Full definition
A maintainability metric representing the average time required to diagnose, repair, and restore a failed system to full operating condition, calculated as: MTTR = Total Repair Time / Number of Repairs. Lower MTTR indicates better maintainability — faster return to production. MTTR includes: fault diagnosis time, spare parts procurement/retrieval, actual repair labor, testing and verification, and restart. It excludes: administrative and logistics delays in some definitions (active repair time only) or includes them in others (total downtime — clarify the definition used). Typical targets: critical equipment MTTR <4 hours (requires pre-positioned spares, trained crew, and documented procedures), general equipment <8 hours. Strategies to reduce MTTR: maintain critical spare parts inventory on-site (safety stock), pre-plan repair procedures (standard work instructions), train maintenance crews on common failure modes, use modular/cartridge-design components (swap entire assemblies rather than repair in place), and implement CMMS work order system with parts lists. MTTR directly impacts production availability: A = MTBF / (MTBF + MTTR). Even with high MTBF, a long MTTR reduces availability. Per IEEE 1413 and ISO 14224 (petroleum/petrochemical reliability data).