If we turn off FD, A/THR reverts to selective mode. So does the FMGS send signals to the FD which sends it to the A/THR?

 

Thanks

Tricky question,

 

but what I can say is if the Flight Directors disengage in certain modes,

then the Auto Throttle reverts from THRUST mode to SPEED mode and tries to attain the speed target.

 

The Auto-Thrust itself though is a function of the FMGS through the Master FMGC normally.

 

If everything is working and set normally the Autpoilot with flight director and auto thrust are linked together.

 

Not sure if that helps.