NASA Procedures and Guidelines
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NPR 8621.1 Eff. Date: June 02, 2000 Cancellation Date: February 11, 2004	NASA Procedures and Guidelines for Mishap Reporting, Investigating, and Recordkeeping
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1.1 Method

1.1.1 The FTA can model the failure of a single event or multiple failures which lead to a single system failure. The FTA is a top down analysis versus the bottom up approach for the event tree analysis. The method identifies an undesirable event and the contributing elements (faults/conditions) that would precipitate it. The contributors are interconnected with the undesirable event, using network paths through Boolean logic gates.

1.1.2 The following basic steps are used to conduct FTA:

a. Define the top event/system failure of interest.

b. Define the physical and analytical boundaries.

c. Define the tree-top structure.

d. Develop the path of failures for each branch to the logical initiating failure.

1.1.3 Once the fault tree has been developed to the desired degree of detail, the various paths can be evaluated to arrive at a probability of occurrence. Cut sets are combinations of component failures causing system failure (i.e., causing the top event of the tree). Minimal cut sets are the smallest combinations causing system failure. The technique is universally applicable to systems of all kinds, with the following ground rules:

a. The undesirable system events which are to be analyzed/abated, and their contributors, must be foreseen.

b. Each of those undesirable system events must be analyzed individually.

1.2 Thoroughness

Primary limitations of the techniques are the presumption that the relevant undesirable events have been identified and the presumption that contributing factors have been adequately identified and explored in sufficient depth. Apart from these limitations, the technique as usually practiced is regarded as among the most thorough of those prevalent for general system application. Significant training and experience is necessary to use these properly. Mastery for the initiated requires from 8 to 40 (or more) hours of study and some practical experience. Prior knowledge of Boolean algebra and/or the use of logic gates is helpful.

1.3 Comments

Application, though time consuming, is not difficult once the technique has been mastered. Computer aids are available and are increasingly used. Unlike Event Tree Analysis and Failure Modes and Effects Analysis, the technique explores only those faults and conditions leading to intolerable losses. The FTA has several strengths. The procedures are well defined and focused on failures. The top-down approach requires analysis completeness at each level before proceeding. It cannot guarantee identification of all failures, but the systematic approach enhances the likelihood of completeness. The FTA addresses effects of multiple failures by identifying inner relationships between components and identifying minimal failure combinations that cause the system to fail (minimal cut sets). The method addresses the effects of design, operation, and maintenance. The FTA can handle complex systems. It provides a graphical representation that aids in understanding these complex operations and interrelationships between subsystems and components. Many standardized computer analysis packages exist to make the process much faster and easier. Finally, FTA provides both qualitative and quantitative (probabilistic) information. Probabilities may be assigned to each subevent and aggregated to determine an overall probability for the top event.

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