The EICAS caution message PRI FLIGHT COMPUTERS shows when all ACEs under power are in direct mode and the PFC is not in the hydraulic OFF self-test mode.
On the status display, the status message FLIGHT CONTROL SYS shows when the PFCS does not operate at minimum dispatch requirement levels. Some of the faults that cause the FLIGHT CONTROL SYS status message are listed below. Other PFCS faults can also cause the FLIGHT CONTROL SYS status message.
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Failure of more than one column, wheel, or speedbrake lever position transducer

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Failure of a pedal transducer

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Failure of a rudder, elevator, aileron, flaperon, or spoiler PCU

*
Failure of a STCM

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Failure of an elevator feel actuator

*
Failure of a PFC and at least one lane in another PFC

*
Failure of an ACE or at least one ACE is in direct mode

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Failure of the stabilizer control

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Failure of the actuator confidence test

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Failure of a PSA, a flight controls ARINC 629 bus, or a WES channel.

The EICAS caution message FLIGHT CONTROLS shows when the flight control system has lost substantial capabilities. Any one of these faults causes the message to show:
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The stabilizer is not operative

*
Two elevator PCUs and one stabilizer path are not operative

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PFCS -OPERATIONAL MODES -FUNCTIONAL DESCRIPTION
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The wheel, column, or pedal position transducers for the captain and first officer disagree by more than 15 percent of full range with the airplane on the ground

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The elevator force fight equalization function is defective.

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PFCS -FUNCTIONAL DESCRIPTION

General

The primary flight control system supplies roll, pitch and yaw control to set the desired airplane attitude.
At the end of each flight, the PFCs do internal tests and select the function of each lane.
At the completion of power-up and with the hydraulic systems pressurized, the PFCs command the surfaces to slowly move to agree with the command from the control wheels, control columns and rudder pedals.
On the ground without the engines running, the system receives electrical power from a secondary source. When the engines start, the system automatically switches to the PMGs as its primary power source.
Roll Control

The roll control causes the ailerons, flaperons and spoilers to move. When the pilots move a control wheel, six position transducers send analog signals to the PFCs through the ACEs. The PFCs select the signal to process. The control laws, protection functions and stability augmentation of the PFCs, calculate the command output signals. After selection of one signal among the three supplied by the PFCs, the PFCs send the command output signal to the ACEs. The ACEs then command the control surface PCUs. PCU position transducers send feedback signals to the ACEs.
EFFECTIVITY
BEJ ALL

The PFCs also control:
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The aileron lockout during cruise

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The aileron and flaperon droop when the flaps extend.

Speedbrake Control
When the pilots move the speedbrake lever, four position transducers send analog signals to the PFCs through the ACEs. The PFCs then calculate the command output signals. After selection of one signal, the PFCs send the selected signal to the ACEs. The ACEs then command the spoiler PCUs. PCU position transducers send feedback signals to the ACEs.
Pitch Control
The pitch control causes the elevators and the horizontal stabilizer to move. When the pilots move a control column, six position transducers send analog signals to the PFCs through the ACEs. The PFCs process these signals and command the elevator PCUs in the same manner as the roll control.
When there is a command to the elevator that lasts longer than a calculated time, the elevator offload function of the PFCs send stabilizer command signals to the ACEs. The ACEs then command the stabilizer trim control modules (STCMs) which hydraulically control the stabilizer ballscrew actuator. The movement of the stabilizer then causes the elevator to fair.
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PFCS -FUNCTIONAL DESCRIPTION

Yaw Control

The yaw control causes the rudder to move. When the pilots move the control pedals, four position transducers send analog signals to the PFCs through the ACEs. The PFCs process these signals and command the rudder PCUs in the same manner as the roll control. The PFCs also control changes in the rudder ratio, gust suppression and wheel-rudder cross-tie functions.
　
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