Maintainability, Maintenance, and Availability
Maintainability basics
Maintainability is the probability that a failed item can be restored within a stated time using stated resources. It is measured by mean time to repair (MTTR), repair rate mu = 1/MTTR, and the maintenance time distribution (often lognormal). Traps: confusing maintainability (ease of repair) with maintenance (the act), and mixing MTBF with MTTR.
Availability
Inherent availability A_i = MTBF / (MTBF + MTTR), assuming only corrective maintenance. Operational availability includes logistics and administrative delay: A_o = uptime / (uptime + downtime). Know which formula the question calls for; using inherent when logistics delay is described is a classic trap.
Maintenance strategies
- Reactive (run to failure): acceptable for non-critical, low-cost items.
- Preventive (time-based): good for wear-out modes; can induce failures if overdone.
- Predictive (condition-based): uses vibration, oil analysis, thermography; targets random and developing faults.
- Reliability-centered maintenance (RCM): selects strategy by failure consequence and mode. Know that preventive maintenance is effective against wear-out, not random failures.
Worked example
MTBF = 500 hours, MTTR = 5 hours: A_i = 500/505 = 0.9901. If logistics delay adds 5 hours, A_o = 500/(500 + 5 + 5) = 0.9804. Small downtime additions cause noticeable availability loss.
Spares and logistics
Spares requirements depend on failure rate, repair time, and desired service level; Poisson models estimate spare demand. Avoid assuming one spare per installed unit.