Re: [SystemSafety] Modelling and coding guidelines: "Unambiguous Graphical Representation"

From: SPRIGGS, John J < >
Date: Thu, 17 Mar 2016 13:09:13 +0000


Hi Matthew,
I do not know the provenance, but I will speculate. Twenty-four hours a day for three hundred and sixty-five days is 8760 hours (I would, of course, have used 8766). It probably arises from someone in committee saying “Surely, a failure-free year is sufficient”. The document was, after all, developed by an industry association of equipment suppliers, I assume that they would not want to make it too hard. I know ED-109 is not for avionics, but if they had read across from the then-current JAR-25 requirements for civil avionics, which did not use risk matrices, preferring hard limits, they would have found a larger number of hours was required for the equivalent assurance level (and then, perhaps, they should be looking for a 95% confidence level rather than saying, “We assume that there was a single failure just after we stopped monitoring at time T, so we will use the quantity 1/T to compare with the limit in the JAR”…

John
From: Matthew Squair [mailto:mattsquair_at_xxxxxx Sent: 17 March 2016 12:45
To: SPRIGGS, John J
Cc: Peter Bernard Ladkin; systemsafety_at_xxxxxx Subject: Re: [SystemSafety] Modelling and coding guidelines: "Unambiguous Graphical Representation"

Strangely that COTS low number has been 'used in anger' on a project of mine. Any idea were it came from?

Matthew Squair

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

Peter wrote: "... let me refer you to the current edition of IEC 61508, Parts 2 and 3. The conditions on "proven in use" for SW are to my mind incoherent. "

IEC61508 is not alone in being incoherent on this matter, EUROCAE Document ED-109 (RTCA/DO-278, if you prefer) sets assurance levels on the basis of the severity of the risk that is being mitigated but, in a note about using service history to support assurance for COTS and the like, it suggests a (low) numbers of failure-free hours that can be used to claim achievement of some assurance levels. But, surely, that is "likelihood", which should be orthogonal to severity.

The newer version, ED-109A (RTCA/DO-278A) does not have this note, which may be why some have said that the COTS requirements are much more onerous than in the original...

John

-----Original Message-----
From: systemsafety [mailto:systemsafety-bounces_at_xxxxxx Sent: 16 March 2016 07:23
Subject: Re: [SystemSafety] Modelling and coding guidelines: "Unambiguous Graphical Representation"



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