SAS 03/ GSFC/SATC-ERAU-DoC Fault Tree Analysis Application for Safety and Reliability Massood Towhidnejad Embry-Riddle University Dolores Wallace & Al Gallo NASA Goddard, SATC NASA OSMA SAS '03

SAS 03/ GSFC/SATC-ERAU-DoC 2 Overview FTA Background SFTA and the System Life Cycle SFTA Examples and Diagrams Modeling System Behavior Advantages & Disadvantages of SFTA Impact of Our Research Summary

SAS 03/ GSFC/SATC-ERAU-DoC 3 Fault Tree Analysis General Hazard Analysis technique started in the 1960s Attributes: –Graphical –Top Down –Analytical Qualitative Quantitative Goal: To identify all conditions that put system in a Hazardous States FTA applied to software –Little work has been done to date –SFTA focuses on the code and requirements –Generally applied to “small” projects (<2000 LOC) Observations: –SFTA impractical at code level –Should be applied to Systems at the early stages of life cycle –Need to address the quantitative analysis

SAS 03/ GSFC/SATC-ERAU-DoC 4 SFTA Applied to System Life Cycle Requirements Phase Design Phase. Coding & Test Highlight requirements for Safety concerns & hazards Perform analysis on elements of the design (i.e., Activity, Sequence, and State diagram) Adjust design to eliminate /mitigate hazardous states... Increase reviews and walkthroughs effectiveness Applied only to critical code Adjust design to eliminate /mitigate hazardous states

SAS 03/ GSFC/SATC-ERAU-DoC 5 SFTA Road Map Requirements, Design, Code and Test Fault Tree Use fault tree diagrams as a graphical communication vehicle for developers, testers, designers, managers and customers Verify and Modify Verify/add missing Customer/Domain expert Software Engineer

SAS 03/ GSFC/SATC-ERAU-DoC 6 Fault Tree Example (Activity Diagram) User enter security data System validate Permission System validate entry Security access denied System update profile Display security screen Update priority: Update Address: Update name: User modifies profile Yes Try > 3 Try <=3 Valid Access No Mod. Accept No Reenter Yes Yes No

SAS 03/ GSFC/SATC-ERAU-DoC 7 Fault Tree Example (Activity Diagram)

SAS 03/ GSFC/SATC-ERAU-DoC 8 Fault Tree Example (Sequence Diagram)

SAS 03/ GSFC/SATC-ERAU-DoC 9 Fault Tree Example (Sequence Diagram)

SAS 03/ GSFC/SATC-ERAU-DoC 10 Fault Tree Example (Sequence Diagram)

SAS 03/ GSFC/SATC-ERAU-DoC 11 Fault Tree Example (State Diagram) Source: Smart Draw

SAS 03/ GSFC/SATC-ERAU-DoC 12 Fault Tree Example (State Diagram)

SAS 03/ GSFC/SATC-ERAU-DoC 13 Modeling System Behavior Hardware –Large sample size –Large collections of historic data –Classification of failure types –Degradation (increase failure rate) Software –Limited sample size (usually one) –Limited availability of software failure data –Classification of cause more relevant –Improvement!!!! (decrease failure rate) –Probability values not available, though subject of research efforts Time Reliability H/W S/W t = release 0

SAS 03/ GSFC/SATC-ERAU-DoC 14 Assessing SFTA Advantages: –Easy to learn –Graphical Representation –Communication vehicle with customer –Partial automatic conversion possible (but not desirable) Disadvantages: –Conversion is labor intensive –Automatic conversion is not attractive –Lack of software reliability data –Timing and Loops need special attention –No dedicated SFTA tool

SAS 03/ GSFC/SATC-ERAU-DoC 15 Impact of Our Research Technology Transfer, Infusion, Recognition & Commercialization –SATC collaborating with commercial vendor of commercial FTA tool Enhancing product to accommodate software uniquenesses Planning to build in paradigm features –GSFC Center Director and SMA Director tracking through center’s Tech Transfer Office SFTA activity and a safety-related tool Assisting in the collaboration / licensing

SAS 03/ GSFC/SATC-ERAU-DoC 16 Summary Applied Fault Tree (FT) to Object Oriented design In addition to fault detection, FT can serve as a communication medium with customer Lack of reliability data reduces the quantitative analysis of the FT Seeking alternative sources of data for quantitative analysis Collaborating with a FT tool vendor to develop software fault tree tool