In maintainability analysis, the exponential distribution applies to maintenance tasks and maintenance actions for which completion times are independent of previous maintenance experience (e.g., substitution methods of failure isolation in which several equally likely alternatives are available and each alternative is exercised, one at a time, until the one which caused the failure is isolated), producing a probability density function given by:
The method used in evaluating the maintainability parameters is similar to that used for analyzing reliability with exponential times-to-failure. The fundamental maintainability parameter is the repair rate, m, which is constant for the exponential distribution. It is the reciprocal of M ct , the mean-time-to-repair (MTTR). Thus, another expression for g(t) in terms of m, the repair rate, is:
g (t) = µ e-µt (Equation 18)
The maintainability function, M(t), is given by:
The MTTR is given by:
If the maintainability function, M(t), is known, the MTTR can also be obtained from:
The median time to repair Ä M ct is given by:
The maximum time to repair is given by:
ke = value of Mcti/ct at the specified percentage point a on the exponential function at which MMaxct is defined.
Values of k e are shown in Table XV.
Table XV - Values of k sub e for Specified