Journal of System Safety
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From the Editor's Desk: Change
No full textAt the last Executive Council meeting of the International System Safety Society (ISSS), a decision was made to temporarily reduce the number of Journal of System Safety issues to three hard copies per year. This measure was necessary in order to reduce expenses that are currently exceeding income. We’ll continue to provide a high-quality journal by including a little more material in each issue to help make up for the loss. If more members would pay their dues and encourage non-members to join the ISSS, our income would increase enough to go back to the normal number of issues
Assessing the Software Control Autonomy of System Functions in Safety-Critical Systems
Get PDFSoftware Control Category (SCC) denotes the degree of control autonomy, command and control authority, and redundant fault tolerance software has over hazardous system functions of safety-critical systems. The use of SCC for determining the software contribution to system risks is a unique feature of the MIL-STD-882E System Safety Standard. A lower SCC designation means that the software system has a greater control autonomy over hazardous system functions, whereas SCC 1 means complete autonomous control. Software with greater control autonomy over hazardous system functions require greater effort to assure reliability and safety. Correct assessment of the SCC level of hazardous system functions is crucial for optimizing the safety property of a system developed under budget, schedule, and resource constraints. Beyond the categorical definitions provided by the MIL-STD-882E Standard, there is little information on conducting an SCC assessment. To close this knowledge gap, we present an SCC assessment method. Our paper will describe in detail the process and rules for assessing SCC. For illustration, we apply our method to assess the SCC of several safety-significant functions of an automobile’s brake-assist system
Incremental Assurance Through Eliminative Argumentation
Get PDFAn assurance case for a critical system is valid for that system at a particular point in time, such as when the system is delivered to a certification authority for review. The argument is structured around evidence that exists at that point in time. However, modern assurance cases are rarely one-off exercises. More information might become available (e.g., field data) that could strengthen (or weaken) the validity of the case. This paper proposes the notion of incremental assurance wherein the assurance case structure includes both the currently available evidence and a plan for incrementally increasing confidence in the system as additional or higher quality evidence becomes available. Such evidence is needed to further reduce doubts engineers or reviewers might have. This paper formalizes the idea of incremental assurance through an argumentation pattern. The concept of incremental assurance is demonstrated by applying the pattern to part of a safety assurance case for an air traffic control system
The Difficulties with Replacing Crew Launch Abort Systems with Designed Reliability
Get PDFAs the space industry continues to innovate and new paradigms arise to challenge the status quo, human spaceflight is now perceived as safer and more accessible than ever before. This has led to a new line of thinking in which crewed launch vehicles should be reusable and reliable like commercial airplanes, forgoing the need for an abort system. This paper will counter that line of thought with an analysis of the spectrum of coverage historical crew abort systems provided during launch and use historical data from launch rate successes and failures to glean insight into what reliability in the human spaceflight industry can expect when designing the vehicles of the future. This historical launch vehicle reliability will then be compared to system safety standards used in the commercial aviation industry to understand if future designs truly need a crew abort system. Through this analysis, the rationale for why these crew abort systems have historically been used can be better understood
Human Reliability Analysis using a Human Factors Hazard Model
Get PDFHuman Reliability Analysis (HRA) has found application within a diverse set of engineering domains, but the methods used to apply HRA are often complicated, time-consuming, costly to apply, specific to particular (i.e., nuclear) applications, and are not suitable for direct comparison amongst themselves.
This paper proposes a Human Factors Hazard Model (HFHM), which builds an HRA method from the tools of Fault Tree Analysis (FTA), Event Tree Analysis (ETA), and a novel model of considering serial Human Error Probability (HEP) more relevant to psychomotor-intensive industrial and commercial applications such as manufacturing, teleoperation, and vehicle operation. The HEP approach uses Performance Shaping Factors (PSFs) relevant to human behavior, as well as specific characteristics unique to a system architecture and its corresponding operational behavior. The HFHM tool is intended to establish a common analysis approach, to simplify and automate the modeling of the likelihood of a mishap due to a human-system interaction during a hazard event.
The HFHM is executed commercial software tools (MS Excel and SysML) such that trade and sensitivity studies can be conducted and iterated automatically. The results generated by the HFHM can be used to guide risk assessment, safety requirements generation and management, design options, and safety controls within the system design architecting process. Verification and evaluation of the HFHM through simulation and subject matter expert evaluation illustrate the value of the HFHM as a tool for HRA and system safety analysis in a set of key industrial applications
System Safety Bookshelf: System Safety for the 21st Century, 2nd Edition
No full textOver many decades System Safety has evolved from a more re-active nature - learning from failures and improving – not really suitable for high consequence enterprises - to today’s more pro-active form. This is now based on better fundamental understanding, better assessment processes, better standards, more comprehensive analysis tools with better audit and regulation procedures. However, unlike ‘set educational subjects’ such as engineering, science, technology and mathematics, there are less opportunities for formal System Safety education and training in academia and elsewhere, even though system safety impacts on all aspects of life. One hopes that this will continue to be rectified.
