What is mean time to repair?
Mean time to repair (MTTR) is the average time required to troubleshoot and repair failed equipment and return it to normal operating conditions. It is a basic technical measure of the maintainability of equipment and repairable parts. Maintenance time is defined as the time between the start of the incident and the moment the system is returned to production. For example, how long the equipment is out of production. This includes notification time, diagnostic time, fix time, wait time (cool down), reassembly, alignment, calibration, test time, back to prod etc. It generally does not take into account lead-time for parts. MTTR reflects how well an organization can respond to a problem and repair it.
How is MTTR calculated?
Expressed mathematically, it is the total maintenance time divided by the total number of maintenance actions over a specific period.
Over the lifetime of an asset, each failure will vary depending on the severity of the issue. Some issues will require a simple parts swap, while others could take days to diagnose and repair. The frequency vs repair time plot follows the lognormal distribution. We will have a large number of repairs that are quick to repair, and a small number that take much longer.
What does it mean for your organization?
For mission critical equipment, MTTR can have a dramatic effect on the organization’s bottom line. Taking too long to repair equipment could lead to missed orders and soured business relationships. To limit the impact of MTTR, organizations have their own maintenance teams hold spare parts onsite, or run parallel production lines.
How can you measure MTTR?
Every time a system is logged down for repair, the timestamp is logged in the database. The CMMS tracks the repair time until the system is returned online back to production. It is then possible to run the MTTR reports to see how it trends over time. If you break up the downtime further into sub components such as waiting technician, waiting part, under repair etc. you can extract a very accurate calculation of MTTR.
What can MTTR tell you?
Prediction of the number of hours that a system or component will be unavailable whilst undergoing maintenance is of vital importance in reliability and availability studies. MTTR yields a lot of information that can help reliability engineers make informed decisions such as repair or replace, hire, optimize maintenance schedules, or store parts onsite. For example, as a system ages, it may take longer to repair. MTTR will trend upwards prompting the repair vs replace decision.
For an accurate calculation of MTTR, however, we must make the following assumptions:
- One technician performs all tasks sequentially.
- Appropriately trained personnel perform the maintenance.
You can also use MTTR to predict performance or the life cycle cost of new systems. Equipment manufacturers are now using a modular design philosophy so parts or subassemblies can be swapped out quickly and easily. Consider being faced with a purchasing decision that involves two similar systems - one has a higher MTTR because repairable items are difficult to remove due to their location. The additional time and costs to maintain should be factored into the life of the system to simplify the purchasing decision.