Windchill Risk and Reliability Desktop Help > Windchill FTA > Fault Tree Object Properties > Gate or Event Properties > Calculation Data Page > Gate Calculation Properties
Gate Calculation Properties
On the Calculation Data page for a gate, the number of properties in which you enter data is minimal. For pass-through gates and remarks gate, you do not specify any properties because no calculation data is associated with these gate types. For most gate types in time dependent and Lambda Tau trees, you specify only the maximum allowable probability. Descriptions follow for all calculation properties that can appear for gates in all types of trees.
Property
Description
Maximum Allowable Probability
The highest probability of failure that still meets acceptance requirements. For a gate in an SAE tree, this value is the maximum allowable probability of failure per flight. If you have a “per flight hour” requirement, you must convert it to a “per flight” requirement. You can enter any value between 0 and 1. The default is 1. For gates in time dependent and SAE trees, you indicate whether failure units are per hour (default) or per mission. For a time dependent tree, the mission time is the primary time point. For a Lambda Tau tree, this time-related input is not needed.
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SAE ARP4761 defines probability values for five failure condition classifications: Minor, Major, Severe Major, Hazardous, and Catastrophic. You use the failure condition classification for the top gate to determine the value to use. When you run FTA calculations, the “per flight” requirement that you enter here is compared to the probability calculation result for the gate to determine if safety criteria are met. If you create a Project from the Civil Aerospace Template for SAE trees, a custom label of Max Allowable Prob is assigned to this data field. Administrators can assign custom labels to the fields in most tables in the Project’s Data Definition file.
Votes
For a voting gate only, the number of TRUE inputs that are required for the output to be true. The default is 0. This field is available only after the voting gate has child events. For more information, see Voting Gate.
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The next three properties are shown only for sequencing AND gates, which can be inserted only in SAE trees. For more information, see Sequencing AND Gate (SAE Fault Trees Only).
Required Order Factor Type
Indicates whether the required order factor, also known as the sequencing factor, is calculated or specified. According to SAE ARP4761, this factor is incorporated to make calculated probabilities less conservative. For events with larger differences in exposure times, also known as ‘at risk’ times, SAE ARP4761 indicates that the required order factor should be calculated and provides examples.
• When Calculated is selected (default), Number of Valid Sequences is available.
• When Specified is selected, Required Order Factor is available.
Required Order Factor
When you want to specify the required order factor for a sequencing AND gate, the number of sequences to consider. The default is 1.
Number of Valid Sequences
When you want to calculate the required order factor for a sequencing AND gate, the number of events that can lead to the failure. For a newly inserted sequencing AND gate, a number sign (#) is shown by default. Prior to running FTA calculations, you must enter a valid value. Otherwise, when you run these calculations, a window opens, indicating that the input value for calculating the required order factor is either missing or invalid.
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The number of valid sequences is represented by k. The k value must be less than or equal to n!, where n is the number of input events. If k is greater than n!, a window opens, indicating that the value is too large. If k is equal to n!, the sequencing AND gate behaves exactly like an AND gate.