The PFC disconnect switch is a guarded toggle switch with two positions: AUTO and DISC. The switch is guarded in the AUTO position. An amber PFC disconnect (DISC) light is next to the switch.
Location

The PFC disconnect switch and light are on the P5 panel in the flight deck.
Functional Description

The PFC disconnect switch sends a discrete signal to each ACE. The ACE then transmits this signal to its on-side PFC through the flight controls ARINC 629 buses.
When the PFC disconnect switch is in the AUTO position, PFCS mode selection is automatic. The pilots can set the switch to DISC to select the direct mode of operation.
The amber DISC light shows when the PFCS is in direct mode, regardless of how the direct mode was engaged.
Indications
The EICAS caution message PRI FLIGHT COMPUTERS shows that all the ACEs that have power are in the direct mode and the PFC is not in the hydraulic OFF self-test mode. The message shows when the PFC disconnect switch is in the DISC position.

EFFECTIVITY BEJ ALL  27-02-00 
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D633W101-BEJ  Sep 05/2002 

777 AIRCRAFT MAINTENANCE MANUAL

PFCS -PFC DISCONNECT SWITCH

EFFECTIVITY BEJ ALL  27-02-00 
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D633W101-BEJ  Sep 05/2002 

PFCS -OPERATIONAL MODES -FUNCTIONAL DESCRIPTION

General
These are the three modes of PFCS operation:
*
Normal

*
Secondary

*
Direct.

During the PFCS normal and secondary mode operation, both the PFCs and the ACEs are in operation. In the secondary mode, the PFC’s operation downgrades. During the direct mode, only the ACEs operate in response to direct analog inputs.
The PFCS selects the mode of operation. On the ground with ground speed less than 40 knots, the PFCs automatically select the highest mode available based on the inputs from the ADIRU and SAARU. In flight, the pilots cycle the PFC disconnect switch on the P5 panel, or move it back to the AUTO position to cause the selection of the highest mode available.
During PFCS mode changes, there is a smooth change to the new commands.
Normal Mode

The PFCS selects the normal mode when all these conditions are met:
* The PFC disconnect switch, on the P5 panel, is in the AUTO position
*
The needed input data is available

*
At least one PFC and three ACEs are in normal mode.

All the control laws, the protection functions, and the stability augmentation of the PFCs are active in the normal mode. The autopilot operates only in normal mode.
Secondary Mode
The PFCS selects the secondary mode when it detects the loss of important air and attitude sensor data from both the ADIRU and the SAARU. The PFCS also selects the secondary mode when two ACEs are in direct mode.
A simplified set of control laws operate the PFCS in secondary mode. The protection functions are not available. Gain schedules and limits are a function of flap data discretes from the ACEs. Pitch stability augmentation uses pitch rate data from the ACEs. Yaw damper augmentation uses inertial data from the ADIRU and the SAARU if available.
Direct Mode
The PFCS selects the direct mode when there are failures that make the normal and secondary modes unreliable. If three ACEs are in direct mode, the PFCS switches the last ACE to the direct mode. The PFC disconnect switch also causes the selection of the direct mode.

EFFECTIVITY BEJ ALL  27-02-00 
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D633W101-BEJ  Sep 05/2002 

PFCS -OPERATIONAL MODES -FUNCTIONAL DESCRIPTION

The PFCs do not control the surfaces in the direct mode. Position transducer signals from the control wheels, columns and pedals go directly to the ACEs which use them to control the PCUs. The protection functions are not available.
When the PFCS is in direct mode, whether automatically or manually, the DISC amber light shows next to the PFC disconnect switch.
Manual Switching

The PFCS goes to the direct mode of operation when the pilots move the PFC disconnect switch to the DISC position.
If the pilots cycle the PFC disconnect switch or move it again to the AUTO position, the PFCS goes from the direct mode to the highest mode available. The PFCS selects the mode based on the validity of available data.
If the PFCS is in the secondary or direct modes and the normal mode becomes available, the PFCS may select the normal mode. To do this in the air, you must cycle the PFC disconnect switch. If the airplane is on the ground and ground speed is less than 40 knots, then the PFCS selects the normal mode automatically.
Indications

The EICAS caution message FLIGHT CONTROL MODE shows when the PFCS is in secondary mode.
EFFECTIVITY
BEJ ALL