Fault Tree Analysis
FTASection 06: System Safety & Functional Safety
Definition
A top-down, deductive analytical method used to determine the combinations of lower-level events (hardware failures, software errors, human errors, environmental conditions, and maintenance actions) that could cause a specific undesired top-level event (typically a failure condition identified in the FHA). The fault tree is a graphical model using Boolean logic gates (AND, OR, NOT, voting gates) to represent the logical relationships between events. Quantitative FTA assigns failure rates to basic events and calculates the probability of the top event using Boolean algebra or numerical methods. Qualitative FTA identifies minimal cut sets — the smallest combinations of basic events that can cause the top event.
Where This Shows Up
FTA is one of the most widely used quantitative methods in aviation safety assessment, particularly for demonstrating compliance with the probability objectives of 25.1309. Fault trees allow engineers to evaluate whether architectural features like redundancy, dissimilarity, and monitoring are sufficient to meet safety targets. Minimal cut set analysis reveals single points of failure and combinations of failures that are safety-relevant. FTA results feed directly into the PSSA and SSA.
Primary Sources
Provides guidance on conducting FTA as part of the safety assessment process, including treatment of common cause failures, exposure time, and latent failures.
Detailed guidance on fault tree construction, gate symbols, and quantification methods.
Artifacts Produced
Graphical representation of the fault tree showing the top event, intermediate events, basic events, and their logical relationships through AND/OR gates.
Related Terms
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