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Maintainability and Availability

1 tool in the Reliability area.

Maintainability and Availability

Maintainability and availability of a repairable system from its maintenance tasks: each task carries a failure rate or MTBF, a repair-time distribution (lognormal, normal or exponential, by its mean or median and spread), a crew size and a logistics delay. The page gives the failure-rate-weighted MTTR of MIL-HDBK-470A and MIL-HDBK-338B, the median and the 90 % and 95 % maximum repair times of the mixture (exact per task, numerical inversion for the mix), the maintainability function, the maintenance man-hours per operating hour, the system MTBF and mission reliability, and the three availabilities side by side: inherent A_i = MTBF/(MTBF + MTTR), achieved A_a with preventive maintenance through MTBM and the mean active maintenance time, and operational A_o with the mean downtime (repair, logistics and administrative delays). A redundant k-of-n group with r repair channels is solved as a continuous-time Markov chain: steady-state availability by Gaussian elimination and the closed product form, mean time to first failure, failure frequency, mean up and down times, and the instantaneous availability A(t) by Runge-Kutta integration. Spares are sized from the Poisson demand over the lead time for an entered fill rate, with expected backorders, and a maintainability demonstration test is planned by MIL-HDBK-470A Appendix B Test Method 1 (sample sizes under the lognormal and distribution-free plans, acceptance threshold and operating-characteristic curves) with the Test Method 9 criteria. The default case is the five-subsystem series system of MIL-HDBK-338B Figure 5.8-1 (MTBF 12.9 h, MTTR 4.26 h, A_i 0.752), a 1-of-2 redundant pair with one repair crew, and the handbook's 30 / 45 minute demonstration example (n = 56 and 43).

1 worked example reproduced, listed on the tool page.

Other Reliability sections

Life Data · System Models · Growth and Testing · Physics of Failure · Safety Analysis