Yaw Dampers — Page 405, Lesson 506

Let's pick up right where the rudder authority question lives, because that's the heart of what a yaw damper actually does in the aeroplane. First, I want to make sure we're anchored on the problem the yaw damper solves. You've got a rate gyro sensing yaw rate, and the aeroplane has a natural oscillatory motion called Dutch roll. Now, the key thing about the filter in this system — and I want you to hold onto this term precisely — is that it's a narrow band pass filter. That means it's designed to pass only signals which change at the frequency of the Dutch roll. So the rate gyro produces outputs for all turns, but only those related to Dutch roll will appear at the input to the servo amplifier driving the rudder servomotor. In other words, the filter is there to reject everything that isn't the Dutch roll frequency, so the rudder only reacts to that specific oscillation, not to normal turns or other disturbances. Now, the DC polarities — and this is a subtle but important point — are greatest when the rate of turn is greatest, and they reverse when the direction of turn, meaning the rate of gyro signal, reverses. So the signal strength tracks the magnitude of the turn rate, and the sign flips with the direction. That's what gives the damper its sense of direction. Let me walk you through the basic signal path, because it's beautifully simple in principle. The rate gyro senses yaw rate. Its output goes through the Dutch roll filter, which is that narrow band pass filter. Only the Dutch roll component survives. That filtered signal then goes to the servo amplifier, and the servo amplifier drives the rudder servomotor. So the chain is: rate gyro → Dutch roll filter → servo amplifier → rudder servomotor. The filter is the gatekeeper that decides what actually gets to move the rudder. Now here's the part I really want you to lock in, because it's a safety and design concept that examiners love: rudder control computing authority. The reason we don't let the yaw damper move the rudder freely is twofold. First, removal of oscillations does not require a large rudder movement. Second, and this is the safety driver, we want to reduce the hazard posed by a yaw damper runaway. So the yaw damper authority is normally only about 3 to 6 degrees left and right of centre. That's the total authority band — the damper can only command the rudder within that small range either side of neutral. And here's the accumulative twist. If 2 yaw damper systems are operating on a single span rudder, their authority is accumulative. So singly, each system could move the rudder by 3 degrees. Together, they can move it by 6 degrees. That's a direct addition — the authorities stack. So if you have two dampers, each limited to 3 degrees, the combined authority is 6 degrees, which still sits inside that 3–6 degree band. That's a deliberate design choice so that even with both systems working, you never exceed the safe authority limit. So the whole philosophy is: small, limited rudder movements, tuned specifically to the Dutch roll frequency, with the authority capped to protect against a runaway failure. That's the yaw damper in a nutshell.