Availability and Maintainability

Availability measures the fraction of time a system is operational. Maintainability measures how quickly a system can be restored after failure. Together with reliability, they form the RAM (Reliability, Availability, Maintainability) triad.

Availability formula

Inherent availability: Aᵢ = MTBF / (MTBF + MTTR) Where: MTBF = Mean Time Between Failures (hours) MTTR = Mean Time To Repair (hours) Example: MTBF = 500 h, MTTR = 5 h Aᵢ = 500 / (500 + 5) = 500 / 505 = 0.990 (99.0%)

Types of availability

Type	Includes in downtime
Inherent (Aᵢ)	Active corrective maintenance only
Achieved (Aₐ)	Corrective + preventive maintenance
Operational (Aₒ)	All downtime including logistics, supply delays, admin

Maintainability M(t)

M(t) = 1 − e^(−μt) Where μ = repair rate = 1/MTTR Probability of completing repair within time t: μ = 1/5 = 0.20 repairs/hour M(2 h) = 1 − e^(−0.20 × 2) = 1 − e^(−0.40) = 0.330 30.3% probability of completing repair within 2 hours.

CQE exam tips

Higher MTBF improves availability; lower MTTR also improves it — the lever that is cheaper to pull depends on the system.
Operational availability is always ≤ achieved ≤ inherent.
MTTR includes diagnosis time, repair time, and verification time — not just the wrench time.

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