Implied service strategies in availability assessments

System availability is often calculated using simple equations, such as A = /sub MTBF+MTTR/-/sup MTBF/, for steady-state availability. Use of this equation has implicit maintenance strategies and capabilities that are sometimes not recognized by the analyst. That is, for each time this equation is used in a system model, it implies another simultaneous service (maintenance) action can be performed. If it is not possible to have more than one service action at a time, then errors are introduced. The magnitude of these errors depends on the failure rates, restoration rates and model configurations (logic). Approximation will always be a useful and important tool in the reliability engineer's toolbox, but it is important to know which approximation should be used and whether it yields conservative or optimistic results. This paper will help the engineer understand the impact of some common approximations used in calculating availability.