[SystemSafety] Difference between software reliability and astrology

[M Ellims]

Hi,

Took a while but knew it was somewhere: I checked to see if it was still current but couldn't quickly find anything about it's withdrawal.

AC.1309-14 6/21/88 System Design and Analysis

Experienced engineering and operational judgment should be applied when determining whether or not a system is complex. Comparison with similar, previously-approved systems is sometimes helpful. Al I relevant system attributes should be considered; however, the complexity of the software used to program a digital computer-based system should not be considered because the software is assessed and controlled by other means, as described in
Paragraph 7i

<snip>

7i In general, the means of compliance described in this AC are not directly applicable to software assessments because it is not feasible to assess the number or kinds of software errors, if any, that may remain after
the completion at system design, development, and test. Advisory Circular 20-115A dated August 12, 1986, "Radio Technical Commission for Aeronautics Document RTCA/D0-178A," or later revisions thereto, provides acceptable means for assessing and controlling the software used to program digital computer-based systems.


-----Original Message-----
From: systemsafety [mailto:systemsafety-bounces at lists.techfak.uni-bielefeld.de] On Behalf Of Prof. Dr. Peter Bernard Ladkin
Sent: 22 August 2024 10:33
To: systemsafety at lists.techfak.uni-bielefeld.de
Subject: Re: [SystemSafety] Difference between software reliability and astrology

On 2024-08-22 10:42 , M Ellims wrote:
> The first thing to note is that the figure given is for electronic/electrical hardware and excludes software.

Not so. 14 CFR 25.1309 can be read at https://www.law.cornell.edu/cfr/text/14/25.1309  and we can 
see that it is not restricted.

Note that the European reg is similar but not identical.

> Guidance from the FAA is the software included in any FTA analysis should be assigned a failure rate of zero. The rational being that software failure rates are in general cannot be reliably estimated and thus the dependence/reliance on DO178.

That may be an "acceptable means of compliance" as EASA would say, but it is not what the regs say 
(see above).

PBL

Prof. i.R. Dr. Peter Bernard Ladkin, Bielefeld, Germany
www.rvs-bi.de




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