(a)
Roll control: Ailerons, one surface each wing (2)
(b)
Pitch control: Elevators, one surface each side (2)
(c)
Directional control (yaw): Rudder, one surface
(2)
Secondary flight controls consist of 28 surfaces for four systems.
(a)
Spoilers: provide lift and drag, augment roll control, six surfaces each wing (12)
(b)
Leading edge slats: provide high lift, five surfaces each wing
(10)
(c)
Trailing edge flaps: provide high lift, two surfaces each wing
(4)
(d)
Stabilizer: augments pitch control, one surface each side (2)
(3)
Actuation of all control surfaces is by hydraulic servos or actuators. Primary flight control actuators are controlled mechanically by both the pilots and the electrohydraulic autoflight servos. Spoilers are controlled only with electrohydraulic actuators. Flaps and slats are driven with hydraulic motors; they also are driven by a backup system using electric motors.
C. Flight Control Inputs (Fig. 2)
(1)
All primary flight control surfaces are physically moved by hydraulic power control actuators (PCAs). Mechanical and hydraulic devices are used to provide normal control system feel. The PCAs are controlled by movement of mechanical linkage. The mechanical linkage is moved as a result of:
(a)
Conventional manual flight controls consisting of control columns, cables and quadrants, etc.
(b)
Servos responding to autopilot command signals.
(2)
Two or more actuators drive each primary flight control surface. Each PCA is powered by only one hydraulic system. This provides dual or triple path control for each primary control surface ensuring control of the airplane in the event of a hydraulic system or actuator failure. The servos and actuators of controlling systems also use hydraulic power.
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757
MAINTENANCE MANUAL
EDGE SLAT (4)
Flight Controls - Introduction
Figure 1
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A
757
MAINTENANCE MANUAL
TO AILERON POWER CONTROL ACTUATORS
TO ELEVATOR POWER CONTROL ACTUATORS
TO RUDDER POWER CONTROL ACTUATORS
RUDDER CONTROL
AIRPLANES WITH CONTROL WHEEL STEERING
Flight Controls Inputs Figure 2
G23010
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757 MAINTENANCE MANUAL
(3) Operation of the autopilot servos is similar except that when electrically engaged the input command is electrical rather than mechanical. The servos electrical feedback (position) neutralizes the control input (command) from the system computer when the new position is reached.
D. Power Control Actuator Reaction (Fig. 3)
(1)
The mechanical inputs to the PCAs provide the commands for movement of the control surfaces. The inputs are neutralized by the reaction of the PCA. For the aileron and elevator systems, the action of the summing linkage maintains the PCA control lever in the neutral position. The linkage sums the motion of the control input and the output piston, which move in opposite directions. The resultant establishes the position of the control lever. For the rudder system, the PCA is basically floating except it is referenced to structure to prevent lateral motion. Longitudinal motion occurs when the rudder is moved in response to a command. That motion, caused by movement of the reaction link, neutralizes the position of the control lever.
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