Agreed.. but is still found in the "Autopilot" chapter of the Maintenance Manual
No, it's not. The yaw damper is found in the autoflight ATA 22 chapter, because it's a flight control function and therefore an autoflight function, but it is not part of the autopilot.
The yaw dampers are autoflight functions, but not autopilot functions. As such, they're a flight control function, without any relationship to the autopilot. While it's fair to say that the yaw damper is part of the autoflight system, it's incorrect to say the yaw damper is part of the autopilot system, because it is not part of the autopilot. It's a two-axis autopilot: pitch and roll (not yaw).
How about defining it by what the pitch and roll computers supply,or do? The pitch and roll computers have no input to the rudders. The yaw damper is entirely independent, and operates with or without the flight director or autopilot engaged, active, or operable.
The yaw damper switches are electrical hydraulic shutoff switches. The switches either shut off hydraulic power to the yaw dampers, or they are an active part of the flight control at all times, as part of the stability augmentation function of the flight control, itself.
The autopilot computers operate the ailerons and elevators. Not the rudder. The yaw damper computers are independent from the autopilot/flight director computers and do not take output from them, nor provide input to them.
The previously cited maintenance manual thus describes the system as "
On ALL EXCEPT 747SP, two identical yaw damper systems control the upper and lower rudders. Each system monitors airplane yaw rate and positions the rudder to compensate for periodic yaw oscillations (dutch roll). Correction signals are applied to the rudder packages during manual and autopilot controlled flight to displace the upper and lower rudders sufficiently to damp out any yaw oscillations of the airplane. Rudder displacement is limited to 3.6 degrees. The yaw damper system also provides a turn coordination feature which improves airplane response during turn maneuvers when the flaps are down at least 1 degree. System gains are also changed as a function of flap position. When the flaps are down, the roll attitude signal from the INS is introduced to provide rudder displacement proportional to roll rate. The roll attitude signal is not used when the flaps are up. The yaw damper system is normally engaged for all flight modes and operates full time."
Essentially, the purpose of the yaw damper is to correct for dutch roll (periodic yaw oscillations). Per the MEL, while no autopilots are required for flight (except for RVSM), at least one of the two yaw damper systems are always required. Only one may be inoperative. The autopilots may be entirely inoperative, but the yaw dampers may not.