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PRIMARY FLIGHT CONTROL SYSTEM -INTRODUCTION
Purpose
The primary flight control system (PFCS) controls the airplane flight attitude in relation to the three basic axes:
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Longitudinal
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Lateral
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Vertical.
Roll Control
The roll control uses the ailerons, flaperons, and spoilers to control the airplane attitude about the longitudinal axis. During a bank of the airplane, the aileron and flaperon on one wing move in an opposite direction from the aileron and flaperon on the other wing. The spoilers move up only on the down wing and do not move on the up wing.
Pitch Control
The pitch control uses the horizontal stabilizer and the elevator to control the airplane attitude about the lateral axis. The stabilizer controls long term pitch changes. The elevator supplies short term pitch control.
Yaw Control
The yaw control uses the rudder to control the airplane attitude about the vertical axis. The rudder has a tab which moves to increase the effectiveness of the rudder.
Speedbrakes
The PFCS also includes the speedbrakes. In addition to roll control, the spoilers also act as speedbrakes in the air and on the ground. They deploy on both wings to increase drag and to decrease the amount of lift the wings supply.
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PRIMARY FLIGHT CONTROL SYSTEM -INTRODUCTION
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777 AIRCRAFT MAINTENANCE MANUAL
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PFCS -GENERAL DESCRIPTION
General
The pilots or the autopilot commands control the PFCS. The pilots can override the autopilot.
Manual Operation
Position transducers change the pilots’ manual commands of the control wheel, the control columns, the rudder pedals, and the speedbrake lever to analog electrical signals. These signals go to the four actuator control electronics (ACEs). The ACEs change the signals to digital format and send them to the three primary flight computers (PFCs).
The PFCs have interfaces with the airplane systems through the three flight controls ARINC 629 buses. In addition to command signals from the ACEs, the PFCs also receive data from:
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The airplane information management system (AIMS)
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The air data inertial reference unit (ADIRU)
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The secondary attitude air data reference unit (SAARU).
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) and the stabilizer trim control modules (STCMs). The ACEs and the PCUs form control loops which control the surfaces based on the PFCs commands.
One, two or three PCUs operate each control surface. One PCU controls each spoiler, two PCUs control each aileron, flaperon, and elevator, and three PCUs control the rudder. The PCUs contain a hydraulic actuator, an electrohydraulic servo valve, and a position feedback transducer.
When commanded, the servo valve causes the hydraulic actuator to move the control surface. 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.
Two STCMs control hydraulic power to the motors and brakes of the horizontal stabilizer.
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PFCS -GENERAL DESCRIPTION
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 in synchronization with the autopilot commands. The movement of the flight deck controls supplies visual indications to the flight crew.
PFCS Modes of Operation
The PFCS has three modes of operation: normal, secondary, and direct.
Normal mode operates when the necessary data are 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.
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