Safety Critical Systems Analysis

This assignment delves into the realm of safety-critical systems, requiring a thorough understanding of their design, implementation, and analysis. Students are tasked with examining various aspects of these systems, including the application of formal methods for ensuring their reliability and robustness. Case studies and best practices will be explored to provide practical insights into developing and managing safety-critical software.

Running head: Safety Critical System 1
Safety Critical System
Name
Affiliate institution

Running head: Safety Critical System 2
Abstract
In today’s systems, dependency on software has greatly increased due to technological
evolutions. Currently, computers control nearly everything including safety critical system such
as hospitals, airports, nuclear reactors among others. Software choice for this systems is very
critical and needs enhance research on requirements and functionality. However, software
programs can have a great influence on safety installations. As much as there are a lot of risks in
depending too much on software, it has exceedingly got into safety critical installations. For
security systems choosing an ideal software to control them is very essential. Choosing either
free software or commercial software is a factor to consider. As much as free software is greatly
customized and is updated frequently, developers may develop loopholes that can be used to get
into the system from the “backdoor”. Commercial software on the other hand can be good for
critical systems because the vendor can be made accountable for any failure. Thorough criticism
has to be done on the two types of software to check on the security and reliability of the
software. As far as dynamic evaluation methods and traditional testing techniques can be good to
identify functional errors, they are insufficient when the software can cause injury or death. In
this regard, safety critical system software must be able to handle the problems determined
through safety evaluation to enhance system safety. Any failure of a software in safety critical
installation can lead to a catastrophe including environmental degradation, injuries or even death.

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Table of Contents
Abstract............................................................................................................................................2
Introduction......................................................................................................................................4
Research problem and objectives.................................................................................................4
Method.........................................................................................................................................4
Scope............................................................................................................................................4
Free software...................................................................................................................................5
Commercial software.......................................................................................................................5
Safety Critical (SC) systems............................................................................................................5
Safety-Critical Systems Technical Best Practices.......................................................................5
Use of mission-thread and quality attribute scenarios analyses.............................................5
Specifying and prioritizing safety-critical requirements..........................................................6
Conduct static and hazard analyses..........................................................................................7
Conclusions......................................................................................................................................8
Reference List................................................................................................................................10

Running head: Safety Critical System 4
Introduction
When dealing with safety critical system, human life, environmental conditions and other
animals live are very important. As such, software for such system should be able to address
problems that risks human and animal life and environment. Free and commercial software
implemented in this systems should be able to enhance the safety of human life. Free or software
can be installed in this systems but enhance analysis and evaluation should be done to them. No
doubt should be left out when it comes to safety of human life. A software installed in an
airplane for example is very critical, in case of malfunction, live are put at risk and may lead to
death. Also in nuclear reactors, the formulas embedded in the software controlling the reaction is
very essential, any malfunction may cause the reaction plant to explode and cause catastrophic
trauma including loss of human life, animals and environmental effects.
Research problem and objectives
The goal of this report is to identify safety assurance of both free and commercial software in
safety critical systems. Some of the issues to be addressed include: if the software critical system
can be acquired from open source or commercial based, quality assurance during development of
the software, requirements to develop and software for the safety critical systems, and how
security and safety precautions are handled.
Method
The techniques and methodologies used in this report are literature study on journal articles,
books and online materials that have discussed software for safety critical systems. It is however
inclined on safety precautions and quality assurance.
Scope
There are various aspects that define software programs for safety critical systems. Guidelines,
laws and standards provides the procedures for determining software requirements, analysis of
risks and documentation. This paper focuses on requirement specification, safety and risk
analysis and quality assurance of both free and commercial software.

