Full definition
Mean Time To Repair — the maintainability metric representing the average time to diagnose, repair, and return a failed system to full operation: MTTR = Total Repair Time / Number of Repair Events. Lower MTTR indicates better maintainability and faster return to production. MTTR includes: fault detection and diagnosis time, spare parts acquisition/retrieval, physical repair labor, testing and verification, and restart/commissioning. Industrial targets: critical equipment <4 hours (requires pre-positioned spares, documented procedures, trained crew on-site), general equipment <8 hours. MTTR is directly influenced by: maintenance crew skill and training (cross-training reduces wait for specialists), spare parts availability (the single largest contributor to MTTR — waiting for parts is often 60-80% of total downtime), equipment design for maintainability (accessibility, modular/cartridge components, standardized fasteners), and quality of maintenance documentation (procedures, drawings, torque values). MTTR reduction strategies: critical spare parts inventory, pre-planned repair kits, modular design (swap assemblies, repair offline), CMMS with procedures and parts lists, and condition monitoring (predictive maintenance gives advance notice to prepare). Per IEEE 1413 and ISO 14224. MTTR combines with MTBF to determine availability: A = MTBF / (MTBF + MTTR).