Is an airplane's control software source code open (or to some extent) to external review?

I don't imagine it is open-sourced, but if it is completely a black box, how can we know that any accident is not due to weather, but system drawback?

  • 4
    $\begingroup$ Even if the source code would be open you usually could not derive the cause of an accident from it. Just because some software has an obvious problem does not mean that the problem triggered the accident. Just because something does not have a problem does not mean that it wasn't involved in the accident - it might for example have been fed with unexpected data. There is usually way more needed then the software to determine the cause of an accident. $\endgroup$
    – Steffen Ullrich
    Commented Jan 28, 2022 at 14:02
  • $\begingroup$ Interesting that we seem to have no single tag designed to encompass the entire flight control system including all its components, but not to also include other aircraft systems -- perhaps we need a new tag-- $\endgroup$ Commented Jan 28, 2022 at 15:44

1 Answer 1


Any safety-critical software, of which aircraft fly-by-wire is an obvious example, must be written to the regulations regarding the SSIL (Software Safety Integrity Level).

At SSIL-4 (Defined as - if the software fails many people may die), it is legally required in most jurisdictions that the software is independently audited. So, the source code and full development process is made available to an outside party. This, of course, is a confidential process.

The exact regulations regarding this process vary between jurisdictions. For example in the USA the software must be validated under DO178C. In Europe and much of Asia, one of the CENELEC standards, e.g. BS EN50128 is used - the bane of my working life!.

So, is it a black box? Not necessarily. Can a fault be traced if an accident occurs? Yes, probably. All relevant standards mandate a mechanism for tracing the software processes in great detail. Even near-misses must be analyzed and faults corrected.

  • $\begingroup$ If military aircraft are exempt from this, then that might be worth noting in this answer. $\endgroup$ Commented Jan 28, 2022 at 15:47
  • $\begingroup$ I can't comment on military aspects of this. $\endgroup$
    – Chenmunka
    Commented Jan 28, 2022 at 18:22
  • $\begingroup$ I believe EASA expects DO-178C to be used for avionics. Not having experience with software cert in Asia, it's interesting to look at different certification standards. $\endgroup$ Commented Jan 31, 2022 at 14:55

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