Fault Tree Handbook with Aerospace Applications

1 Fault tree Handbook with Aerospace ApplicationsVersion TTTrrreeeeee HHHaaannndddbbbooooookkk wwwiiittthhh AAAeeerrrooossspppaaaccceeeAAAppppppllli iicccaaatttiiiooonnnsssPrepared forNASA Office of Safety and Mission AssuranceNASA HeadquartersWashington, DC 20546 August, 2002 Fault tree Handbook with Aerospace ApplicationsVersion TTTrrreeeeee HHHaaannndddbbbooooookkk wwwiiittthhh AAAeeerrrooossspppaaaccceeeAAAppppppllli iicccaaatttiiiooonnnsssNASA Project Coordinators:Dr. Michael Stamatelatos, NASA HeadquartersOffice of Safety and Mission AssuranceMr. Jos Caraballo, NASA Langley Research CenterAuthors:NASADr. Michael Stamatelatos, NASA HQ, OSMALead Author:Dr.

2 William Vesely, SAICC ontributing Authors (listed in alphabetic order):Dr. Joanne Dugan, University of VirginiaMr. Joseph Fragola, SAICMr. Joseph Minarick III, SAICMr. Jan Railsback, NASA JSCF ault tree Handbook with Aerospace ApplicationsVersion TTTrrreeeeee HHHaaannndddbbbooooookkk wwwiiittthhh AAAeeerrrooossspppaaaccceeeAAAppppppllli iicccaaatttiiiooonnnsssAcknowledgementsT he project coordinators and the authors express their gratitude to NASA Office of Safety andMission Assurance (OSMA) management (Dr. Michael Greenfield, Deputy AssociateAdministrator and Dr. Peter Rutledge, Director of Enterprise Safety and Mission Assurance) andto Mr.

3 Frederick Gregory, NASA Deputy Administrator, for their support and encouragement indeveloping this document. The authors also owe thanks to a number of reviewers who providedconstructive tree Handbook with Aerospace ApplicationsVersion TTTrrreeeeee HHHaaannndddbbbooooookkk wwwiiittthhh AAAeeerrrooossspppaaaccceeeAAAppppppllli iicccaaatttiiiooonnnsssForewordNASA has been a leader in most technologies it has employed in its programs over the of the important NASA objectives is now to add Probabilistic Risk Assessment (PRA) to itsrepertoire of expertise in proven methods to reduce technological and programmatic tree Analysis (FTA) is one of the most important logic and probabilistic techniques usedin PRA and system reliability assessment to perform risk and reliability assessment in the early 1960s originated in US aerospaceand missile programs.

4 Fault tree analysis is such an example that was quite popular in the midsixties. Early in the Apollo project the question was asked about the probability of successfullysending astronauts to the moon and returning them safely to Earth. A risk, or reliability,calculation of some sort was performed and the result was a mission success probability that wasunacceptably low. This result discouraged NASA from further quantitative risk or reliabilityanalysis until after the Challenger accident in 1986. Instead, NASA decided to rely on the use offailure modes and effects analysis (FMEA) and other qualitative methods for system safetyassessments.

5 After the Challenger accident, the importance of PRA and FTA in systems risk andreliability analysis was realized and its use at NASA has begun to nuclear industry began to utilize probabilistic risk assessment to assess safety following theThree Mile Island accident in 1979. In 1981, the US Nuclear Regulatory Commission (NRC)issued the Fault tree Handbook , NUREG-0492. Over the past two decades, this document hasbecome the leading technical information source on how FTA should be performed. Althoughoriginally intended for nuclear power Applications , the Fault tree Handbook has beenextensively used in all fields where this powerful systems analysis methodology was the past two decades, probabilistic risk assessment and its underlying techniques, includingFTA, has become a useful and respected methodology for safety assessment.

6 Because of itslogical, systematic and comprehensive approach, PRA and FTA have been repeatedly provenFault tree Handbook with Aerospace ApplicationsVersion of uncovering design and operational weaknesses that escaped even some of the bestdeterministic safety and engineering experts. This methodology showed that it was veryimportant 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 multiplehigh-probability and nearly benign events. Contrary to common perception, the latter isoftentimes more detrimental to safety than the foremost strength of PRA and its underlying analysis techniques, including FTA, is that it is adecision support tool.

7 In safety Applications , this methodology helps managers and engineers finddesign and operational weaknesses in complex systems and then helps them systematically andefficiently uncover and prioritize safety order to best benefit from PRA and FTA in management decisions, it is important thatmanagers and their support staffs be familiar with the value and application of these methods. Inaddition, there should be a small but robust group of in-house technical experts that understandthe methods used in a PRA or FTA study, can explain its meaning and applicability to givenproblems to management and serve as in-house technical advisers to the management decisionprocess for safety improvement.

8 If these in-house experts do not exist initially, they should behired or groomed through training and transfer of technology, becoming part of the corporateresources and memory that will help shape the organization, taking advantage of the PRA andFTA methods and results and the expert knowledge of the external consultants. In-house expertswill help build risk-based knowledge and experience and stimulate cultural changes so that aprogressive organization can use these resources to make sound and cost-effective safetyimprovement support of this, NASA has recently began to implement the following important riskassessment enhancement principles in its programs and projects.

9 Transfer quantitative risk assessment technology to NASA managers and practitioners assoon as possible, Develop or acquire quantitative risk assessment expertise and state-of-the-art softwareand data, Encourage ownership in quantitative risk assessment methods, studies and results in orderto use them effectively in the management decision process, Develop a corporate memory of the risk assessment project results and data on which tobuild future capabilities and experience, and Develop risk awareness in programs and projects that will eventually help NASA developa risk-informed culture for all its programs and this end, and in support of the Risk Management Program, NASA began to develop trainingand practitioner documents on how to perform quantitative risk assessment and utilize importanttechniques like FTA.

10 One such document is a Procedures Guide for performing PRA foraerospace Applications . The other is this document, the re-issue of an updated version of theFault tree Handbook for Aerospace considerable amount of material on PRA methods and Applications has been written over thepast three decades. Several university and practitioner textbooks and sourcebooks currently existFault tree Handbook with Aerospace ApplicationsVersion they focus on application of PRA in industries other than Aerospace . Although some of thetechniques used in PRA originated in work for Aerospace and military Applications , nocomprehensive reference currently exists for PRA Applications to Aerospace systems.