How Do the FACs Control Rudder Trim in the A320?

In the A320, the rudder trim can be operated in two ways: manually by the human pilot, or automatically by the Autopilot.

In this snack-sized training, we’ll see just how the FACs control the A320’s rudder trim function.
What functions does the rudder trim accomplish in the A320?
The rudder trim system carries out a number of ruddery-related functions.

First, during manual flight, the system executes the trim order requested by the pilot through the RUD TRIM rotary selector (pictured above).

This moves the neutral point of the artificial feel unit by the equivalent of 1 degree of rudder travel per second.

Remember that the rudder can easily be reset to zero as well, simply by using the RUD TRIM RESET PB.

Secondly, when the autopilot is flying, trim orders originate within the FMGS and are then sent to the FACs.

This means that both the RUD TRIM rotary selector and the RUD TRIM RESET PB are inactive whilst the autopilot is engaged.

For those who like to use the pedals to stretch their legs, it may be useful to know that with the AP engaged, if the rudder is pushed out of trim by more than 10 degrees, the AP will disengage.
How do the FACs control the rudder trim in the A320?

The A320 has two electric motors that trim the rudder.

These are the same motors used to control the artificial feel unit.

During normal operation, FAC 1 controls Motor 1 to power the rudder trim.

FAC 2 controls motor #2 and remains synchronized with FAC 1, ready to act as a backup.

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What happens to the rudder trim if both FACs fail?

A failure of both FAC 1 and FAC 2 means that there’s no one left to control the rudder trim.

This means that you will no longer be able to control the rudder trim.

You’re FAC’d!

Don’t believe me?

Check it out!

How do guys screw up using the rudder trim?

In most cases, pilots will only need to deal with the rudder trim on the A320 when a failure requires its use.

Powerplant failures are a common example of this, and require rudder trim to combat the resulting thrust asymmetry.

Here are some helpful tips based on the observation of hundreds of crews:

A common mistake made by players of all ages is to try to engage the autopilot while the rudder is out of trim.

Here’s what it looks like:

  1. Engine fails.
  2. Pilot steps on and holds rudder, but doesn’t trim it out.
  3. Pilot engages autopilot and then promptly releases rudder pressure.
  4. Aircraft experiences a violent yaw swing and autopilot disengages.
  5. The Check Captain’s iPad goes flying through the simulator, hits the back wall and shatters.
  6. Check Captain is most unpleased and begins taking notes.

It’s a pretty sloppy look and is bound to earn you some special comments.

So there you go.

I hope you’ve enjoyed our look into how the FACS control the rudder trim in the A320.

As far as flight control computers go, the FACs don’t always get the attention they deserve, but make no mistake, they’re pretty important and anyway, every little bit of info helps to make your understanding of the machine clearer and more complete.

And that’s a good thing.

Be kind, be smart, fly well.

Until next time…

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FYI. Another interesting point on Rudder Trim is that it also continues to work on the ground, and thus it can directly control Nose Wheel Steering… If you find you are constantly holding in a small amount of Left or Right turn input while taxiing, it is possible to trim that required input out with the Rudder Trim, or, also visa versa, if you see you have a significant Rudder Trim input, that rudder trim will steer the nose wheel exactly in the indicated direction.

Also of interest, there is a Normal Procedures, Supplementary Procedure for Operation with Nosewheel Steering Offset that advises to use the Rudder Trim to determine the actual degree of offset angle in the nose wheel.

Caution! Rudder Trim reset must be performed before takeoff.

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