Maintainability Calculations
The following table describes the calculations that you can select on the Maintainability page in the Calculate window. All calculations are listed in the order in which they appear on the Maintainability page. This table also describes how each of these results is calculated. Preventative maintenance is the practice of replacing components or subsystems before they fail to promote continuous system operation. Corrective maintenance restores a failed system to operational status by repairing or replacing failed component(s).
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The criticality of the selection you make for Repair level on the Maintainability page is explained in Maintainability Calculation Properties.
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Calculation
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Description
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Calculation Method
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Percent isolation to a single replaceable item
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The percentage of faults detected that can be isolated to a single part or replaceable item (RI). Faults can be stored in Maintainability FD&I Library files.
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1. Calculate the sum of the Failure Rates (FRs) of all outputs that result in isolation to a single RI.
2. Divide this value by the sum of the FRs for all outputs.
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Percent isolation to a group of replaceable items
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The percentage of faults detected that can be isolated to n or fewer RIs. You enter a value for Quantity, which appears below this calculation option.
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1. Calculate the sum of the FRs of all outputs that result in isolation to n or less RIs.
2. Divide this value by the sum of the FRs for all outputs.
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Mean maintenance manhours per repair (MMH/Repair)
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This calculation is identical to the MTTR calculation except manhour values are substituted for time values.
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For more information, see MTTR.
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Mean maintenance manhours per maintenance action (MMH/MA)
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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.
F2n is the frequency of occurrence of Type 2 false alarms for the RI.
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1. Calculate the sum for all RIs of the (MMH/Repair for each RI) * (FR of each RI) * (1 + F2n).
2. Calculate the sum for all RIs of the (MMH associated with Type 1 false alarms) * (FR of each RI) * (Frequency of occurrence of Type 1 false alarms).
3. Calculate the sum for all RIs of the (FR of each RI) * (1 + F2n).
4. Calculate the sum for all RIs of the (FR of each RI) * (Frequency of occurrence of Type 1 false alarms).
5. Calculate the MMH/MA by dividing the sum of steps 3 and 4 above by the sum of steps 1 and 2 above.
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Mean maintenance manhours per operating hour (MMH/OH)
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The sum of all corrective, preventive, and false alarm repair times divided by total flight hours.
F2n is the frequency of occurrence of Type 2 false alarms for the RI.
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1. Calculate the sum for all RIs of the (MMH/Repair for each RI) * (FR of each RI expressed in failures per operating hour) * (1 + F2n).
2. Calculate the sum for all RIs of the (MMH associated with Type 1 false alarms) * (FR of each RI expressed in failures per operating hour) * (Frequency of occurrence of Type 1 false alarms).
3. Calculate the sum for each preventive maintenance action of the (MMH per Preventive Maintenance Action) * (Frequency of the Preventive Maintenance Action).
4. Calculate the MMH/OH by adding the results of steps 1, 2, and 3.
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Mean maintenance manhours per flight hour (MMH/FH)
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The sum of all corrective, preventive, and false alarm repair times divided by total flight hours. This calculation is the same as that for MMH/OH except flight hours is substituted for operating hours.
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The value entered for Percent flight hours, which appears beneath this option, is used to convert total hours to flight hours (FH/OH * 100).
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Max corrective maintenance time for sigma percentile
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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 result is the time within which 90% of all maintenance actions can be accomplished. This calculation is taken from MIL-HDBK-472, Procedure III.
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The value entered for Percentile, which appears beneath this option, is the sigma percentile to use for this calculation. Both the sigma value and this result are calculated per equations in MIL-HDBK-472. You can generate an ancillary report that outputs a summary of Mmax sigma values by selecting the Report Design file named Maintainability Mmax Summary.
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Mean corrective maintenance time
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The mean time required to perform all corrective maintenance actions necessary to return a failed system back to operation status. Corrective maintenance includes the repair or replacement of failed components. Once these values are computed, they summary results are stored in the Maintainability MCMT Percentile Summary Table.
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1. Calculate [the sum of (frequency of occurrence of corrective maintenance tasks) * (Manhours)].
2. Divide this value by the [sum of frequency of occurrence of corrective maintenance tasks].
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Mean preventive maintenance time
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The mean time required to perform all preventive maintenance actions. Preventive maintenance is the practice of replacing components or subsystems before they fail to promote continuous system operation This calculation is taken from MIL-HDBK-472 Procedure 2.
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1. Calculate [Sum of (Frequency of Occurrence of Preventive Maintenance Tasks) * (Manhours)]
2. Divide this value by the [Sum of Frequency of Occurrence of Preventive Maintenance Tasks].
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Mean active maintenance time
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The average time required to perform all corrective and preventive maintenance actions. As indicated earlier, corrective maintenance restores a failed system to operational status by repairing or replacing failed components, while preventative maintenance is the practice of replacing components or subsystems before they fail to promote continuous system operation. The following equation is taken from MIL-HDBK-472 Procedure 2:
(Corrective Time + Maintenance Time) / Number of Maintenance Tasks
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1. Calculate the (Total FR for all RIs) * (Total MMH/Repair) * (Operating Time During t j (ti)).
2. Calculate the (Sum of all Frequencies of Preventive Maintenance Tasks for all RIs) * (MPMT) * (Calendar Time in Operating Inventory (t j)).
3. Calculate the (Total FR for all RIs) * ti.
4. Calculate the (Sum of all Frequencies of Preventive Maintenance Tasks for all RIs) * t j.
5. Calculate the Mean Active Maintenance Time (MAMT) by taking results (1) + (2) divided by results (3) + (4).
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Maintainability index
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The total maintenance effort, including both preventative and corrective maintenance, that is required to maintain a product so that it remains operational 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 calculation.
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When this calculation option is selected, the following fields become available:
• T j: calendar time in operating inventory (hrs)
• Ti: operating time during tj (hrs)
• T: operating time (hrs)
The calculation equation for the maintainability index is:
1. Calculate the Sum of (1) + (2) from the MAMT calculation.
2. Divide this value by the operating time (t).
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