Originally posted by Owain Glyndwr ...
Philosophically, the yaw damper is a follower and shouldn't be driving the motion unless the system designers really fouled up and produced an unstable system (which they did not).
Agreed, and thanks for your generous reply. However, it is somewhat difficult to follow what inputs the YD is getting, and what effect NCD data may be having on the Ny Accelerometer inputs to the YD calculation.
The DFDR Lateral Acceleration (sideslip) trace is reflected in the YD output and its amplitude appears to have increased marginally during the descent and has a natural period of around 7 - 8 seconds which only gets interrupted by prolonged left bank commands. As pointed out earlier, the clockwise heading change is greatest when the Ny driven oscillation is dampened by hard over and held left SS inputs. So my feeling is that the PF managed to dampen the Ny oscillations that could have easily resulted in a total LOC.
Having written the above, then noted your latest comments, I am no further ahead in resolving what I still consider is the YD driving the Rudder and "leading" the Roll/Bank. I've had a look at the Pitch Attitude trace and find that maximum yaw coupled with side-slip tended to occur when pitched up with a RH roll bias.
You say that, "
In Alt2 the rudder deflection is controlled by pedal angle and yaw damper commands", and yet in the period we have been looking at, only YD commands have been presented to the rudder. How or why does the Lateral Acceleration trace then synchronize with the YD / Rudder traces? To answer my own question, I have previously posited that the aircraft was effectively 'fish tailing'.
Its not important to the outcome, though helpful in understanding what is actually happening in what appears on the surface to be an aircraft in a benign stall and retaining some lateral stability, i.e. avoiding the 'spiral dive'.
