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SW Safety Principles Conclusions and References

SW Safety Principles Conclusions and References is the sixth and final blog post on Principles of Software Safety Assurance. In them, we look at the 4+1 principles that underlie all software safety standards.

We outline common software safety assurance principles that are evident in software safety standards and best practices. You can think of these guidelines as the unchanging foundation of any software safety argument because they hold true across projects and domains.

The principles serve as a guide for cross-sector certification and aid in maintaining comprehension of the “big picture” of software safety issues while evaluating and negotiating the specifics of individual standards.


These six blog posts have presented the 4+1 model of foundational principles of software safety assurance. The principles strongly connect to elements of current software safety assurance standards and they act as a common benchmark against which standards can be measured.

Through the examples provided, it’s also clear that, although these concepts can be stated clearly, they haven’t always been put into practice. There may still be difficulties with their application by current standards. Particularly, there is still a great deal of research and discussion going on about the management of confidence with respect to software safety assurance (Principle 4+1).

[My own, informal, observations agree with this last point. Some standards apply Principle 4+1 more rigorously, but as a result, they are more expensive. As a result, they are less popular and less used.]

Standards and References

[1] RTCA/EUROCAE, Software Considerations in Airborne Systems and Equipment Certification, DO-178C/ED-12C, 2011.

[2] CENELEC, EN-50128:2011 – Railway applications – Communication, signaling and processing systems – Software for railway control and protection systems, 2011.

[3] ISO-26262 Road vehicles – Functional safety, FDIS, International Organization for Standardization (ISO), 2011

[4] IEC-61508 – Functional Safety of Electrical / Electronic / Programmable Electronic Safety-Related Systems. International Electrotechnical Commission (IEC), 1998

[5] FDA, Examples of Reported Infusion Pump Problems, Accessed on 27 September 2012,

[6] FDA, FDA Issues Statement on Baxter’s Recall of Colleague Infusion Pumps, Accessed on 27 September 2012,

[7] FDA, Total Product Life Cycle: Infusion Pump – Premarket Notification 510(k) Submissions, Draft Guidance, April 23, 2010.

[8] “Report on the Accident to Airbus A320-211 Aircraft in Warsaw on 14 September 1993”, Main Commission Aircraft Accident Investigation Warsaw, March 1994,  Accessed on 1st October 2012.

[9] JPL Special Review Board, “Report on the Loss of the Mars Polar Lander and Deep Space 2 Missions”, Jet Propulsion Laboratory”, March 2000.

[10] Australian Transport Safety Bureau. In-Flight Upset Event 240Km North-West of Perth, WA, Boeing Company 777-2000, 9M-MRG. Aviation Occurrence Report 200503722, 2007.

[11] H. Wolpe, General Accounting Office Report on Patriot Missile Software Problem, February 4, 1992, Accessed on 1st October 2012, Available at:

[12] Y.C. Yeh, Triple-Triple Redundant 777 Primary Flight Computer, IEEE Aerospace Applications Conference pg 293-307, 1996.

[13] D.M. Hunns and N. Wainwright, Software-based protection for Sizewell B: the regulator’s perspective. Nuclear Engineering International, September 1991.

[14] R.D. Hawkins, T.P. Kelly, A Framework for Determining the Sufficiency of Software Safety Assurance. IET System Safety Conference, 2012.

[15] SAE. ARP 4754 – Guidelines for Development of Civil Aircraft and Systems. 1996.

Software Safety Principles: End of the Series

This blog post series was derived from ‘The Principles of Software Safety Assurance’, by RD Hawkins, I Habli & TP Kelly, University of York. The original paper is available for free here. I was privileged to be taught safety engineering by Tim Kelly, and others, at the University of York. I am pleased to share their valuable work in a more accessible format.

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