Windchill Risk and Reliability Desktop Help > Windchill FTA > Fault Tree Object Properties > Gate or Event Properties > Calculation Data Page > Time Dependent Event Properties
Time Dependent Event Properties
On the Calculation Data page for a time dependent event, properties are listed under three headings: Failure properties, Maintenance properties, and Failure rate adjustments.
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For a spare event, the Maintenance properties heading is replaced by the Standby properties heading. Additionally, only two model selections are available: Constant probability and Failure rate/MTBF.
Property
Description
Failure properties
The properties under this heading specify the failure data to use for calculating results. For more information, about calculations for time dependent events, see Event Input Parameters.
Model
The model to use. The selection you make here determines the other properties beneath under this heading. It also determines the properties that are shown and/or available under the two additional headings. The model choices for time dependent events are:
• Constant probability. When selected, Probability is shown. You enter a value from 0 to 1 to specify the constant probability of failure for the event over time.
• Failure rate/MTBF. When selected, you must select Failure Rate (default) or MTBF to indicate what type of value you are entering in the field to the right. You then enter a value greater than 0. For an MTBF value, you must also select Hours (default) or Minutes to indicate the units.
• Frequency. When selected, Frequency is shown. You enter a value greater than or equal to 0 to specify the average number of failures that occur each hour.
• Failure with repair. When selected, you must select Failure Rate (default) or MTBF to indicate what type of value you are entering in the field to the right. You then enter a value greater than 0. For an MTBF value, you must also select Hours (default) or Minutes to indicate the units. Additionally, under the Maintenance properties heading, you should enter a value greater than 0 for Mean Corrective Time. You then select Hours (default) or Minutes to indicate the units.
• Failure with periodic inspection. When selected, you must select Failure Rate (default) or MTBF to indicate what type of value you are entering in the field to the right. You then enter a value greater than 0. For an MTBF value, you must also select Hours (default) or Minutes to indicate the units. Additionally, under the Maintenance properties heading, you should enter a value greater than 0 for Inspection Time. You then select Hours (default) or Minutes to indicate the units.
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The values that you enter for failure rate and frequency are affected by the failure rate multiplier, which is specified by General > Failure Rate Multiplier in the System file properties.
Distribution
The distribution to use to specify the uncertainty in the parameter. Choices for the Constant probability model are: Fixed, Beta, and Uniform. Choices for all other models are: Fixed, Lognormal, Normal, Gamma, and Uniform.
Standard Deviation
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 value of 0 is used.
Failure Sequence
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 left 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 left 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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When the exact method or the simulation method is used for calculating FTA results, the order restriction for the events is imposed locally for each AND gate. For cut set-based methods, the order restriction is imposed at the cut set level. If the events with first and last positions are not the repeated events, all approaches produce the same results. However, if the events with first and last positions are repeated events, the results obtained from the exact method and simulation method differ slight from the results for the cut set-based methods.
Maintenance properties
The properties under this heading specify maintenance data to use for calculating results for the event. Availability depends on the model.
Mean Corrective Time OR Inspection Time
The average time it takes to repair or inspect the component associated with the event.
• When Failure with repair is the model, Mean Corrective Time is shown. You enter the average time it takes to repair a failed component and return it to an operational state during corrective maintenance.
• When Failure with periodic inspection is the model, Inspection Time is shown. You enter the average time it takes to inspect the component during preventative maintenance.
Distribution
The distribution to use to specify the uncertainty in the parameter. The choices are: Fixed, Lognormal, Normal, Gamma, and Uniform.
Standard Deviation
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 value of 0 is used.
Standby properties
For a spare event only, this heading replaces the Maintenance properties heading. The properties under this heading specify data about the spare component.
Dormancy Factor
The ratio of failure rate in the spare mode and failure rate in the operational mode. The default is 0.
• If the dormancy factor of all spares (events) connected to a spare gate are 0, then the spare acts like a cold spare.
• If the dormancy factor of all spares connected to a spare gate is 1, then the spare acts like a hot spare.
• If the dormancy factor of all spares connected to a spare gate are the same (and are between 0 and 1), then the spare acts like a warm spare.
Spares Pool
The number of identical instances of the spare event. The default is 1. However, if there are two or more identical spare components of the spare event, you must enter the appropriate value.
Failure rate adjustments
The properties under this heading specify data to use to adjust the failure rate calculated for the event. Availability depends on the model.
Failure Rate Percentage
The percentage of the failure rate to use for the event. The default is 100.
Exposure Time Percentage
The percentage of the overall mission time specified as the exposure time, or ‘at risk’ time associated with the event. This percentage is the fraction of the mission time for which the event is valid. The default is 100.
Use Approximate Formula
Indicates whether to calculate exact or approximate failure probability results. SAE ARP4761 and ISO 26262 supply the same approximate formula. Use of this formula is suggested when λt is less than or equal to 0.1. Consequently, you might want to select this checkbox for small λt values.
Event Calculation Properties