Key Maintainability Indicators

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Written By Functional Safety Expert

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In the context of reliability engineering, maintainability is a crucial element. It refers to the ease with which a system can be maintained or repaired when a failure occurs. Unlike maintenance, which is an operational activity carried out during the operational lifecycle of a system, maintainability is integrated from the design phase to minimize downtime and maximize system availability throughout its use.

Maintainability directly influences the availability of a system, making it a key component of functional safety. By optimizing maintainability from the design phase, not only is the time needed to repair a failure reduced, but it is also ensured that the system is ready to resume operations as quickly as possible. This article presents five essential indicators that allow for the assessment of a system’s maintainability from its design.

  1. MTBF (Mean Time Between Failures):
    • Definition: MTBF measures the average time between two successive failures of a piece of equipment. During the design phase, it quantifies the reliability of a system. The higher the MTBF, the less likely the equipment is to fail frequently.
    • Calculation:
      • MTBF = Total Operational Time / Number of Failures
    • Example: If a machine operates for 1,000 hours and fails 4 times, the MTBF would be 250 hours, indicating a failure occurs on average every 250 hours.
  2. MTTR (Mean Time To Repair):
    • Definition: MTTR is an indicator of maintainability. It measures the average time required to repair a system after a failure. A low MTTR indicates good system design, allowing for quick and efficient repairs.
    • Calculation:
      • MTTR = Total Repair Time / Number of Failures
    • Example: If a system fails 3 times, and each repair takes 2, 3, and 4 hours respectively, the MTTR would be 3 hours, reflecting how quickly failures are typically resolved.
  3. MDT (Mean Down Time):
    • Definition: MDT measures the total time a system is out of service, including both wait time and repair time. It is a key indicator that shows the overall duration of system unavailability after a failure.
    • Calculation:
      • MDT = Total Downtime / Number of Failures
    • Example: If a system has been down for 40 hours due to 5 failures, the MDT would be 8 hours on average per failure.
  4. MUT (Mean Up Time):
    • Definition: MUT is a complementary indicator to MTBF. It measures the time during which a system is operational and available before a new failure occurs. It assesses the system’s capacity to function without interruption.
    • Calculation:
      • MUT = Total Operating Time / Number of Operating Periods
    • Example: If a machine operates for 1,200 hours spread over 6 operating periods, the MUT would be 200 hours, indicating the system operates for an average of 200 hours before each stop.
  5. Preventive vs. Corrective Maintenance Rate:
    • Definition: This rate compares the proportion of preventive maintenance interventions (planned) to corrective maintenance (unplanned). In design, the goal is to minimize corrective maintenance by implementing suitable preventive maintenance strategies.
    • Calculation:
      • Preventive/Corrective Rate = Number of Preventive Interventions / Number of Corrective Interventions
    • Example: If a team performs 12 preventive interventions and 6 corrective interventions, the rate will be 2, meaning that for every failure requiring repair, two preventive actions were taken to prevent future failures.

Maintainability is an essential component of functional safety as it directly influences the availability of systems. Unlike maintenance, which is executed during the operational phase, maintainability is an inherent property of the system introduced during the design phase to ensure quick and effective maintenance throughout its lifecycle.

Indicators such as MTBF, MTTR, MDT, MUT, and the preventive/corrective maintenance rate allow engineers to design systems that are not only reliable but also maintainable. These indicators provide valuable tools to predict a system’s behavior in case of a failure and minimize downtime, thereby ensuring continuous and effective service.

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