Fault Tree Handbook with Aerospace Applications Version 1.1
Foreword
capable of uncovering design and operational weaknesses that escaped even some of the best
deterministic safety and engineering experts. This methodology showed that it was very
important to examine not only low-probability and high-consequence individual mishap events,
but also high-consequence scenarios which can emerge as a result of occurrence of multiple
high-probability and nearly benign events. Contrary to common perception, the latter is
oftentimes more detrimental to safety than the former.
A foremost strength of PRA and its underlying analysis techniques, including FTA, is that it is a
decision support tool. In safety applications, this methodology helps managers and engineers find
design and operational weaknesses in complex systems and then helps them systematically and
efficiently uncover and prioritize safety improvements.
In order to best benefit from PRA and FTA in management decisions, it is important that
managers and their support staffs be familiar with the value and application of these methods. In
addition, there should be a small but robust group of in-house technical experts that understand
the methods used in a PRA or FTA study, can explain its meaning and applicability to given
problems to management and serve as in-house technical advisers to the management decision
process for safety improvement. If these in-house experts do not exist initially, they should be
hired or groomed through training and transfer of technology, becoming part of the corporate
resources and memory that will help shape the organization, taking advantage of the PRA and
FTA methods and results and the expert knowledge of the external consultants. In-house experts
will help build risk-based knowledge and experience and stimulate cultural changes so that a
progressive organization can use these resources to make sound and cost-effective safety
improvement decisions.
In support of this, NASA has recently began to implement the following important risk
assessment enhancement principles in its programs and projects:
• Transfer quantitative risk assessment technology to NASA managers and practitioners as
soon as possible,
• Develop or acquire quantitative risk assessment expertise and state-of-the-art software
and data,
• Encourage ownership in quantitative risk assessment methods, studies and results in order
to use them effectively in the management decision process,
• Develop a corporate memory of the risk assessment project results and data on which to
build future capabilities and experience, and
• Develop risk awareness in programs and projects that will eventually help NASA develop
a risk-informed culture for all its programs and activities.
To this end, and in support of the Risk Management Program, NASA began to develop training
and practitioner documents on how to perform quantitative risk assessment and utilize important
techniques like FTA. One such document is a Procedures Guide for performing PRA for
aerospace applications. The other is this document, the re-issue of an updated version of the
Fault Tree Handbook for aerospace applications.
A considerable amount of material on PRA methods and applications has been written over the
past three decades. Several university and practitioner textbooks and sourcebooks currently exist
