High-Level Modeling
The FTA module uses a common user interface for both explicit and implicit modeling. To use explicit modeling, you generally specify failure rates and exposure times for a specific flight, usually the worst case scenario. For instance, you can explicitly model the examples in Appendix L of SAE ARP4761 by entering an exposure time for an item that is greater than the average flight duration.
In some cases, particularly when an item is used in a specific phase of flight, exposure time can be less than flight duration. In the FTA module, it is possible to use both explicit and high-level modeling concepts within a tree. However, in such cases, calculating the worst case probability is appropriate.
The following differences exist between SAE trees and time dependent trees:
• SAE trees do not support dynamic gates. However, they consider the sequence or order-dependent failure behavior described in SAE ARP4754A and ARP4761 in two ways:
◦ You can supply sequencing data related to latent failures and monitor failures for an event on the Latency page in the Properties window.
◦ You can specify the ordering of failures within a flight by inserting sequencing AND gates. This type of gate can be inserted only in SAE trees. In the Properties window for this gate type, you adjust the probability occurrence for the gate based on the required order factor. You can enter specify the required order factor or calculate it based on a number of valid sequences.
• SAE trees do not support implicit common cause failure (CCF) models such as the Alpha and Beta models. For more information, see Common Cause Failure Models. However, they do support explicit modeling of common cause failures via repeated events and transfer gates.
• Because SAE trees do not support spare gates, which are dynamic gates, they also do not support spare events.
Additionally, the models for events in SAE trees are modified so that you can specify the failure characteristics of system items for different flights. You specify the failure properties of a basic event within a flight by selecting either Constant probability or Failure rate/MTBF as the model on the Calculation Data page. You then supply the appropriate value. For more information, see SAE Event Parameters.
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If you supply a failure rate, you must consider the units specified by > in the System file properties. For example, assume that the default value of 1,000,000 units is in use. Entering a failure rate of 1000 is equal to 1000/1,000,000 or = 0.001 failures per hour. For more information, see General System File Properties.
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When you supply a failure rate or MTBF value, the probability of failure per flight is calculated using the exponential distribution. The exact formula for the failure probability within a flight is Pf = 1-exp(-λτ) where λ is the failure rate in hours and τ is the flight time. You enter the flight time, which is the same for all events within the tree, on the > page in the Calculate window. For descriptions of the properties under the Reliability/availability evaluation heading, see General Page for SAE Fault Trees.
In some examples in SAE ARP4761, failure rate is calculated using an approximate formula. Therefore, to match the equations and results in this SAE document, the Use Approximate Formula checkbox is available on the Calculation Data page in an event’s Properties window. However, SAE ARP4761 indicates that you should use the approximate formula only when λτ is less than or equal to 0.1. Consequently, you should select this checkbox only for small λτ values.
The following topics provide additional information about high-level modeling for SAE trees:
• Monitors