Re: [SystemSafety] Software Safety Assessment

From: Matthew Squair < >
Date: Thu, 9 Jul 2015 14:31:36 +1000


Carl,

My perspective is as follows. In summary I don't think your question or the answers have much to do with 'safety', per se, but a lot to do with role of formalism in engineering.

  1. Is it acceptable to use an obsolete (but still valid) safety standard to assess new software?
    1. Yes, unless there exist latter standards that specifically identify and fix inadequacies in the former, thereby invalidating it. See for example the issue of DO-178C, which resolved a number of recognized problems with its predecessor 178B.

Remember that complying to a standard is not an argument about whether you're safe per se but about compliance with a defined protocol. It's similar to the principle of legal formalism whereby the law is deemed to define itself by rigorous adherence to the statutes even (and perhaps especially) when it cuts against common sense. This is one view of the law by the way.

As to how close the standard gets to safety or the law gets to justice, who knows. But from a formalism perspective we're not arguing that are we?

2. Is the SIL1 claim for 10 year old Project A invalid because the checklist could have been better?

  1. No, from the same perspective, if it's part of the same standard and that has not been explicitly superseded by another. So it's a perfectly valid formalism, go to it.
  2. If Project B used the old checklist from Project A would that be adequate?
  3. From this perspective, yes as long as there was no superseding standard that obsoleted it for that reason (e.g poor checklists).

>From a practical perspective, now that you know there's a problem?

Maybe... But do you? Really?

Matthew Squair

MIEAust, CPEng
Mob: +61 488770655
Email; Mattsquair_at_xxxxxx
Web: http://criticaluncertainties.com

On 8 Jul 2015, at 10:40 pm, Carl Sandom <carl_at_xxxxxx

  It's complicated and I was trying to avoid too much detail to get to the central questions.

It has been 'fielded' and is being 'used' during extended V&V activities (in parallel with the old system) but it is not yet considered fully operational. Safety assessment of some software aspects continues on Program A but not the 'process-based' software development assessment which was the subject of Standard X and the original checklist in 2004. For the scenario, take it as read that Standard X tools and techniques *are still valid* even though it is now obsolete.

My original questions slightly modified are:

  1. Is it acceptable to use an obsolete (but still valid) safety standard to assess new software?
  2. Is the SIL1 claim for 10 year old Project A invalid because the checklist could have been better?
  3. If Project B used the old checklist from Project A would that be adequate?

Cheers

Carl

*From:* Drew Rae [mailto:d.rae_at_xxxxxx *Sent:* 08 July 2015 13:15
*To:* Carl Sandom
*Subject:* Re: [SystemSafety] Software Safety Assessment

Carl,

You may need to clarify here.

The software was assessed 10 years ago.

The project has not yet been fielded.

There must be a missing detail for this not to be 10 year old unused code. Has it been used in some context other than Project A during that time?

On 08/07/2015, at 10:08 PM, Carl Sandom wrote:

  Some important clarifications:

Project A has not yet been fielded but the software was assessed against Standard X 10 years ago.

The techniques applied to Project A were appropriate and fulfilled the requirements of Standard X......at that time and now. But the evidence from the checklist could have been better.

No idea why you assumed unused 10 year old code but that's not the case here.

Cheers

Carl

*From:* Drew Rae [mailto:d.rae_at_xxxxxx *Sent:* 08 July 2015 12:57
*To:* Carl Sandom
*Cc:* systemsafety_at_xxxxxx *Subject:* Re: [SystemSafety] Software Safety Assessment

"Acceptable" either comes from some form of social consensus, or from a belief that the particular techniques applied are appropriate for that particular piece of software. The way you've phrased the question, it sounds like there is significant doubt that the techniques applied on Project A were appropriate for Project A. If Project A hasn't been previously deployed, that's like saying

"I've got this piece of old flex cable sitting under the house. It doesn't meet current electrical safety standards - in fact it wouldn't have met 10 year old safety standards except they were a bit vague and there was a loophole - but I should be allowed to use it anyway. And since I'm using dodgy flex anyway, you don't mind if I apply the same standards to my new wiring as well, do you?".

Compliance with a standard is typically the _minimum_ required for safety. Safety requires compliance, but compliance doesn't give safety. If there's doubt that the checklist for Project A was good enough, no amount of weaselling about standards is going to make it good enough.

As others have said though, if you just want acceptability as a social consensus, then it's not a question that can be answered in the abstract, only in terms of the supplier, customer, and any contract or regulator involved.

Incidentally - someone is resurrecting 10 year old code that's been sitting unused, and significantly hacking it around, and they intend to use it for a safety application? And that's not enough to make people run screaming for cover? I can understand a need to modify legacy embedded code that's been in use, but unused 10 year old code?

On 08/07/2015, at 7:53 PM, Carl Sandom wrote:

   Consider the following scenario:

In 2004 Project A software was assessed against a safety standard (let's call it Standard X). Standard X had a very prescriptive list of software safety requirements and a simple checklist was used for assessing SIL1 compliance.

In 2014, Project B began to integrate significant new functionality into Project A. Standard X, which was by 2014 an obsolete standard, was used to assess the significantly smaller software baseline of Project B. Under modern scrutiny, the simple Standard X checklist used for Project A in 2004 was not as explicit as it could have been and it was decided to use an improved checklist for Project B.

A couple of important questions can be raised with this scenario:

  1. Is it acceptable to use an obsolete safety standard to assess software?
  2. Is the SIL1 claim for 10 year old Project A invalid because the checklist could have been better?
  3. If Project B used the old checklist from Project A would that be adequate?

I've been having some interesting discussions with the Project Managers involved, any thoughts?

Regards

Carl


Dr. Carl Sandom CErgHF CEng PhD

Director

iSys Integrity Ltd.

+44 7967 672560

carl_at_xxxxxx

www.isys-integrity.com




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