Re: [SystemSafety] Software reliability (or whatever you would prefer to call it)

From: David MENTRE < >
Date: Wed, 11 Mar 2015 08:50:41 +0100

Dear Mr. Ladkin,

Le 10/03/2015 12:16, Peter Bernard Ladkin a écrit :
> The standard resolution of questions as to whether math is correctly or incorrectly used is to
> provide a proof or a counterexample.

I'll try a counter-example ;-) or more exactly I would like to see your reasoning on software reliability and probabilities on the following example.

Consider reuse of a Inertial Reference System module from Ariane 4 to Ariane 501.

In Ariane 4 this module was working flawlessly. There was a track record for its use, i.e. several successful flights (and I assume numerous tests before those flights).

In Ariane 501 this module was reused, I assume mostly unmodified, because is worked very well in Ariane 4. However on 501 flight an out-of-expected-range input value (horizontal velocity) triggered an out-of-range exception in a float to integer conversion routine. This exception resulted in a diagnostic pattern appearing on a bus and misinterpreted by another software module that leaded to Ariane 501 incorrect trajectory and thus explosion.

(All details in:

For me, until Ariane 501, the module could be characterized a pretty reliable with a high probability.

Then, from Ariane 501, the same module exhibited a very low reliability due to a systematic software failure in the new environment. The systematic failure was latent already in Ariane 4, but never triggered because the horizontal velocity was in expected range. Moreover, the overall software architecture also had a potential failure because a diagnostic pattern could be interpreted as a value. Once again, in Ariane 4 such scenario apparently did not occur.

Would your proposal on Annex D address this case? Under which conditions a software module can be assessed reliable and reused?

Sincerely yours,
D. Mentré

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