Reliability and maintainability (R&M) are often considered to be complementary disciplines. To understand why, consider the equation for inherent
availability (equation 1). Inherent availability reflects the percent of time a product would be available if no delays due to maintenance, supply, etc. (i.e.,
not design-related) were encountered.
where MTBF is the mean time between failure
and MTTR is the mean time to repair
If the product never failed, the MTBF would be infinite and Ai would be 100%. Or, if it took no time at all to repair the product, MTTR would be zero and again the availability would be 100%. As shown in Figure 1, a given level of Ai (see the next section for a discussion of availability) can be achieved with different values of R&M. As reliability decreases, better maintainability is needed to achieve the same availability and vice versa.
This complementary relationship is important because it means that trades can be made between the two requirements when the end objective is a given availability. For example, if achieving a given level of reliability is too costly or technically difficult, it may be possible to achieve a given availability by increasing the maintainability requirement, and vice versa. Also, some reliability analyses, such as the Failure Modes and Effects Analysis (FMEA), provide data needed by the maintainability engineer. If for no other reason than these, the maintainability and reliability engineers must work hand-in-hand to ensure that the product meets the R&M requirements.
Figure 1 -
Different Combinations of MTBF and MTTR Yield the Same Inherent