DO-178 & DO-254 Traceability
DO-178C traceability for software and DO-254 traceability for hardware is mandatory, from the top down and also the bottom up. Understand DO178C and DO254 traceability facts and how to deploy the best practices including automation.
PER DO-178 AND DO-254
Traceability is a key aspect of most modern professions. Accountants, researchers, and forensic scientists alike all apply traceability. For high quality and safety-critical engineering development efforts however, traceability is a cornerstone not just for achieving success, but to proving it as well.
Most software, and now hardware, engineering standards require varying degrees of traceability. Why? Traceability is a readily understood means of ensuring that implementation corresponds to specification. Traditionally, traceability has meant simply ensuring that each unique requirement has corresponding test cases which verify the requirement was implemented. In such a simplistic application of traceability, one need merely develop tests to cover each requirement then associate, or “trace” those tests to that requirement. When all requirements were thus “traced” to specific test cases, traceability, and likely the testing, were deemed complete as follows in Figure 1:
As can be seen from the figure below, ARP4754A, DO-178C & DO-254 traceability essentially forms a closed circuit and is bi-directional. It is closed-circuit because traceability is top-to-bottom via requirements to code and tests, as well as bottom-to-top, from code and tests, to the associated requirements. Also, DO-178C and DO-254 traceability should provide for one-to-many and many-to-one associations. What does this mean? Simple: each traceability item can trace to multiple lower-level items, while multiple higher-level items can trace to the same lower-level item. For example, a single ARP4754A System level requirement can (and likely will) be decomposed into multiple lower level requirements; thus that System requirement traces to each of those lower-level requirements. Conversely, a software requirement per DO-178 may need several test cases to verify it; each of those DO-178 test cases thus traces up to that single software requirement to provide aggregate coverage of that requirement. The following traceability figure applies to ARP4754A, DO-178 and DO-254, as excerpted from AFuzion’s DO-178C training and DO-254 training:
Avionics Traceability per ARP4754A, DO-178C, and DO-254 per AFuzion
With modern safety-critical software and hardware engineering processes such as those needed for FAA certification and EASA Certification, traceability is used to answer the following questions for ARP4754A, DO-178C, and DO-254, at a minimum:
- Where is each requirement implemented?
- Is every requirement allocated?
- Is the implementation compliant with the requirements?
- Is the requirement related to Safety, and if so, are traces to related and parent Safety requirements clearly denoted for continual safety analysis impact?
- What verification test cases will be used to verify a given requirement?
- What is the aggregate impact of changing any requirement or code?
- Is the project complete and ready for certification?
- Is the deliverable product indeed what was originally requested and specified?
Do traceability tools need to be formally qualified per DO-330 for FAA or EASA certification? Such would appear so at first glance because they automate traceability aspects required by DO-178C/DO-254. However, if the outputs of the traceability tool are manually verified, qualification is not required. Since most traceability tool users do perform a manual verification of traceability as part of SOI #4, qualification of the traceability is not required or performed.
As a final check, be sure your traceability tool can support a typically basic traceability paradigm such as that depicted below in the figure below taken from AFuzion’s Training for software; note that hardware also needs traceability and ideally would be supported by a common tool/database.
For the remaining 8 pages of this AFuzion Aviation Traceability Technical Whitepaper, please download below.
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