Figure D8
In SAE ARP4761, Figure D8 is an example of a fault tree where two item failures cause a loss of a function. One of these items can fail latent. The order of failure for these two items does not matter.
• Item 1 can fail at any point between when it is checked initially and when it is next checked.
• Item 2 is known to be operating at the start of each flight and never fails latent.
In the FTA module, this two-item tree might look like the one that follows.
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, use 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.