Yaw Dampers — Page 413, Lesson 511

We're looking at the tail end of the yaw damper chapter now, and this is the part that deals with testing and monitoring the system. Let me walk you through it. First, the monitoring. The system is designed to detect a failure in one of the yaw dampers, and when it does, it takes that inoperative yaw damper out of the loop. The purpose is to avoid spurious rudder inputs — meaning, if a yaw damper has failed, we don't want it sending random, false commands to the rudder. So the monitoring circuitry isolates it. Now, there's a small instrument involved in this monitoring: the rudder trim indicator. There's one of these for each yaw damper system fitted. It shows the demands being placed on the rudder by moving left and right of centre. So if the rudder is being commanded to the left, the indicator moves left of centre; commanded right, it moves right. Its main use is during testing of the yaw dampers in the preflight checks. Here's how the test works. There's a yaw damper test switch. When you operate it for one of the yaw dampers, a test signal is generated. That signal operates a small torquing coil on the yaw damper's rate gyro. Now, the rate gyro is the sensor that detects yaw rate — the rate at which the aircraft is rotating about its vertical axis. The torquing coil physically moves the gyro, and this fools the yaw damper system into thinking a yaw condition actually exists, even though the aircraft is sitting still on the ground. A pass at test is indicated by the position indicator moving in the direction tested and then returning back to centre. So, if you move the test switch to the left, that simulates a yaw in one direction, and the position indicator should move to the left as well. If you operate the switch to the right, then the rudder should move to the right. The key point is the indicator moves in the direction you tested, then returns to centre — that confirms the system is working. Now, not every aircraft has these test switches. Some aircraft rely on the yawing movement of the aircraft during taxi to test the yaw damper system. So, as you taxi and make a left turn, the aircraft yaws left. The yaw damper signal will move left, the yaw damper will then move the rudder left, and then the signal will return to the neutral position. That natural yawing during taxi serves as the test signal. So the whole idea here is verification — either through a synthetic test signal via the torquing coil, or through real yawing during taxi — and the rudder trim indicator is your readout to confirm the system responds correctly and returns to neutral.