Availability
How does Availability differ from Reliability?
Availability measures access/uptime (can I use it?), while Reliability measures consistency without failure over time (will it work continuously?).
Can Availability be 100%?
Practically no; even 'five nines' (99.999%) systems have ~5 minutes of annual downtime.
Why is Availability a function of Reliability and Maintainability?
Because an asset must not fail (Reliability) and be quickly repaired (Maintainability) to remain available.
In reliability engineering and CMMS, Availability is the probability that an asset is operable and committable at the start of a mission when called at a random time, calculated as uptime divided by total time (uptime + downtime). It is expressed as a percentage, such as 99.9%, representing the ratio of time an asset is operational versus its total scheduled operation time.
On the shop floor, Availability tracks whether equipment is ready to run when needed, regardless of performance while running. In CMMS, it is calculated using MTBF and MTTR via the formula Availability = MTBF / (MTBF + MTTR). Maintenance teams use this metric to prioritize reduce-downtime strategies, such as improving repair speed or extending failure intervals, to ensure assets are operational when required for production.
Ignoring planned downtime: Treating only unplanned downtime as a failure inflates Availability and masks reliability issues.
Over-relying on high Availability without assessing performance: An asset may be available but running inefficiently or producing defective output, misleading reliability assessments.
Neglecting MTTR reduction: Failing to improve repair processes keeps MTTR high, directly lowering Availability even if MTBF is strong.