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MIL-HDBK-338 15 OCTOBER 1984 As illustrated in Figure 5.3.1-1, maintainability can be expressed either as a measure of the time (T) required to repair a given percentage (P%) of all system failures, or as a probability (P) of restoring the system to operational status within a period of time (T) following a failure. Some of the commonly used maintainability engineering terms are portrayed graphically in Figure 5.3.1-2 as a maintainability "function" derived as illustrated for the case where the pdf has a lognormal distribution. Points (1), (2), and (3) shown in the figure identify the mean, median, and maximum corrective time-to-repair, respectively. Points (1), (2), and (3) are defined as follows: (1) Mean Time to Repair, Mct - the mean time required to complete a maintenance action, i.e., total maintenance downtime divided by total maintenance actions for a given period of time, given as: M ct K^Mcti) (5.66) where A.-J = failure rate for the ith repairable element of the item for which maintainability is to be determined, adjusted for duty cycle, catastrophic failures, tolerance and interaction failures, etc., which will result in deterioration of item performance to the point that a maintenance action will be initiated ^cti = average corrective time required to repair the ith repairable element in the event of its failure (2) Median Time to Repair, Mct - the downtime within which 50% of all maintenance actions can be completed (3) Maximum Time to Repair - the maximum time required to complete a specified, e.g., 95%, percentage of all maintenance actions. These terms will be described in more detail in the following sections, in terms of the form that they take, given the statistical distribution of time-to-repair. 5.3.2 STATISTICAL DISTRIBUTIONS USED IN MAINTAINABILITY MODELS A smaller number of statistical distributions is used for maintainability analysis than for reliability analysis. This may be due to the fact that maintainability has traditionally lagged reliability theory in development. The most commonly used distributions for maintainability analysis have been the normal, lognormal, and exponential. In fact, as the exponential distribution has been the one most widely used in reliability analysis of 5-40

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