A simple graphical approach for comparing reliability trends of different units in a fleet

Consider a fleet of repairable systems made up of different line replacement units (LRU) in which the maintenance and repair policy is based on fail-replace-repair-use cycles for all types of LRU within the fleet of repairable systems. We model the failure process of a system by a piecewise HPP model which accounts for random age mix in the pool of LRU. Using the model, we propose a graphical approach for comparing reliability trends of different units in a fleet of repairable systems and between different fleets with identical sets of such units. The applicability of the proposed approach is demonstrated using simulated data as well as actual field data.