Perhaps more importantly, there's a pretty long engineering history of assuming that "similar" means "don't need to test as much" not working out. Any time you make a change, you can and should be testing the parts as though they were a new design. I mean the most recent example of that was the Boeing MCAS.
I wouldn't call Ariane 5 an evolution of Ariane IV.
Code and digital system re-use in aerospace systems is not uncommon. After all, the fly-by-wire computer system on board the Space Shuttle was derived from the original Apollo flight computer, and they are two very different space vehicles..
Right but the point is it doesn't let you make assumptions that tests aren't needed, just that you expect them to be likely to pass. The design still has to be tested as though its a new system, it's just the re-use hopefully saved some development time and the testing hopefully finds fewer issues.
A new system requiring extensive testing would have alerted the FAA that something was off, and possibly led to a more costly re-certification they were trying to avoid.
That aircraft should never have been allowed to fly.
