During direct mode operation, the ACEs directly control the rudder trim actuator.
When the selector is beyond the 25 degree position, the ACEs cause the rudder trim actuator to move the rudder. The trim rate is 1.4 units per second. The trim range of the rudder in direct mode is a function of whether or not the flaps and slats are retracted.
The manual trim cancel switch function does not operate in direct mode.
Indications
The warning message CONFIG RUDDER shows on the EICAS display when all of these conditions occur:
*
The rudder trim is not within 2 units from center

*
Either the left or right engine thrust is more than the takeoff threshold

*
The airplane is on the ground

*
TAC is not active.

The advisory message THRUST ASYM COMP shows when the rudder trim system is defective.
The status message FLIGHT CONTROL SYS shows when the rudder trim system has failed.
The rudder trim indicator on the center aisle stand is blank when the rudder trim system is defective.

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RUDDER CONTROL -FUNCTIONAL DESCRIPTION -RUDDER TRIM

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RUDDER CONTROL -RUDDER RATIO CHANGER

Purpose

The rudder ratio changer reduces the travel of the rudder. This keeps the effectiveness of the rudder constant and reduces the air loads on the rudder.
Normal Mode Rudder Ratio

In normal mode, the control laws in the PFCs reduce the rudder input command as the computed airspeed (CAS) increases. The rudder ratio changer schedule is fixed.
At airspeeds below 130 kts, the ratio is 1.00 and full rudder pedal movement causes 27 degrees of rudder movement under no load. As the airspeed increases, the ratio decreases. At airspeeds above 430 kts, full rudder pedal movement causes 2.9 degrees of rudder movement under no load.
Secondary Mode Rudder Ratio

In secondary mode, the PFCs reduce the rudder input command by a fixed ratio based on the flaps and slats position. When the flaps and slats are not retracted, full pedal movement causes 27 degrees of rudder movement under no load.
When the flaps and slats are retracted, full pedal movement causes 10 degrees of rudder movement under no load.
Direct Mode Rudder Ratio
In direct mode, the ACEs reduce the rudder input command by a fixed ratio based on the flaps and slats position. When the flaps and slats are not retracted, full pedal movement causes

23.5 degrees of rudder movement under no load.
When the flaps and slats are retracted, full pedal movement causes 10 degrees of rudder movement under no load.

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777 AIRCRAFT MAINTENANCE MANUAL

RUDDER CONTROL -RUDDER RATIO CHANGER

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RUDDER CONTROL -FUNCTIONAL DESCRIPTION -THRUST ASYMMETRY COMPENSATION

General

The thrust asymmetry compensation (TAC) helps control the airplane during asymmetrical engine thrust conditions. The TAC protection function operates in the PFCS normal mode only.
The pilot can disarm the TAC function with the TAC switch on the P5 panel.
Input Signals

The TAC function receives thrust data for the two engines from the engine data interface units (EDIU). The EDIUs use the N1 rotational speed signals from the electronic engine controls (EECs) to calculate the thrust values. The EECs also send other engine data to the EDIUs to make sure the N1 signal is accurate. The EDIUs send the engine thrust data to the PFCs through the AIMS.
TAC also uses the CAS signal from the ADIRU, flap position from the FSEUs and the TAC switch position from the C ACE.
Functional Description

The TAC function arms manually or automatically. TAC arms manually when you move the TAC switch from OFF to AUTO and all these conditions occur:
*
All the signals used by TAC are satisfactory

*
The rudder trim can operate

*
The PFCS is in normal mode

*
The L1 ACE and R ACE are not in direct mode.

When the TAC switch is in the AUTO position, the TAC function arms automatically. The conditions for TAC automatic arm are the manual arm conditions and these conditions:
*
The PFCs find that the airplane is on the ground

*
CAS is less than 70 knots

*
One or both engines are running with the thrust levers at the idle position or the engines are not running.

The TAC function disarms when the TAC switch is in the OFF position. It also disarms for any one of these conditions:
　
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