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Fault Tree Analysis

Fehlerbaum Zweistrahliges FlugzeugFocused on faults, fault tree analysis is the leading method for demonstrating safety.
In contrast to FMEA, fault tree analysis is not limited to single and independent failures. Instead, fault tree analysis can handle complex fault scenarios and special system behavior.
Most safety related systems are either fault tolerant, or require a specific sequence of faults in order to become potentially dangerous. For such systems, fault tree analysis is the method of choice because it is designed for handling such specific dependencies.
Like in earlier paragraphs, the twin engine aircraft example may give sufficient insight.
Twin engine aircraft are designed to fly safely with only one engine. Upon engine failure, the aircraft must fly directly to the nearest eligible alternate airport with its remaining engine.


The fault tree on the right doesn't account for the fact that flying on one engine would increase the failure rate of that engine. Depending on the power of the fault tree software, fault tree analysis basically could potentially handle that, however with some difficulties.

Over all, the right fault tree example with its three AND gates (red symbols) demonstrates that this method is way more specific than FMEA (which is basically just a collection of single and independent failures).
The downside of fault tree analysis is that, e.g. it there are 10 different fault scenarios, 10 individual fault trees have to be created (while just one FMEA for the whole system would be sufficient)

Although there are fault tree standards available (e.g. NUREG-0492, free in the www), the fault tree methodology itself tends to be self explaining. The above example has only AND gates in order to demonstrate the difference to FMEA: Fault tree analysis can handle AND-ed events, but FMEA can not. In practice however, fault trees normally have more OR gates than AND gates. .


Fault tree analysis begins on the top with the so called top event. The wording of the top event must be as precise as possible. During analysis, the fault tree grows downwards towards the so called basic events. Basic events are such events that cannot or need not be divided any more. The basic events and the top event are connected via lboolean operators, basically AND and  OR.

In most safety analyses, input data for the basic events comes either from FMEA or MTBF calculation.

The main difficulties with fault tree analysis are:
  1. Top event wording not precise. This provides unneccesary room for interpretation.
  2. Logical tree diagram not correct. This is not a mathematical problem, but rather a consequence when the system behavior is not understood properly.
    • The same problem exists in FMEA, too, but in fault tree analysis it is a prevailing issue.
  3. Fault tree diagram being too detailed, and/or counter-intuitive.
    • Fault tree diagrams shouldn't be too comprehensive. Instead of OR-ing many "atomic" events, it is more practical to keep the sum of these events in a single event. This keeps fault tree diagrams slim without sacrificing any information.

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