Windchill Risk and Reliability Desktop Help > Windchill FTA > FTA Calculations > FTA Calculation Properties > General Page > General Page for SAE Fault Trees
General Page for SAE Fault Trees
The following table describes the properties that appear on the FTA > General page for SAE fault trees.
Property
Description
Qualitative analysis only
When selected, only qualitative analysis is performed. Any data entered for quantitative analysis is ignored. A qualitative analysis results in the listing of cut sets and the propagation of True/False conditions.
Reliability/availability evaluation
The properties under this heading specify information needed for SAE calculations.
Flight time
The average flight time in hours for the aircraft. This value must be greater than 0. The default is 1.
• When developing a new aircraft, you usually use the value stated in the customer requirements.
• When modifying an existing aircraft, you usually use actual average flight time based on in-service data.
Flights per mission
The number of flights per mission. This value must be greater than 0. The default is 100.
Perform exact calculations
Indicates whether to use exact calculations. When this checkbox is cleared, cut sets are used to calculate an approximation based on properties you supply on the FTA > Advanced page. For more information, see Advanced Page. When this checkbox is selected, gate logic is used to calculate an exact solution.
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Selecting Perform exact calculations is the same as selecting Exact as the calculation method on the Advanced page.
Start calculating at
The gate at which to begin the calculation for the selected tree. Choices are:
• Topmost gate. When selected (default), the calculation originates at the top most gate of the active tree.
• Current gate (top of current page). When selected, the calculation originates at the top gate on the current tree page.
Calculations
The properties under this heading specify the FTA calculations to perform for SAE trees.
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Probability calculations assume a typical flight of average duration and consider the appropriate exposure and ‘at risk’ times. Both per flight and per flight hour results are calculated for average and worst case probability calculations.
Average probability
The average probability of the top event occurrence per flight and also per flight hour for the failure condition.
Worst case probability
The worst case probability of the top event occurrence per flight and per flight hour for the failure condition. This conservative analysis is sometimes used to show compliance rather than the more complicated average probability. If no failure is latent, the average probability and worst case probability are identical.
Mission probability
The probability of the top event occurrence during the mission.
Perform uncertainty analysis
When selected, uncertainty analysis is performed. This checkbox is present for all three types of fault trees. Uncertainty analysis is used for trees with top events that rarely occur, which is frequently the case for safety analyses. Failure properties, such as failure rate, are specified using statistical distributions or confidence bounds. A standard error is then entered to indicate the uncertainty associated with the resulting failure rate. If Qualitative analysis only is selected at the top of the page, the checkbox for this property is cleared and unavailable.
When calculating the next three results, the FTA module uses the values calculated for the type of probability selected for Based on, which appears to the right. Choices are Average probability and Worst case probability. The default is Average probability.
Cut sets
A cut set identifies which unique combination of component failures and/or events can cause the top event to occur. A minimal cut set is the smallest set of events, which, if they all occur, cause the top event to occur. When this checkbox is selected, minimal cut sets are generated, although they are referred to simply as cut sets in the application.
Reliability importance measures
Reliability importance measures rank the basic events in the tree to show how consequential they are in regard to causing the top event to occur. When this checkbox is selected, importance measures are calculated for the events that are inputs to gates. For more information, see Reliability Importance Measure Calculations.
Meets safety criteria
When selected, the calculated failure probability is compared to the maximum allowable probability to see if safety criteria are met. If the failure probability is equal to or less than the value specified for maximum allowable probability, Meets safety criteria is set to Yes. Otherwise, it is set to No. If Qualitative analysis only is selected at the top of the page, the checkbox for this property is cleared and unavailable.
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When Meets Safety Criteria is inserted in the FTA Table, you can use the filter bar to show only gates that meet or do not meet safety criteria. This field appears in the FTA Table by default for Projects created from the FTA Civil Aerospace Template. If you did not create your SAE Project from this template, you can use the Format Builder to insert this field.
Calculate top gate only
When selected (default), calculation results are generated for only the top gate.
Calculate all gates
When selected, calculation results are generated for all gates.
Ignore cut sets with probabilities less than
The probability value that determines whether to use a cut set in calculations. Choices are: 0, 1e-30, 1e-25, 1e-20, 1e-15, 1e-10, and le-5. The default is 0. You can also type a probability value in this field.
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This property and the next one do not apply to exact calculations. Their availability depends on selections for Qualitative analysis only, Perform exact calculations, and Cut sets. You use these two properties to reduce the number of cut sets to consider in the generation of cut set results. Ignoring cut sets with low probabilities or a large number of events generally has a minimal impact on the results and can greatly increase the speed at which calculations run.
Ignore cut sets with an order greater than
The number of events that determines whether to use a cut set in calculations. The default for this field is a number sign (#), which indicates that no limit exists on the number of events. You can enter any integer greater than or equal to 1 to set a limit.
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