Inspection Intervals for Condition-Maintained Items Which Fail in an Obvious Manner

Replacement theory suggests items with non-increasing hazard rate should be left to fail. This ignores the possible value of inspections to obtain warnig of pending failure when restorative maintenance can be performed more cheaply than failure repair. Equations are developed for inspection schedules for items with known base distribution of self-announcing failures. In optimization, two simplifying assumptions are made: 1) Equal conditional probability of failure in any interval between inspections, given survival to start of interval (equal interval-risk) gives a near-optimal schedule. 2) The fraction of failures prevented by inspection is a function only of the interval-risk under failure-only maintenance. Results tend to justify traditional schedules and urge better supervision of maintenance.