What would happen to the rudder of an A320 if both Flight Augmentation Computer (FAC) systems completely failed during mid-flight? Would the auto pilot immediately pass control back to the pilots or if they failed would that be it? Would it be impossible for the cockpit to communicate with the mechanisms which control the rudder or is manual operation possible in an A320?

Also, I know this maybe a more difficult/impossible questions but what would cause the malfunction of one of the FAC computers? Are there any likely scenarios or previous cases?

  • This is mostly a guess, but I think the rudder trim wheels would still work. Also the only way I could imagine both FAC's failing is if they power to them was switched off. Avionics software and hardware is incredibly reliable. – raptortech97 Jan 30 '15 at 17:33
  • @raptortech97: According to the diagrams here, rudder trim is purely electronic. – Henning Makholm Jan 30 '15 at 17:38
  • @HenningMakholm ah, you're right. I must have been thinking of the elevator. – raptortech97 Jan 30 '15 at 17:40

The A320 has a mechanical link from the rudder pedals to the hydraulic rudder actuators. Computer-generated rudder commands for yaw damping and turn coordination are added to the mechanical signal from the rudder pedals, but in the event of a two-sided computer failure the rudder can still be controlled.

The elevator trim wheels are also mechanically connected, so rudimentary control of the aircraft remains even in the event of a total electronic failure, as long as there's hydraulic power.

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    @Federico: Hm, I may be misreading the diagrams, but to me it looked like the only thing that actually grips the rudder surface are three hydraulic actuators. If the hydraulic pressure goes away, what good is it that the pedals are mechanically connected to their controlling valves? – Henning Makholm Jan 30 '15 at 18:38
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    I went to check and you're correct, also the THS requires hydraulic power, I remembered otherwise. – Federico Jan 30 '15 at 18:49
  • @Federico: The hydraulic systems will still work for some time if they stop being powered because some power is accumulated in the reservoirs. They won't work if they are all punctured and there is no backup for that. – Jan Hudec Jan 31 '15 at 16:56
  • Thanks everyone this is all really helpful, so just to clarify even if both FAC systems fail, or direct power is pulled so they can no longer operate, manual rudder control is still possible as long as the hydraulic pumps connected to the rudder remain operational? Thanks again – user7077 Feb 2 '15 at 11:51
  • @user7077: Yes. – Henning Makholm Feb 2 '15 at 12:48

The Flight Augmentation (FAC) performs the following functions:

  1. Yaw damper Rudder travel limiting
  2. Monitoring of the flight envelope and computation of maneuvering speed
  3. Yaw autopilot order
  4. Bite fuction of the AFS

The trim actuator, the yaw damper servoactuators and the travel limitation unit are normally controlled by the flight Augmentation Computer (FAC 1) and FAC 2 being in standby. The rudders are controlled either mechanically from the pedals or electrically when the auto pilot is engaged. Therefore if both FACs fail, the rudder control remains unaffected. However the autopilot is no longer available.

Only the Rudder has a mechanical link which remains functionnal enough even with electrical commands failure. Landing with it together with engine control only has been tried during certifications.

  • this is partially incorrect. The mechanical backup includes the THS trimwheel. – Federico Apr 9 '15 at 8:55

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