Lambda Tau Event Parameters
On the Calculation Data page for a Lambda Tau event, properties are listed under three headings: Failure properties, either Maintenance properties or Mission properties, and Failure rate adjustments. The label for the second heading depends on the model selected in the failure properties.
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Property
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Description
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Failure properties
The properties under this heading specify the failure data to use for calculating results.
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Model
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The model to use. The selection you make here determines whether the next property is Probability or Failure Rate (λ). For additional information, see Modeling Lambda Tau Events.
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Probability OR Failure Rate (λ)
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The constant probability or failure rate value to use in the calculations.
• When Constant probability is the model, Probability is shown. The availability is equal to the constant probability value that you enter. You can enter any value from 0 to 1. The default is 0. For this model is selected, properties are available only under the first heading.
• When any other model is selected, Failure Rate (λ) is shown. The Lambda parameter (λ) represents the failure rate for the event. You can enter any value from 0 to 1. The default is 0. Depending on the model, you enter the appropriate value for the Tau (τ) parameter under the second heading. The Tau (τ) parameter depends on the model.
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Distribution
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The distribution to use to specify the uncertainty in the parameter. Choices for the Probability model are: Fixed, Beta, and Uniform., Choices for all other models are: Fixed, Lognormal, Normal, Gamma, and Uniform.
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Standard Deviation
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If you want to use a standard deviation for the distribution, the value to use. The default is 0. When Fixed is the distribution, this field is unavailable. The default of 0 is used.
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Failure Sequence
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Indicates the allowable position of the event in a cut set for a time sequence of event occurrences. This property is available in time dependent and Lambda Tau trees only if the parent of the event is an AND gate. Choices are:
• None. When selected (default), no order sequence is specified for the event.
• First. When selected, the event is the first to occur. In the diagram, the first icon is shown beneath the right side of the text box.  • Last. When selected, the event is the last to occur. In the diagram, the last icon is shown beneath the right side of the text box.  Nothing in the software prevents you from designating multiple events as the first event and the last event. However, when FTA calculations run, an error occurs if multiple first and/or last events are present. The error message indicates that only one event can be marked as the first event and the last event for any AND gate. It then lists the identifiers for conflicting events. For repeated events, all instances of the repeat share the value selected for the failure sequence. While this field is available for all repeated events, it is shown only for instances where the repeated event is the child of an AND gate. |
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Maintenance properties
This heading appears and displays available properties when one of the following models is selected: Average unavailability (approx), Asymptotic unavailability, or Average unavailability. The label for the first property depends on the model. All properties under this heading specify maintenance data to use for calculating results for the event.
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Time Between Tests OR MTTR
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The value you enter depends on the model.
• When Average unavailability (approx) or Average unavailability is the model, Time Between Tests is shown. You enter the average time between consecutive tests.
• When Asymptotic unavailability is the model, MTTR is shown. You enter the average time it takes to repair the failed component and return it to an operational state.
You then select Hours (default) or Minutes to indicate the units.
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Distribution
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The distribution to use to specify the uncertainty for the MTTR value. Choices are: Fixed, Lognormal, Normal, Gamma, and Uniform.
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Standard Deviation
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If you want to use a standard deviation, the value to use. The default is 0. When Fixed is selected for the distribution, this field is unavailable. The default of 0 is used.
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Mission properties
This heading appears and displays available properties when one of the following models is selected: Lambda Tau or Probability of failure. The label for the first property depends on the model. All properties under this heading specify mission data to use for calculating results for the event.
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Mission Time/MTTR OR MTTR
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The value you enter depends on the model:
• When Lambda Tau is the model, Mission Time/MTTR is shown. For a non-repairable event, you enter the mission time. For a repairable event, you enter the average time it takes to repair the failed component and return it to an operational state.
• When Probability of failure is the model, MTTR is shown. You enter the average time it takes to repair a failed component and return it to an operational state during corrective maintenance.
You then select Hours (default) or Minutes to indicate the units.
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Distribution
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The distribution to use to specify the uncertainty for the MTTR value. Choices are: Fixed, Lognormal, Normal, Gamma, and Uniform.
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Standard Deviation
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If you want to use a standard deviation, the value to use. The default is 0. When Fixed is selected for the distribution, this field is unavailable. The default of 0 is used.
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