The 727 didn't have a FCC, rather a lousy two-axis Sperry auto-pilot. The two yaw dampers were independent of that lousy auto-pilot in all respects.
I never said the 727 had FCC's.
Strait out of the B-722 SP-150 Maintenance Manual.
B.The autopilot and yaw damper system, hereafter referred to as the autopilot, provides automatic control in yaw, roll and pitch axes. Each axis is connected electromechanically to the hydraulic power units of the flight control system.
(1)Control of the yaw axis is by a dual yaw damper system, which is shown in (figure) and consists of the following: upper and lower yaw damper couplers, related rudder power units, rudders, rudder position sensors, engage switches, a yaw damper ground test switch, an overall autopilot ground test switch, an indicator that displays dual rudder position and system engagement, and relays located in an autopilot accessory unit. An air data computer provides airspeed modulated attenuation of yaw damper rate gyro error signals.
(2)Engagement of the system yaw damper stabilizes the airplane by using yaw rate error signals characteristic of the natural yaw frequency of the airplane. These rate signals, when processed through the system, individually deflect the two rudders a maximum of ± 5 degrees in opposition to yaw rate error. Yaw damper interlocks within the autopilot require engaging either yaw damper prior to engaging the autopilot. Normally, both couplers are engaged when airplane power is applied since the engage switches are lock guarded to ON.
(3)The pilot may yaw the airplane by rudder pedal action when the yaw damper system is engaged without feeling excessive pedal opposition from the system. This is possible since the two inputs to the rudder power units (manual rudder pedals and couplers) are independent and neither input is fed back into the other. Thus, the airplane may be manually yaw maneuvered in a normal manner without input opposition from the yaw damper system. Simultaneous dual inputs of manual and coupler yaw cammands are algebraically summed within the rudder power unit to produce the required rudder displacement.
Granted is no IRU/FCC set up but it is designed on the same but crude principals. Not the Hydro-mechanical Damper mentioned before.