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Free software
Free software is the software built by programmers where the source code is licensed without
any charges encouraging changes and improvements. The users or individuals are not restricted
on the usage of the software and the charges are zero. (Ramachandran & Nair, 2012)
Commercial software
Commercial software is any program structured and built for sale or licensing to the consumers.
Examples of commercial software are the off-the-shelf programs like games or those licensed in
stores with computer specialty or grocery and music stores. Some of the commonly known
examples of commercial software are the products of Microsoft like MS Office and Windows
Operating System. (Kelty, 2008).
Safety Critical (SC) systems
Software systems are becoming crucial to the daily activities. Safety critical systems failure may
cause death, serious injury, loss of equipment, or environment harm. Examples of safety critical
system include; systems that help fly commercial airliner, enhance application of brake in
vehicle, manages trains flow on rails, controls shutdown of nuclear reactors and infusion of
medications to patients. (O'grady, 2015)
Safety-Critical Systems Technical Best Practices
Use of mission-thread and quality attribute scenarios analyses.
Requirements of SC are filed via a combination of workshops of mission-thread and scenarios of
quality attribute. A scenario of quality attribute is a use case that is extended and focuses on a
quality attribute like testability, performance, maintainability, availability, safety and security. A
mission thread is an end to end sequence of events and activities outlined as series of steps that
completes one or more capabilities execution that the system supports. (Berg, 2008).
Product returns analyses and surveys and legal actions can assist in identifying related and safety
operational concerns with the current products. SC systems may malfunction but the failure must
be managed with cautiousness to safeguard the main assets like environment, human lives and

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property. For instance, in cases of failure of infusion pump system treatment should stopped
while in cases like chemotherapy and intravenous feeding it may be more dangerous to halt the
treatment fully than reducing the volume. As such, scenarios of different failure may need
different results. (Popp, 2015)
QAW (quality attribute workshop) is one technique of drawing out the scenarios of SC quality
attributes and recognizing and specifying requirements of SC.
Challenging mission-critical requirements are the SC requirement principal source that develop
the necessity for novel solutions. For instance, military aircraft that is of high performance like
the B-2 Spirit flying wing and F-117 Nighthawk are structured to be highly maneuverable and
aerodynamic, qualities gotten by transmitting stability requirements to the software of flight
control from the pilot. (Dale & Anderson, 2009)
Specifying and prioritizing safety-critical requirements.
For the safety critical system, specify the following; mission-critical requirements that is the
performance, function and behavior. For instance, behavior state machine representations like
state flow of Simulink, charts of UML state or scenario driven threads through functions of
systems to aid in deriving requirements of system behavioral. Secondly is the safety critical
requirements such as security, safety and reliability. (Smith & Simpson, 2010).
Quality attribute specification inherent is a type of measure of the outcome that is desired which
helps to specify the aimed outcome in a greater clarity scenario and evaluating success with
greater goal. Scenarios of quality attribute need some unit of measure. It is essential to use some
performance or behavior measure as the first step to threshold setup. Such measures can be
developed by choosing the requirements of the current approach. For instance, on flight control
the likelihood of both the co-pilot and pilot experiencing heart attack over a 10 hour mission is
around 10^(-9) and this develops a software reliability threshold. (Dale & Anderson, 2011).
The system architecture should identify which each system requirement item in the system
applies to, acknowledging that in some scenarios multiple items may require to collectively meet
a requirement.

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Requirements should be reviewed and identify safety critical requirements and requirements that
are non-safety-critical and which are the most essential. The requirements that need most
attention are the ones that deal with events that have a high likelihood of occurrence or the ones
with most dangerous impacts.
Conduct static and hazard analyses
Static analyses to the system specification should be applied including partial implementation,
mission threads, architecture, quality attribute scenarios, and requirement to assist in determining
what can be wrong and how to control it. it evaluates the output in design points for the safety
critical component.
For instance, in infusion pump, the designs look simple. A pump is required to manage the rate
of motor and a keypad used to key-in the frequency and the dosage, among other things. But
when manufacturing for market that are large is considered. One need to put into consideration
what can go wrong and file situations that will be required to be handled. Such considerations
may miss from the original concept of infusion pump. (Sterpone, 2008)
For instance, embolisms can come as a result of air bubbles entering a patient beyond a certain
level. Certain system components need to be designed to prevent air from entering in infusion
pump line and thus protect the patient. Some kind of sensor is needed from a hardware
standpoint to identify certain size of air bubbles. From some software standpoints if detection of
an air bubble is made, the pump will be required to be shut down and an alarm raised. Similarly,
these actions should be ensured that they occur which means redundancy will be required or
some technique of fault-tolerance to ensure that these actions occur. (Singh & Jain, 2017)
In general, SC system development must counter various operational challenges including how
to address system failure. In other words, the system must control its functions to identify when a
fault is happening, signal that failure is in process, and then make sure that it fails in the correct
way through design techniques of fault tolerance. A lot of redundancy may be required on both
the software and hardware depending on the level of criticality, to make sure that the portion of
fail-safe of the system executes. Another methodology is to implement system of SC as a state

