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Software developed for aircraft needing TA certification pretty much has to be done using a process following RTCA/DO-178C. My question regards low-level (sometimes called "derived") requirements:

Low-level requirements either refine high-level requirements, or express design decisions. I've been told a low-level requirement should be traceable to some statement or small block of code, and at DAL C and above, each piece of code needs to be traceable to a low-level requirement.

Do DERs want to see a test corresponding to each low-level requirement, or is that only so for the high-level requirements? Would it be OK to do testing at a function level ("unit testing") as low-level requirement testing, and are the DERs happy with that?

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  • $\begingroup$ DERs? what is it? $\endgroup$
    – Federico
    Apr 5, 2016 at 7:46
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    $\begingroup$ Design Engineering Representatives. These are people who don't work for the FAA but are authorized by the FAA to guide a product through the safety assurance process. In practical terms, if the DER says you have to do something or supply a certain documentation, you have to do it or you won't be approved. The closest analogue in EASA to the DER is a Panel 10 Coordinator. $\endgroup$ Apr 5, 2016 at 14:48
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    $\begingroup$ uhm, then I probably suggest to rephrase the question. As we do not sit in the head of a DER, we cannot know how they think or what they want, making this question slightly "opinion based". If you can formulate it so that you require only references to documentation or regulations, I think that it might be easier to answer. $\endgroup$
    – Federico
    Apr 5, 2016 at 14:52
  • $\begingroup$ Actually, @Federico, the question is about the current state of the (subjective) DERs' interpretation of the rules. So it is not asking for the answerer's opinion, it is asking for an assessment of the opinions of the people whose opinions count (the DERs). One data point: The consulting DER we have spoken to has said that the low-level requirements do have to be tested ("verified") even at DAL C. $\endgroup$ May 3, 2016 at 12:52
  • $\begingroup$ That does not change much. Answerer's opinions or someone else's opinions, still opinions are. It will be more likely to receive an answer if you would ask "what rules/directives/documents the DER has to base its decision on". $\endgroup$
    – Federico
    May 3, 2016 at 13:16

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One thing I would start with is that low level requirements are not the same as derived requirements, although low level requirements can be derived. Derived requirements are requirements that cannot be traced to a higher level requirement.

For verification of DO-178C Level C software, your SVCP will need to completely cover high level and low level requirements as well as attain 100% statement coverage of your code.

As far as what the DER for your project will approve, there really isn't any way for us to know.

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  • $\begingroup$ I can't think of any better words that this to describe how the 178 objectives are satisfied, great answer. Higher level testing is preferred as it tests the software on the unit connected to real interfaces but where you can't get full condition/decision/statement coverage from High Level tests, you should supplement these with low level/unit tests. $\endgroup$
    – scotty3785
    Feb 10, 2022 at 14:14

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