Using Priority AND Gates in SAE Fault Trees For Sequencing Gates

When using an AND gate in either conventional or SAE ARP 4754A / ARP4761 fault tree analysis, the logic used in calculations is that the order of occurrence of the underlying events does not matter. However, in some cases, failure order dependent events must be accounted for properly.

SAE ARP4761 specifies that a Required Order Factor (ROF), or Sequencing Factor, should be used to handle failure order dependent events.

When performing fault tree analysis in Relyence Fault Tree based on SAE ARP4754A / ARP4761, failure order dependent events can be modeled using the Priority AND gate.

In the Properties for the Priority AND gate in an SAE ARP4754A / ARP4761 Fault Tree, the Calculation Parameters section allows you to define:

• Required Order Factor Type
• Required Order Factor
• Number of Valid Sequences

To specify the Required Order Factor, set the Required Order Factor Type to Specified. Then enter a value for the Required Order Factor.

To calculate the Required Order Factor, set the Required Order Factor Type to Calculated. In this case, Relyence calculates the Required Order Factor based on the number of input Events to the Priority AND gate and the defined Number of Valid Sequences. When there are n input Events and k valid sequences, the Required Order Factor is calculated as (k/n!).