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machine whereby once a failed state is reached by the device, transitions to a safe state
automatically occurs. In the example of fly-by-wire, both failing into a state that is safe and
redundancy are used. Such systems of fly-by-wire have been structured with; fourfold
redundancy which needs voting logic and monitoring to handle disagreement among repeated
subsystems and reversion that is automatic to backups controls that are mechanical and manual.
(Singh, Sharma & Saxena, 2015)
Static analysis techniques like FMEA (failure mode and effect analysis) and hazard analysis
offer proven and broad approaches to system reliability assessment. There are various FMEA
forms but they all go through approaches that are systematic to detect failures, their cause,
control for the chosen root causes, and the type of management needed to identify failure and
mark their control. The FMEA result can cause the need for additional structure like adding a
sensor to aid in identifying failure indication in its control followed by another FMEA iteration
to re-measure new priorities of risks and risk exposure. Hazard analysis result is characterized by
high priority hazard of risks and mitigation including impact severity, likelihood, detection and
prevention of hazard. (Hilbrich & Dieudonné, 2013)
Other methods of analysis focus on the response of the system in resource situations or
corruption of communication. These involves scheduling analyses and timing studies
Conclusions
Technologist and end users should be given priority since they can be able to evaluate the
likelihood of failure and the effect of failure in the context of specific mission. Prioritizing and
specifying requirements need clear understanding of stakeholders in place, why, how and when
they can be engaged in the project.
It is advantageous to research for alternatives in requirement allocation to components since
alternatives may provide high cost especially when options for architectures is put into
consideration. This research is put into consideration to help achieve fail-safe operation. For
instance, various alternatives may help in exploring the use of redundancy.

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It is sometimes unlikely to achieve the requirements set correctly the first time. So some iteration
should be expected through requirements and requirement allocation adjustment specifically as
the tradeoffs, architectures and priorities emerges.

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Reference List
RAMACHANDRAN, H., & NAIR, A. S. (2012). Scilab: (a free software to Matlab). New
Delhi, S. Chand & Company.
KELTY, C. M. (2008). Two bits: the cultural significance of free software. Durham, Duke
University Press.
O'GRADY, S. (2015). The software paradox: the rise and fall of the commercial software
market. http://public.eblib.com/choice/publicfullrecord.aspx?p=3564556. Sebastopol, CA :
O'Reilly Media
BERG, N. (2008). Secrets to a successful commercial software (COTS) implementation. New
York, iUniverse, Inc.
POPP, K. M. (2015). Best Practices for commercial use of open source software Business
models, processes and tools for managing open source software. Norderstedt, Books on
Demand.
SMITH, D. J., & SIMPSON, K. G. L. (2010). Safety Critical Systems Handbook A
STRAIGHTFOWARD GUIDE TO FUNCTIONAL SAFETY, IEC 61508 (2010 EDITION) AND
RELATED STANDARDS, INCLUDING PROCESS IEC 61511 AND MACHINERY IEC 62061
AND ISO 13849. Burlington, Elsevier Science.
DALE, C., & ANDERSON, T. (2009). Safety-critical systems: problems, process, and practice :
proceedings of the Seventeenth Safety-Critical Systems Symposium, Brighton, UK. Berlin,
Springer.
DALE, C., & ANDERSON, T. (2011). Advances in systems safety: proceedings of the nineteenth
Safety-Critical Systems Symposium, Southampton, UK, 8-10th February 2011. London, Springer.
http://public.eblib.com/choice/publicfullrecord.aspx?p=645662.
STERPONE, L. (2008). Electronics system design techniques for safety critical applications.
Berlin, Springer. http://public.eblib.com/choice/publicfullrecord.aspx?p=418293.
Singh, M. & Jain, V. (2017) An Augmented Framework for Formal Analysis of Safety Critical
Systems. Journal of Software Engineering and Applications, 10, 721-733.
doi: 10.4236/jsea.2017.108039.

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Singh, M. , Sharma, A. & Saxena, R. (2015) Why Formal Methods Are Considered for Safety
Critical Systems?. Journal of Software Engineering and Applications, 8, 531-538.
doi: 10.4236/jsea.2015.810050.
Hilbrich, R., & Dieudonné, L. (2013) "Deploying Safety-Critical Applications on Complex
Avionics Hardware Architectures," Journal of Software Engineering and Applications, Vol. 6
No. 5, pp. 229-235. doi: 10.4236/jsea.2013.65028.