Good question, I don't know the answer. But I can give you some more info on this project: the company, one of the largest freighters operating out of Schiphol already had such software but they had lost the source code. The assignment was to create a new piece of software that produced
exactly identical output. That was the bar to clear. If not for a friend (who got me the job in the first place!) who managed to decrypt the original (it was apparently written in some variation of BASIC with an obfuscation layer but once you had it reversed it would actually list as source) I would likely have never managed to complete the assignment. But once I had that in hand it was much, much easier because I could read all of the original tables.
One of the bigger problems I encountered was that the original software wasn't perfect, so the first step was to replicate it, bugs and all and only then to fix the bugs. So the FAA never entered into it but the Dutch equivalent did. Also, I think there is a big difference in stuff in use in GA versus that which is used for commercial aviation. We're talking early 90's or so, so the details are a bit hazy by now.
Interesting job though, that's for sure, lots of good lessons from that one. I'd never been sent back four or five times by an auditor because my work wasn't up to their standards and every time they sent me back they had a valid point, it wasn't bs just to check a box somewhere, they had valid concerns and I had to rework some of it quite extensively. At the level of "I don't care if the answer is correct, I need to understand why it is correct".