FBW architecture - an ex-pilot's viewpoint
Quote from PBL [Feb23/0913], re. A320 FBW architecture:
The FACs, ELACs and SECs run in parallel on the inputs. The outputs cannot be determined by voting (you can't vote with only two processors!) but I don't know how the checks work.
[Unquote]
Although some of your post went over my head, as an ex-A320 driver (from airline launch), found it fascinating and succinct. For a given computer type, we always wondered how genuinely independent the "command" SW programmer-team could be from the "monitor" team. After all, they presumably each have the same basic mission? One is minded of a murder trial, where the jury has to be isolated from outside information sources while reaching its verdict.
To suggest an answer to your question, my simplistic understanding was that, in the event of an anomaly between the command channel and the monitor channel, the computer concerned merely shut itself down. If it was an ELAC or SEC, not much of a problem for us: there were 4 more remaining. And we were allowed an attempt at reset. If it was a FAC, that was 50% of the flight-data calculation gone, but I don't think it was particularly serious. [I write in the absence of my FCOM, which is now way out of date anyway.]
Notwithstanding my second paragraph, and the gloomy predictions that were banded about with such relish when we put it into service 20 years ago, the A320 is now a mature design, and - so far - the worst scenarios expected have been conspicuous by their absence.
Let's hope the same can be said for the B777 in 10-years' time, despite the radically different philosophy adopted by Boeing.
PS: Isn't it remarkable that, 20 years on, the SECs may still be employing an Intel 80186 chip, now ancient history in the home-PC world? We were all buying PCs with 80286/80386 chips, even as the A320s were first going into service.