Abstract A system such as missiles and spare parts of aircrafts has to perform a normal operation in a severe environment at any time when it is used. However, missiles are in storage for a long time from the delivery to the usage and its reliability goes down with time. Thus, a system in storage should be inspected and maintained at periodic times NT (N=1,2,…) to hold a higher reliability than a prespecified value q. The following inspection model is considered: A system has two types of units, where unit 1 is maintained and unit 2 is not done. A system is also overhauled if its reliability becomes equal to or lower than q. The number N ∗ of inspections and time ( N ∗ T+t 0 ) until overhaul are derived. Using these results, the average cost C(T) is obtained and an optimal inspection time to minimize C(T) is discussed.
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