777 AIRCRAFT MAINTENANCE MANUAL
PFCS -FUNCTIONAL DESCRIPTION
EFFECTIVITY BEJ ALL 27-02-00
Page 53
D633W101-BEJ Sep 05/2002
PFCS -AUTOPILOT -GENERAL DESCRIPTION
General
With the autopilot engaged, the PFCS receives command inputs from the AFDCs. The PFCS processes these commands in the same manner as manual inputs from the pilots.
The autopilot function of autopilot flight director system (AFDS) supplies automatic control of the:
*
Altitude
*
Vertical speed
*
Flight path angle
*
Heading
*
Track
*
Airspeed
*
Approach
*
Go-around.
The AFDS does not have servos to move the primary flight control surfaces. The autopilot commands go to the PFCs. The PFCs use these commands to supply surface commands to the ACEs. The PFCs also supply backdrive commands to the AFDCs.
Autopilot Components
The components of AFDS are:
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One mode control panel (MCP)
*
Three autopilot flight director computers (AFDCs)
*
Six backdrive actuators
*
Two control wheel disengage switches.
Autopilot Engagement
There are two autopilot engage switches on the MCP. When the pilot pushes either momentary action switch, the PFCs receive a request to engage the autopilot function. If conditions are correct, the PFCs permit all the available AFDCs to engage in the pitch and roll axes during cruise.
The autopilot yaw axis engages only during the approach and autoland phase of flight. In yaw, the autopilot can control adverse yaw (engine out) and crab angle.
EFFECTIVITY BEJ ALL 27-02-00
Page 54
D633W101-BEJ Sep 05/2002
777 AIRCRAFT MAINTENANCE MANUAL
PFCS -AUTOPILOT -GENERAL DESCRIPTION
EFFECTIVITY BEJ ALL 27-02-00
Page 55
D633W101-BEJ Sep 05/2002
PFCS -AUTOPILOT -FUNCTIONAL DESCRIPTION
General
The autopilot flight director computers (AFDCs) calculate roll, pitch and yaw commands to control the airplane automatically. Based on pilot selection, the AFDCs also calculate pitch/roll engage request, yaw engage request and other discrete signals.
Normal Operation
The autopilot commands and discrete engage signals go from the AFDCs to the PFCs through the flight controls ARINC 629 buses. Each PFC receives these inputs from the three AFDCs. The PFC selects a command signal by mid-value selection. This process selects the middle value of the three, not the average value.
The PFC selects the discrete signals by majority selection.
The PFCs filter and limit the autopilot command signals before their use in the control laws. The PFCs process and change the autopilot command signals to surface commands and backdrive commands. The surface commands from the PFCs go to the ACEs to move the flight control surfaces.
Autopilot commands go to the rudder only during automatic approach and landings.
In manual control (no autopilot engaged), the PFCs supply bank angle protection through use of the AFDS wheel backdrive actuators.
Backdrive Operation
The PFCs send column, wheel and rudder pedal backdrive commands to the AFDCs to operate the backdrive actuators. In cruise, one wheel and one column backdrive actuator operate. Actuator selection alternates at each autopilot engagement. In autoland all backdrive actuators operate. When commanded, the actuators move the control columns, control wheels and
rudder pedals to a position that represents the AFDC
commands. These movements supply tactile and visual
feedback to the pilots.
Position transducers on the columns, control wheels and rudder pedals supply position feedback to the PFCs and AFDCs through the ACEs. The AFDCs use the signal to make sure that the backdrive actuators operate correctly.
Non-normal Operation
For both command and discrete signals, if an input is not valid or failed, the PFC removes it from the calculation. If a second input is not valid, the PFC uses the remaining input. This permits the system to operate with a single AFDC.
When the PFC detects a pilot override in any axis, it disengages all engaged AFDCs.
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