How Do the FACs Control Yaw Damping in the A320?

In the A320, the FACS control several aspects concerning aircraft yaw.
In this snack-sized training, we’ll see how the FACs control and impact the yaw damping process.
What is yaw damping in the first place?
Simply put, in the A320, yaw damping:
  • Stabilizes the aircraft in yaw.
  • Prevents Dutch roll.
  • Coordinates turns.

All of which makes for a much smoother ride, with less side to side yawing.

And from a practical, pilot-centric point of view, it means that whenever you make a lateral stick input, you don’t have to worry about coordinating your turns by stepping on the rudder because the A320 automatically does it for you.

As if that weren’t enough, if you’ve got the AP engaged during takeoff and go-around, the yaw damping function also assists with rudder application after an engine failure.

How come I don’t feel yaw damper or turn coordination inputs on my rudder pedals?
This answer is pretty simple. You don’t feel it because there is no feedback to the rudder pedals from the yaw damper and turn coordinating functions.

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How are yaw damping and turn coordination achieved in the A320?

Good Question.

Let’s start by remembering that the A320 has one rudder surface which controls the aircraft’s yaw.

This rudder surface is moved by three independent servo-jacks, controlled by the Green, Yellow and Blue hydraulic systems.

The ELACs compute the required yaw order for yaw damping and turn coordination and then send these to the FACs.

The FACs then transmit this yaw damping/turn coordination order to the rudder.

To do this, the FACs first send the order to a Yaw Damper Servo Actuator or YDSA.

This YDSA is usually controlled by the Green hydraulic system but is also backed up by the Yellow hydraulic system.

Once the YDSA has received the yaw damping or turn coordinating order from the FAC, it will then drive the three independent hydraulic servo-jacks to move the rudder appropriately.

Remember, there is no feedback to the rudder pedals from the yaw damping and turn coordination functions.In other words, you will not notice any of these movements through the rudder pedals.

The YDSA uses Green and Yellow hydraulic pressure. 

What happens if the G + Y systems fail?

Well, among all of the other shit that will be going wrong, with respect to the FACs, the Yaw Damper Servo Actuator will not be able to do anything since it uses Green Hydraulic pressure and is backed up by Yellow system.

With both of these pressure sources unavailable, the Yaw Damper becomes inoperative!

Don’t believe me?

Check it yourself!

So there you go.

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

Don’t forget the FACs also control other yaw related functions such as the A320’s rudder trim and rudder travel limiter.

We’ll look at these in another training.

Be kind, be smart, fly well.

Until next time…

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