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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.

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  • $\begingroup$ Can you clarify what the actual behavior is versus the expected behavior? You're asking people to explain the logic behind "this" but I'm not sure what "this" is. $\endgroup$ – user71659 May 13 at 3:09
  • $\begingroup$ FD1+2 Failed on approach.; (A) Go-around was executed, then FMA went blank (SPD/ / ) with AP engaged. (B) The second Cat3 approach was successful with FD1+2 failed, but no ROLLOUT after touchdown. I would like to know why the FMA indicated like A and B with FD1+2 Failed. $\endgroup$ – mach May 13 at 6:12
  • $\begingroup$ FD and Autopilot share common FMA modes, so failure of FD 1+2 has no effect on FMA modes as long as AP(s) are engaged. The key information missing here is if G/A was flown manually (sounds so, as FMA went blank; ie both AP/FD disconnected) and if auto-rollout was attempted, which if I recall correctly should not be done without FDs. Again, after APs are off with FDs failed the FMA will go blank, including no ROLLOUT mode. $\endgroup$ – busdriver May 13 at 6:16
  • $\begingroup$ It was the AUTOMATIC G/A. And, on the second attempt, the AP remained engaged until he saw no ROLLOUT and disengaged AP. $\endgroup$ – mach May 13 at 8:26
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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.

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  • $\begingroup$ FDs stay shown with FMA indication when FAC1+2 Fault happens. So, it doesn’t make sense to me that the PARTIAL FAC failure caused him a loss of FDs and FMA when a G/A was executed.. I was not in the sim and have no clue how his IP set up this malfunction. But, I believe that IP just set him up for merely both FD channels fault and gave him those consequences (If it were caused by FAC fault, he should have seen FAC ECAM message). $\endgroup$ – mach May 13 at 8:40
  • $\begingroup$ @mach Because "FAC 1+2 fault" is another partial FAC fault representing loss of flight control functions. Computers in the A320 have segregated functions, often via hardware on different cards. This is also why there's a difference between the FAC reset PB and the FAC circuit breaker. The pilots didn't realize this, which caused the AirAsia crash. $\endgroup$ – user71659 May 13 at 15:03

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