Right, let's pick this up with the series yaw damper — Page 413, Lesson 509

Right, let's pick this up with the series yaw damper. We've got the transfer valve driving the yaw damper actuator, which in turn drives the main actuator. Now, here's a key limit to remember: the maximum deflection of the rudder from this system is just 3° to 4° in either direction. That's a small authority, and it's deliberate — the yaw damper is there to smooth out oscillations, not to fly the aircraft for you. Now, let's look at the feedback loop that keeps this thing honest. We have the Linear Variable Differential Transformer — the LVDT — which senses the rudder's actual position. That position feedback is applied to a summing point, which we call SP2. Its job is to cancel out the processed rate gyro signal. Think about that: the rate gyro tells the system how fast the aircraft is yawing, and the LVDT tells it where the rudder actually is. When the rudder position change is appropriate for the rate of yaw change, the feedback cancels the gyro signal, and the system is satisfied. But there's more. Position feedback is also applied through the energized relays of RL2, and that same feedback is fed back to SP2. The purpose of this whole arrangement is to ensure the rudder will always return to the neutral position. That's the fail-safe behaviour — the rudder must not stay deflected. Now, here's where it gets interesting, because the rudder can be affected by crosswinds. Imagine a strong crosswind holding the rudder off-centre. The position feedback voltage may not be large enough on its own to drive the rudder back to the central position. So what happens? The position feedback voltage now causes INT 2 to ramp up. INT 2 is an integrator — it builds up its output over time. That increasing voltage goes to SP2, and this increase in position feedback starts to drive the rudder back to neutral. And as the rudder returns, the position feedback voltage decreases, which allows the integrator to run down. So it's a self-correcting loop: the integrator provides the extra push when the basic feedback isn't enough, and it relaxes once the rudder is back where it belongs. Let me show you the schematic so you can see how these components connect. Now, let's talk about testing the yaw damper. When you actuate the yaw damper test switch — either to the right or to the left — you apply a voltage to the yaw damper rate gyro torquing coil. That coil torques the rate gyro, which simulates aircraft movement. So you're fooling the gyro into thinking the aircraft is yawing. The rudder position indicator responds to this, because there's an output from the position transducer. Here's the specific behaviour you'd see. If you move the switch to the left, the indication will first move to the left, and then back to the centre. Then, on release of the switch, the indication will move to the right and back to the centre. The reverse happens if you first move the switch to the right. So it's a symmetrical test — you're verifying the system responds in both directions and returns to neutral. Finally, let's look at the indications in the cockpit. On a typical aircraft fitted with two yaw dampers, the controls and indications are fairly simple. You'll have a panel, or part of a panel, that contains an on/off switch, a test switch or button, and a failure light for each of the yaw dampers. That failure light is important, because it indicates many different faults in the system. It could mean loss of hydraulic or electrical power. It could mean a logic failure in the yaw damper computation system. Or it could mean loss of input from the rate gyro. So the light is a general warning — it doesn't tell you which specific fault, just that something in the system isn't right. And that's why the switch is there: once the failure light shows, you can use the switch to isolate the faulty yaw damper.