Definitions
Reliability
The ability of an item to perform its required function, under stated conditions, for a stated interval without failure. Note the two qualifiers: conditions and interval. Reliability without a stated duration is not a meaningful statement.
Availability
The proportion of a defined period in which the item was in a state able to perform its required function. Availability combines failure behaviour with restoration behaviour into one ratio of time.
How they relate
Inherent Availability = MTBF / (MTBF + MTTR) × 100
MTBF carries the reliability side of that expression and MTTR carries the maintainability side. Because they appear together, availability can be held up by fast repairs even when reliability is deteriorating. That is exactly why availability alone is a weak health indicator.
Worked example
Two machines, each observed over 720 hours.
- Machine X: 36 stoppages, 0.5 hours each, 18 hours downtime total.
- Machine Y: 1 stoppage, 18 hours long, 18 hours downtime total.
Both: Availability = 702 / (702 + 18) × 100 = 97.50%
Identical availability, very different assets. Machine X interrupts the process 36 times — each restart carrying quality risk, operator load and possible product loss — and its MTBF is roughly 19.5 hours. Machine Y ran 702 hours failure-free before one long outage that could be planned around. Reporting only availability would hide the distinction completely.
Common mistakes
- Using the words interchangeably. They are different properties and lead to different actions.
- Setting an availability target and calling it a reliability programme. Availability can be defended with faster repairs while failure frequency worsens.
- Quoting reliability without an interval. “95% reliable” means nothing until you say over what duration and under what conditions.
- Confusing inherent and observed availability when comparing design expectations against plant performance.