Reliability prediction models to support conceptual design

During the early stages of conceptual design, the ability to predict reliability is very limited. Without a prototype to test in a lab environment or without field data, component failure rates and system reliability performance are usually unknown. A popular method for early reliability prediction is to develop a computer model for the system. However, most of these models are extremely specific to an individual system or industry. This paper presents three general procedures (using both simulation and analytic solution techniques) for predicting system reliability and average mission cost. The procedures consider both known and unknown failure rates and component-level and subsystem-level analyzes. The estimates are based on the number of series subsystems and redundant (active or stand-by) components for each subsystem. The result is a set of approaches that engineers can use to predict system reliability early in the system-design process. Software was developed (and is discussed in this paper) that facilitates the application of the simulation-based techniques. For the specific type of system and mission addressed in this paper, the analytic approach is superior to the simulation-based prediction models. However, all three approaches are presented for two reasons: (1) to convey the development process involved with building these prediction tools; and (2) the simulation-based approaches are of greater value as the research is extended to consider more complex systems and scenarios     

Evaluating the impact of cannibalization on fleet performance

Organizations that utilize fleets of expensive repairable equipment are faced with numerous challenges related to the specification of maintenance policies and the allocation of maintenance resources. The associated maintenance decisions can have a drastic impact on fleet performance. Due to the significant acquisition costs associated with the components that comprise the units of equipment in the fleet, cannibalization is often used in the absence of available spare parts to enable fleet maintenance managers to satisfy fleet performance constraints such as readiness requirements. This research is focused on the development and analysis of a closed‐network, discrete‐event simulation model that is used to assess the impacts of cannibalization, small spare parts inventories and maintenance‐induced damage on a fleet of systems. Using numerical examples, we demonstrate the comparison between cannibalization and the investment in limited spare parts inventories. We evaluate fleet performance using average readiness and total maintenance cost. Specifically, we demonstrate that investments in spare parts inventories can reduce the need for and value of cannibalization. However, our results also support the use of cannibalization as a low‐cost alternative to investing in expensive spare parts. We also explore the impact of damage induced by maintenance on fleet performance. Specifically, we demonstrate that maintenance‐induced damage can reduce the benefit of cannibalization while drastically increasing maintenance expenditures. Copyright © 2006 John Wiley & Sons, Ltd.