Yaw Damper 747-400

[747MC]

-400 Yaw Damp

Quote - AMM - "The yaw damper provides damping via the upper and lower rudders to eliminate dutch roll. When the airplane is rolled into a turn, a yawing moment is generated by changing lift vectors due to the changes in relative wind. This moment produces a forward component (reduced drag) on the downgoing wing and a rearward component (increased drag) on the upgoing wing. The result of the yaw moments produce yaw opposite to the intended turn direction. Rudder deflections to counter roll induced yaw are required to achieve a coordinated turn.

The yaw damper modules provide self-tests, full flight regime yaw damping control, and turn coordination. Yaw correction and turn coordination are computed from yaw rate, lateral acceleration, indicated angle of attack, roll angle, roll rate, and true airspeed

Each yaw damper is limited to ± 1 degree of rudder, whereas the total output command of the control law from each yaw damper module may never exceed ± 4 degrees rudder. The authority is further limited as the number of valid ADC and IRU inputs decreases."

The simple answer to your question is each YD system is independent of the other, and has the ability to entirely control the yaw, as a single system and simplify a co-ordinated turn. together they work together to reduce the input that would be singularly required by the other - either upper or lower. Due to the surface area and moment arm the deflection of either is not linear ... only applied to null the forces on read on the the combined sensors.

As far as the ANC03IA001 event is concerned the yaw dampner itself had nothing to do with the failure it was a failure of the PCM " rudder power control module manifold fractured, allowing the yaw damper piston to travel beyond its normal position. This resulted in a full left command input to the main control valve. The main control valve drove the rudder to the full left position. The only association to the YD system is that is contained in the same housing as the PCM and fine tunes the output of the main piston actuators to add or subtract to the input according to the inputs above. As a direct result of this incident an AD was issued on the PCM assembly.