Event Modeling for SAE FTA

Relyence Fault Tree supports various input models for accurately describing your fault tree events. To select the most appropriate model for your event, click on the event to select it, then dropdown the list for Input Model in the Properties pane.

Select the appropriate Input Model for calculating the event probability and other quantitative metrics:

Constant Probability: time-independent, based on probability
Failure Rate: time-dependent, based on an exponential distribution of the Failure Rate or Mean Time to Failure
Failure Rate (approximate): time-dependent, based on an approximate formula for the exponential distribution of the Failure Rate or Mean Time to Failure
Mean Unavailability (approximate): time-independent, based on an approximation of average unavailability based on the Lambda Tau method

Relyence Fault Tree will prompt you for the corresponding data parameters needed based on your Input Model selection.

If the Input Model is set as Constant Probability, you must enter the Probability for the Event.
If the Input Model is set as Failure Rate, you must enter the Failure Rate Type for the Event as well as the Failure Rate or Mean Time to Failure for the Event. You may also set Exposure Time details and Latent Failures details, as explained below.
If the Input Model is set as Failure Rate (approximate), you must enter the Failure Rate Type for the Event as well as the Failure Rate or Mean Time to Failure for the Event. You may also set Exposure Time details and Latent Failures details, as explained below.
If the Input Model is set as Mean Unavailability (approximate), you must enter the Lambda for the Event. You may also set Exposure Time details, as explained below.

Exposure Time

If you have and want to set the Exposure Time, select Set Exposure Time and enter the Exposure Time value.

Latent Failures

If you have and want to specify details for latent failures, select Latent Failures and enter the Latent Parameters, including:

Detection Method - defines the procedure used to detect the latent failure; choices include:
- Self Test - a procedure that is performed, often during power up or pre-flight, by the equipment to uncover the occurrence of a latent failure; when Detection Method is Self Test, latent and active failures are distinct and disjoint occurrences
- Monitored - a Monitored latent failure describes a failure with a method to detect its occurrence, for example hardware circuitry, software, or other test methods
- Unmonitored - an Unmonitored latent failure is one in which there is no method to uncover the occurrence of the latent failure because there is no monitor; when Detection Method is Unmonitored, latent and active failures are indistinguishable
Failure Detection Percent - the percentage chance that the check will uncover the latent failure once it occurs
Check Interval - the period of time, in hours, between inspections of the latent failure

Assign Monitor Parameters if Detection Method is Set to Monitored

If Detection Method is set to Monitored, the following additional parameters can be set in the Monitor Parameters section:

Monitor - click the selector button and choose a Monitor from the Monitor Library; if a Monitor is selected, its Monitor Parameters stored in the Monitor Library will be assigned
When a Monitor is selected, the Non-Repeat Monitor is also available; if you use the same Monitor associated with multiple Events in your fault tree, Relyence assumes it to be repeat Monitor, or the same Monitor used associated with multiple Events; however, if it is a unique Monitor, just with the same Monitor Parameters, select the Non-Repeat Monitor to treat the Monitor as a unique monitor
Failure Rate - failure rate; note that a nonzero Failure Rate implies the Monitor can fail where the event would otherwise be detectable
Scrub Time - the interval at which Monitor operations are verified
Scrub Verification Percent - the percentage of time that the scrub verifies and corrects Monitor failures
Check Interval - the interval details when Latent Monitor failures are corrected in the cases where the scrub does not detect a Monitor failure