[SystemSafety] Fwd: Rethinking Safety and Security/11 PCB Layout Myths

[Peter Bernard Ladkin]

David and Mario,

Dictators, monarchs and elected prime ministers are "comparable but not the same". They all run the
country. Would you like to argue that there aren't essential differences?

Suppose you have some software whose performance you have assessed through operational experience
and testing. You know to 95% confidence that the software has a failure likelihood of less than 1 in
10^(-6) per operational hour.

Question 1. Can that qualify the software for a DAL?

Question 2. Can that qualify the software for a SIL?

Answer to 1: No. A DAL varies the confidence level (higher DAL; more confidence needed), but the
requirement in 14 CFR 25.1309 and CS-25.1309 is that any component performs as specified.

Answer to 2: Maybe or maybe not. The reliability requirement for SIL 1 is satisfied. But there is
nothing in the standard about what confidence level is needed. Maybe 95% is OK with some assessors
and not OK with others.

PBL

Prof. Peter Bernard Ladkin, Bielefeld, Germany
MoreInCommon
Je suis Charlie
Tel+msg +49 (0)521 880 7319  www.rvs-bi.de





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