All the posts show that we only look at the failures of the aeronautical skills, without interest at software reality.
To be sure of the way of flying of such an aircraft with software beside the Captain and the FOs, it is MANDATORY to use computer science rules.
The safety of any software is very complex to establish, but possible.
Find there an short but very important extract of the Inquiry Board report after the first flight of Ariane 5 launcher who ended in a failure (Chairman of the Board J.L. LIONS, Académie des Sciences, Médall Fields) :
"The view had been taken that software should be considered correct until it is shown to be at fault. [...]
The Board is in favour of the opposite view, that software should be assumed to be faulty until applying the currently accepted best practice methods can demonstrate that it is correct".