Prediction Calculation Results
When prediction calculations are finished running, the View Calculation Results window displays system results for failure rate, MTBF, reliability, availability, and MTTR. Once you close this window, you can open it again at any time by selecting > .
At the bottom of the page are Excel and Print buttons, which you can use to save table results to a Microsoft Excel spreadsheet or to print them to a printer. For more information, see Excel and Print Buttons.
The failure rate calculated for each part and assembly is stored in Failure Rate, Predicted. This result field can be inserted in the System Tree Items table and Parts Table. It can also be inserted on any Prediction form. You can view the results calculated for an individual assembly in reports.
The following table describes all results that can be calculated and shown in the View Calculation Results window for a prediction.
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Calculation
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Description
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Failure Rate, Predicted
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The number of failures likely to occur over a time period, usually defined in units of failure per million hours (FPMH) or failures per billion hours (FITs). For example, if the failure rate value for a unit is 2 and the units is FPMH, the unit is likely to fail twice in a million hours. Failure rates are predictive values computed using equations based on statistical field data analysis.
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MTBF, Predicted
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The number of hours a unit will operate between failures. If a unit is not repairable, the MTBF (mean time between failures) is equivalent to the MTTF (mean time to failure).
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Reliability, Predicted
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The probability that the system will remain operational until a specified time. Reliability is a time-based probability value, so it is has a value between 0 and 1. A reliability of 0 means that a system is infinitely unreliable, or never functioning. A reliability value of 1 indicates that a system is infinitely reliable, or always operating. In predictions, the equation used to compute system reliability is R = e−λt, where λ is the failure rate in failures per hour and t is the mission time.
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Availability
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The probability that the system is operating properly at a specific time point. Availability, like reliability, is a time-based probability metric, so it has a value 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). The common term “five nines” refers to a system that has an availability of 0.99999. Such a system is operational 99.999% of the time. In predictions, the equation used to compute system reliability is:
Availability = MTBF / MTTR + MTBF
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MTTR
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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. MTTR is also referred to as MCT (mean corrective time). The MTTR is calculated by rolling up the average repair times for lower-level components to subassembly and system repair values. If the Maintainability module is also in use, the MTTR values from it can be used by the Prediction module.
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Additionally, if mission calculations were performed, the View Calculation Results window includes a set of mission results. Prior to running mission calculations, you insert the phases that your system experiences during a mission and the phase durations in the Project’s Mission Profile. For example, if your system is an airplane, you might insert three phases: Take-off, Flight, and Landing. For each of these phases, you specify a duration, environment, and set of operating conditions. You also indicate if the system is dormant in any of these phases. The mission results factor in the lower failure rates experienced in the phases where the system is not in operation. For more information, see Mission Reliability Calculations.