Yaw Dampers Vs. T.A.C.

[tom775257]

Hi all,
Sadly another one of my stupid questions:
The way I understand it (I fly only GA aircraft) the yaw damper moves the rudder to prevent unwanted yaw, and to keep the aircraft in balance. Does the yaw damper have a limited authority over the rudder?....if it doesn't have limited authority, how come on say a 767, with an engine failure, rudder application by the pilot is needed...surely if the yaw damper has full authority, incase of assymetric thrust, it would just compensate for this. Secondly is thrust assymetry compensation essentially just a yaw damper with more authority over rudder movement?? Do any other aircraft apart from the 777 have T.A.C.???
Thanks for any answers.
tom.

[Intruder]

Yes, the yaw damper has limited authority. It depends on the airplane, but likely the damper can put less than 5% of full throw on the rudder, and usually much less than that. Remember, the damper is designed to reduce the _rate_ of yaw, not necessarily the yaw itself.

The 747-400 has what you describe as T.A.C. only in the Approach mode, when all 3 autopilots are working simultaneously. In normal single-autopilot mode, the autopilot does not control the rudder (though the dual yaw dampers are still operating).

[Volume]

Tom,

you are confusing two completely different things :
The yaw damper is not a sideslip angle controler, it compensates movement around the vertical axis, not misalignment with flight direction. The yaw damper is either using the inertial platform or a lateral accelerometer at the nose or the tail of the plane to detect movement around the vertical axis (yaw) and gives adverse rudder to stop this movement. It is designed to avoid dutch rolling of the plane and not to avoid sideslip angles. If the plane is flying stable with 5° sideslip (ie. due to engine thrust asymetry), the yaw damper is not doing anything.
In case of sudden engine failure a yaw will occur and the yaw damper will give adverse rudder, but limited autority will produce a certain sideslip angle. If this angle is stabilized, the yaw damper will not react anymore to the situation.

[tom775257]

Thank you,
That information from you two clears that one up for me.
One final question then (assuming I am understanding this correctly): If the yaw damper responds to acceleration around the vertical axis, what information inputs does the T.A.C. need? Can it gain information on whether a side-slip is occuring from the inertial system?
Cheers,
Tom. :)