Getting Started with Windchill Risk and Reliability > Windchill Maintainability > Understanding Maintainability Calculations
Understanding Maintainability Calculations
Windchill Maintainability computes a number of maintenance-related values, such as MTTR (mean time to repair), MCMT (mean corrective maintenance time), and MPMT (mean preventive maintenance time).
A sampling of supported calculations is outlined below and brief descriptions are provided.
* 
Some of these calculations reference preventive maintenance and/or corrective maintenance. Corrective maintenance restores a failed system to operational status by repairing or replacing failed components. Preventive maintenance is the practice of replacing components or subsystems before they fail to promote continuous system operation.
For more information about these calculations, see the guide or help for Windchill Risk and Reliability.
Calculation
Description
Maintainability index
The total maintenance effort, including both preventive and corrective maintenance, that is required to maintain a product in operational status during a specific time period. The maintainability index is calculated based on MIL-HDBK-472 Procedure 2. This calculation uses the values from both (1) and (2) of the MAMT (mean active maintenance time) calculation.
MTTR
The average time it takes to repair a failed unit and return it to an operational state, based on the average repair time for its replaceable components. The MTTR (mean time to repair) is calculated by rolling up the average repair times for lower-level components to compute subassembly and system repair values. Units for MTTR values are typically hours or minutes. MTTR is also referred to as MCT (mean corrective time).
* 
If you use this module in conjunction with Windchill Prediction, you can use the Maintainability-computed MTTR values for components in your reliability prediction.
Availability
The probability that a system is operating properly at a specific time point. Availability, like reliability, is a time-based probability metric between 0 and 1. Availability is a function of both reliability (how quickly a system fails) and of maintainability (how quickly the system can be repaired).
Mean corrective maintenance time (MCMT)
The mean time required to perform all corrective maintenance actions. The MCMT is the average time required to restore a failed system to operational status by repairing or replacing failed components.
Mean active maintenance time (MAMT)
The average time required to perform all corrective and preventive maintenance actions. The calculation is taken from MIL-HDBK-472 Procedure 2.
Max corrective maintenance time (MAXCMT) for sigma percentile
The time needed to accomplish the specified percentage of potential corrective maintenance tasks, frequently the 90th or 95th percentile. For example, if you specify a value of 90, the MAXCMT is the time within which 90% of all maintenance actions can be accomplished. The calculation is taken from MIL-HDBK-472, Procedure III.
Mean maintenance manhours per repair (MMH/Repair)
This calculation is identical to the MTTR calculation except manhour values are substituted for time values.
Mean maintenance manhours per maintenance action (MMH/MA)
This calculation is similar to the MMH/Repair calculation except that the time spent as a result of false alarms is also taken into account.
Mean maintenance manhours per operating hour (MMH/OH)
The sum of all corrective, preventive, and false alarm repair times divided by the total operating hours.
Mean maintenance manhours per flight hour (MMH/FH)
The sum of all corrective, preventive, and false alarm repair times divided by the total flight hours.
Mean preventive maintenance time (MPMT)
The mean time required to perform all preventive maintenance actions. The MPMT is the average time required to replace components or subsystems before they fail to promote continuous system operation. The calculation is taken from MIL-HDBK-472 Procedure 2.
Percent isolation to a single replaceable item
The percentage of faults detected that can be isolated to a single replaceable item.
Percent isolation to a group of replaceable items
The percentage of faults detected that can be isolated to n or fewer replaceable items, where n is a specified quantity.