The 777 has a 'normal' control column unlike the Airbus which uses a side stick and Boeing elected to have the column respond to autopilot inputs like a conventional cable controlled aircraft.
Position transducers change the pilots' manual commands of the controls to analog electrical signals. These signals go to four actuator control electronics (ACEs). The ACEs change the signals to digital format and send them to the primary flight computers (PFCs).
The PFCs calculate the flight control commands based on control laws and flight envelope protection functions. The control laws supply stability augmentation in the pitch and yaw axes and flight envelope protections in all three axes. The digital command signals from the PFCs go to the ACEs.
The ACEs change these command signals to analog format and send them to the power control units (PCUs). The ACEs and the PCUs form control loops which control the surfaces based on the PFCs commands.
The PCUs each contain a position feedback transducer.
When commanded, PCUs move the control surfaces. The position transducer sends a position feedback signal to the ACEs. The ACEs then stop the PCU command when the position feedback signal equals the commanded position.
Autopilot Operation
The PFCs receive autopilot commands from all three autopilot flight director computers (AFDCs). The PFCs use the autopilot commands in the same manner as the pilots' manual commands. In addition, the PFCs supply the backdrive signals to the backdrive actuators through the AFDCs. The backdrive actuators move the control wheels, control columns, and rudder pedals. The movement of the flight deck controls supplies visual indications to the flight crew.
PFCS Operation
The PFCS have three modes of operation: normal, secondary, and direct.
Normal mode operates when all the necessary data is available for the PFCs and the ACEs. All the control laws, protection functions, and the AFDCs operate.
When the PFCS detects the loss of important air and attitude data, the PFCS operation changes to secondary mode. The PFCs and the ACEs operate but the PFC control laws and protection functions downgrade. The autopilot cannot operate in secondary mode.
In direct mode, the PFCs are not used. The ACEs set the position of the control surfaces in direct response to analog pilot input.
Also, if an attempt is made to fly the aircraft outside of the normal flight envelope (in normal mode) a 'clunk' will be heard as the backdrive actuators attempt to fightback for envelope protection. It can be overridden but the effort required increases.