*
The rudder trim is defective

*
The PFCS is not in normal mode

*
The L1 ACE or R ACE is in direct mode

*
The L1 ACE or R ACE status signal is not available.

The TAC function is active when it is armed and all of these conditions occur:
*
CAS is more than 70 knots

*
The thrust reversers do not have a command to deploy

*
The thrust difference between the two engines is more than 6000 lbs, about 10 percent of the maximum rated thrust.

EFFECTIVITY BEJ ALL  27-21-00 
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RUDDER CONTROL -FUNCTIONAL DESCRIPTION -THRUST ASYMMETRY COMPENSATION

When TAC is active, the PFCs use the difference between the thrust signals from the EDIUs to calculate rudder and rudder trim commands. The TAC function initially calculates a high rate rudder command to make allowance for the yaw moment caused by the thrust difference. This command goes to the ACEs to move the rudder PCUs.
The TAC function also sends rudder trim commands to the ACEs. The rudder trim actuator movement then backdrives the rudder pedals and the pedal position transducers. The PFCs make allowance for the input signals from the pedal position transducers to keep a near constant rudder PCU command.
The PFCs limit the TAC authority to 60 percent of available rudder.
If TAC is active when there is a command for thrust reverser deployment during landing or refused takeoff, the PFCs automatically ramp the TAC trim commands to zero trim. The trim actuator ramps to zero at a speed of 6 units per second.
When TAC is active, the pilot can use the rudder pedals or the rudder trim switches. The PFCs add these command signals to the TAC commands to control the rudder PCUs. If the pilot pushes on the manual trim cancel switch, the PFCs send a command to the rudder trim actuator to remove any input made with the rudder trim selector. The TAC command to the rudder trim actuator is not removed.
EFFECTIVITY
BEJ ALL

If TAC is active during autoland operation of the autopilot, the engine out correction of the autopilot is disabled. This is done so this function of the autopilot does not control during TAC operation. If the pilot moves the pedals, TAC stays active but the autopilot disengages.
The TAC function stops to be active when the thrust difference between the two engines is less than 3 percent of the maximum rated thrust.
If the airspeed data becomes unreliable, the TAC function uses the flap position signal to limit the gain of the rudder and rudder trim command.
Indications
The amber OFF light shows in the TAC switch on the P5 panel and the advisory and status messages THRUST ASYM COMP show when one of these conditions occurs:
*
The rudder trim is defective

*
Loss of the thrust data from 2 of 3 channels in one EDIU

*
Loss of left and right engine running data

*
The two engines are not running in the air

*
The R, L1 or C ACE is failed or in direct mode

*
The PFCS is not in normal mode

*
The TAC switch is in the OFF position

*
The TAC switch position signal is lost.

27-21-00

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RUDDER CONTROL -FUNCTIONAL DESCRIPTION -THRUST ASYMMETRY COMPENSATION
When the TAC function is armed after it was disarmed, the above indications do not show.
EFFECTIVITY BEJ ALL  27-21-00 
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