MTTR
MTTR (mean time to repair) is the most common maintainability calculation performed. It is the average time to it takes to repair a failed unit and return it to an operational state. It is calculated by summing the average time to perform corrective maintenance on all replaceable items (RIs) in the unit. MTTR is also referred to as MCT (mean corrective time). Only corrective maintenance is considered in the MTTR calculation. Periodic maintenance, which includes preventive maintenance and inspections, are not considered in the MTTR calculation. Units for MTTR values are typically hours or minutes.
If the Maintainability model is used with the Prediction module, MTTR values calculated for maintainability predictions can be shared with reliability predictions. When MTTR is available during reliability prediction calculations, it can be used to calculate availability, which is the probability that an item is in operating property at a specific point in time. Availability, like reliability, is a time-based probability metric with a value between 0 and 1. It is a function of both reliability (how quickly a system fails) and maintainability (how quickly the system can be repaired). Because the Maintainability and Prediction modules are integrated, all of these calculations can easily be run.
The calculation method for MTTR follows:
1. For the first RI, calculate the sum of the FR* Percentage * Time for all FD&I outputs.
2. For the first RI, calculate the sum of the FR* Percentage for all FD&I outputs.
3. Get the Mean Repair Time for the first RI by dividing the results of step 2 into the results of step 1.
4. Calculate the sum of the Mean Repair Time (from steps 1 through 3) * FR for each RI.
5. Calculate the sum of the FRs for each RI.
6. Get the MTTR by dividing the results of step 5 into the results of step 4.
If the MTTR for an assembly or part is specified in the
Calculation Data window for the Prediction module, the specified MTTR is used in all other maintainability calculations and ignores any entered maintainability data for calculating MTTR. However, an exception exists Procedure 5A calculations are being performed. For more information, see
MTTR Calculations per Procedure 5A.
When a part or assembly has a specified MTTR (and 5A Procedure is cleared), no tasks or FD&I outputs need to be set. It is assumed that when MTTR is specified, other maintainability calculations, such as maximum corrective maintenance time, and mean preventive maintenance time, are not needed.
Also, having a specified MTTR of 0 is different than having the MTTR set to calculated but with no tasks or FD&I outputs. When MTTR is specified, it is assumed that it is a corrective action and included in the total MTTR calculations. However, when MTTR is calculated but no corrective maintenance items are assigned, it is not included in the total MTTR calculations.