Statistics for Maintenance Modelling: Repairable Systems

# Availability¤

Reliability and maintainability are combined in the term availability which is the proportion of 'up' time. The long-term or steady-state availability, , is given by:

= .

It should be noted that provides no information about the number of repairs. It is most likely that 'down' time involves more than the time to repair. Factors might include the time to diagnose, spare part location and testing. For example, the expression for availability therefore often involves mean downtime (MDT) rather than the MTTR, so is revised as:

=

The unavailability, is given by 1 - .