Figure D10
In SAE ARP4761, Figure D10 is a a typical monitor situation, where the top event is an erroneous output rather than a loss of function. In this fault tree, two item failures cause the occurrence of the top event. One item can fail latent. The failures are order-dependent.
• Item 1 can fail latent. If it fails before Item 2, the top event occurs.
• Item 2 is known to be operating at the start of each flight and never fails latent.
This particular example depicts the transmission of incorrect data. A required order factor is used to indicate the order dependency. In the FTA module, this two-item tree might look like the one that follows.
In the gate properties for the sequencing AND gate, you enter calculation data as follows:
• On the Calculation Data page:
◦ For Required Order Factor Type, select Calculated.
◦ For Number of Valid Sequences, enter 1.
In the event properties for Item 1, you enter calculation data as follows:
• On the Calculation Data page:
◦ Select Failure rate/MTBF as the model.
◦ Select Failure rate and enter 1000.
◦ Select Use Approximate Formula.
• On the Latency page:
◦ Select Latent Failures.
◦ For Check Interval, enter 1000.
◦ For Failure Detection Percent, sue the default value of 0.
◦ For Detection Method, select Self checking.
In the event properties for Item 2, you enter calculation data as follows:
• On the Calculation Data page:
◦ Select Failure rate/MTBF as the model.
◦ Select Failure rate and enter 2000.
◦ Select Use Approximate Formula.
• On the Latency page:
◦ Ensure Latent Failures is cleared.