I hope that someone can answer my question. My friend had “FD 1+2 Failure” in simulator during the low visibility approach. Flight Directors on PFD disappeared, but no FMA change and no approach capability downgrade. When a go-around was executed, lateral and vertical guidances of FMA went blank (SPD/ / ). On the next approach, he captured G/S and LOC with both FDs failed, but no ROLLOUT indication after touch down. Anybody can explain the logic behind this? Thank you.
On the A320, the FAC takes commands from the FMGC to compute the FD bars. The FD bars and certain FMA annunciations and speeds are provided to the DMC for display on the PFD.
Your FD loss scenario is a partial FAC failure which is why AP continued to work, since the FG function of the FMGC was operational, but the FMA annunciations are faulty. Yaw control is also a function of the FAC which affects ROLLOUT.
See Airbus Safety First Magazine 23 for a similar scenario: the aircraft was operating with an ADR channel disabled under MEL, encountered windshear causing discrepant AOA measurements, the FACs only had two measurements so they tripped off causing the crew to lose FD, characteristic speeds, and FMA.