This leads us directly to the importance and value of this book, which gives a complete insight into the nature of what System Safety is all about, including its approaches, methodologies and tools, and which provides guidance on the successful application of a comprehensive, pro-active approach for ensuring safe system design
TBD
Get PDFThis series of events and hospital stays has given me a lot of time to observe the patient-staff interactions of a small slice of the healthcare system from the “System Safety” point of view. There are many excellent papers on many safety aspects of the healthcare industry, including Dev Raheja’s extensive writings about many of the system safety issues found with health care facilities. His many “System Safety in Healthcare” articles in the Journal of System Safety cover a wide range of topics, from the importance of system safety analyses for medical hardware to the “softer” human factors issues of providing healthcare. His articles are an excellent source of information, considerations and recommendations for improvements across the field of healthcare. In this article I am looking at the problems from a slightly different perspective, that of a “user” of the healthcare system
Quantification of Benefits for Medical Devices
Get PDFOne of the most prominent challenges in safety risk management of medical devices is the Benefit-Risk Analysis. This paper proposes a methodology to quantify benefits, thereby creating more consistency, and explainability in the evaluation of benefits and the benefit/risk ratio.
Leveraging the guidance from the FDA, we define four Dimensions for appraising benefits. The product of the rankings of a benefit in all four Dimensions is used as a quantitative measure of a benefit.
The quantitative score for the overall benefit of a medical device would be the sum of the scores of the individual benefits
Model Based Functional Safety – How Functional Is It?
Get PDFAs the engineering world embraces Model Based System Engineering (MBSE), the system safety discipline should also enfold and support MBSE methodology and approaches. The need for Model Based Functional safety, as part of the established system safety and software safety process, is becoming apparent due to existing and developing system design complexity. This paper is intended to show how valuable Model Based Functional Safety approaches can be when evaluating safety signification functions of complex software-intensive integrated systems. Using models can improve the accuracy during the Functional Hazard Analysis (FHA) and can help validate Fault Tree Analyses (FTA) and subsequent system safety analysis (SSA) process and results because the model focuses on the architecture, the physical system, the computer system, as well as the applicable software/middleware/Programmable Logic Devices (PLDs). Model Based Functional Safety may utilize use cases, structural architecture models, activity diagrams, sequence diagrams, functional flow diagrams, and state/mode models to depict safety attributes and to influence explicit safety requirements. SysML may be used to depict critical functions, functional threads, safety features, and expected behavior. Such augmented models (safety models) can also be used to analyze potential off nominal failure conditions and system behavior for various scenarios when conducting FHAs and subsequently detailed system and software safety analyses. This paper will provide an example of the MBSE framework and concepts for tool use in the functional safety analysis and the utilization of attributed models and artifacts to supplement system safety documentation
Letters to the Editor
Get PDFSoftware Safety vs Software Reliability
While looking back through Vol. 56, No. 1 (Summer 2020) of Journal of System Safety, I finally took the time to read Nathaniel Ozarin’s article “Lessons Learned in a Complex Software Safety Program.” The article is quite interesting and thought provoking, comparing what actually occurs while implementing a system safety program to the idealized descriptions found in documents such as MIL-STD-882, JSSSEH and AOP-52. While I found the article interesting and informative, I noted that the author consistently characterizes the “software safety problem” as a “reliability” problem, focused on finding and preventing “failures” and ensuring high “reliability.”
Some Thoughts on the Probabilistic Criteria for Ensuring Safe Airplane-System Designs
We have been employed in the risk sciences for a total of 86 years, including 62 years in reliability engineering and safety engineering positions at The Boeing Company. For many of those years, Yellman was the designated “Risk-Analysis Focal” (person) for Boeing’s 707, 727, 737 and 757 airplane models. For several decades, the United States government has published the same criteria, created by the U.S. Federal Aviation Administration (FAA), intended to ensure that the systems on large (transport-category) aircraft have been designed to be safe [Refs. 1 and 2]. But we believe that the criteria have failed to prevent certain aircraft accidents, and we think that the reasons for that should be better understood. We hope that this discussion will contribute to a better understanding by examining the part potentially played in those accidents by the FAA’s criteria that are defined probabilistically