Originally Posted by
777Supremecist
Then in a way, would the pitch axis of the C*U be considered as kind of a semi open-looped system? Considering how it only maintains the reference speed, and sacrifices the aircraft pitch/attitude to maintain that.
No, it's very much closed loop. It's just a closed loop on a different parameter (airspeed) than the one you were previously thinking about (G or pitch rate). It can also be many others, like speed (in a cruise control) temperature (in a thermostat) pressure (in some industrial tank system) etc.
Actually, probably the feeling that you're getting at is that the outer loop (speed) disturbs what the inner loop (blend of G and pitch rate) is holding. Only what the outermost loop commands, is ultimately held. The inner loop works in service to the outer loop. This way, a gust that disturbs the pitch rate, can be reacted to by that loop at the appropriate (quick) rate at the short period mode while giving the other loop a stable platform from which to act, which is much slower against speed. If there was only a speed loop, then it would either wallow forever (if acting slow), or pitch back and forth in multiple overcorrections (if acting fast.) It couldn't react to the separate parameters at their own rates simultaneously.
Also, I thought the Airbus fcc still maintains path stability in direct law, right?
Only in a very vague manner of speaking. It reacts directly to a blend of G and pitch rate (this blend called C*), which results in a path, as a consequence. And it doesn't hold a pitch, it only zeroes out the rate. So if a big gust moves the pitch (wasn't successfully cancelled), then it stays at the new pitch.