a single component subject to replacements at failure
instances and two types of preventive maintenance opportunities: scheduled,
which occur due to periodic system reviews of the equipment, and unscheduled,
which occur due to failures of other components in the system. Modelling the
state of the component appropriately and incorporating a realistic cost
structure for corrective maintenance as well as condition-based maintenance
(CBM), we derive the optimal CBM policy. In particular, we show that the
optimal long-run average cost policy for the model at hand is a control-limit
policy, where the control limit depends on the time until the next scheduled
opportunity. Furthermore, we explicitly calculate the long-run average cost for
any given control-limit time dependent policy and compare various policies
numerically.
