Originally Posted by
BDAttitude
For new systems for sure. However the question to what extend one can change legacy systems without upgrading them to be fully state of the art (i.e. fulfilling latest standats) is sometimes hazy and aviation industry is not the only industry having skeletons in the closest. But I think you know ;-)
Agreed for hardware. It's usually possible to do a sensible risk assessment and say look, replacing this greased bearing with a ball race is fine because here is a stack of evidence and testing.
But software? No. Changing a single line of legacy code can open a can of worms. Race conditions, overflows, deadlocks... Google that lot. It's like changing the bearing on a door and finding the cabin heater stops working.
It's usually ok to leave legacy code in place on the basis that if it aint broke don't fix it. But a single change (including if you change the hardware it's running on) and really you need to be rebuilding the lot to modern safety critical standards.