grounded27:
Yaw Damp in modern aircraft is a calculation in the FCC that constantly corrects rudder position, if you saw the rudder in flight it would look like it was fluttering constantly. Even in the 727 yaw damp is more or less a function of the auto pilot. If I remember older systems were less rate/gyro driven but simply tightened up rudder authority with airspeed hence the above statement about switching off on the 707 for t/o and landing.
Your latest post referencing the SP-150 seems to indicate it and the two yaw dampers were completely separate. That is how we were trained in any case. We could dispatch with an inop SP-150 but both yaw dampers had to be operating.
BTW, our last 10 727-200s were the "advanced" model with, I believe, the Sperry 250. It would do a poor man's CAT three quasi-auto-land. It would flare but not de-crab nor provide roll-out guidance. We were trained on it in the sim but no one in his right mind would actually use it.
