Originally Posted by
msbbarratt
Safety critical analogue control systems are far easier to maintain and repair over extended periods of time than their digital equivalents.
Hardware-wise, maybe. In most other aspects, absolutely not - otherwise the transition from analogue to digital would not have happened.
There's also no need for triplication for a start, at least not from the point of view establishing correct system output.
"Triplication"? I'm unsure as to what you're referring to. If you're referring to the two disparate software implementations used in the Airbus FBW systems of the A320 and her descendants, then there were only two - not three - distinct implementations, and they were not so much a necessity as a "belt-and-braces" failsafe, given that the A320 was the first implementation of its type.
All that an analogue control system is doing is implementing a series of differential equations.
Software likewise, as AirborneAgain alludes to.
The problem with software systems is that they're way too complex
Not necessarily - see AirborneAgain's post.
Analogue control circuits are also largely immune to component selection ... a capacitor is still a capacitor. Obsolescence is a significantly reduced problem.
But in a software-based system, the logical functions can be replaced simply by replacing a ROM IC or by re-writing to an EPROM IC - a much less problematic process than re-jigging discrete hardware across hundreds of airframes.
We won't be seeing A380s, etc. flying once the spares run out.
Airbus/Boeing FBW systems use hardened versions of obsolete commodity hardware - the suppliers won't stop making them as long as there's a demand.