This paper introduces the concept of a pending-failure state in order to consider usual operating and wearout periods of engineering systems and proposes a 3-state system model. System reliability is evaluated in terms of mean time-to-failure (MTTF) and the ratio of ``transition rate from pending failure to failed'' to ``that from good to pending failure state''. The analysis is rather general and applies to many systems, regardless of mean duration of pending-failure state. The reliability and availability of this system are higher than a 2-state system model having same MTTF and repair rate.
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Khan et al. (1985) studied this question.
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