757 MAINTENANCE MANUAL
Airplane hydraulic systems left, right, and center move the elevators in response to manual or electrical control inputs. Manual inputs are applied to either control column and electrical inputs come via the autopilot servos.In the manual mode, fore and aft movement of the control columns transmits motion from the forward quadrants to the aft quadrants by means of two sets of cables. As the control columns are displaced farther from neutral, the elevator feel unit imparts an increasing centering force to the control system.The feel force is controlled by the feel computer. The feel computer independently develops two feel pressures, using the right and center hydraulic systems. The feel unit uses the developed feel pressure to pressurize the two independent pistons of the feel actuator. This provides tension in the kinematic links of the feel unit. A difference in generated feel pressure will result in the feel actuator bottoming out on the piston with the lower pressure. The higher pressure will be used to develop the tension forces.The kinematic linkages develop the desired force gradient on each of the captain's and first officer's cranks. The forward and aft overrides couple the captain's and first officer's feel unit crank force gradients. This provides the desired column centering and force gradients. A loss of a single hydraulic pressure system will not affect the captain's or first officer's column force gradient. A loss of both hydraulic systems will result in reversion to the force gradient obtained from the full time, normally locked out, mechanical feel system. The feel force is transmitted through the aft quadrants back to the control columns.The aft quadrants transmit elevator inputs to the power control actuator (PCA) input bellcranks by control rods. The motion to each PCA summing link produces an error signal between the input and output positions. This error signal moves the servovalve, directing high pressure hydraulic fluid to one side of the piston, while venting the opposite side of the piston to return pressure. The resultant force unbalance moves the piston and elevator surface while the feedback summing link positions the servovalve input to reduce flow to the piston. Ultimately, the elevator surface reaches the desired position as the servovalve shuts off hydraulic flow to the piston. Control input stops at the captain's and first officer's aft quadrants limit maximum elevator that can be commanded without bottoming the PCU's.
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 27-31-00
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A

757
MAINTENANCE MANUAL

 (6)  
Elevator asymmetry between captain's and first officer's control system is limited at the aft quadrants by an electrical actuator. The actuator uses an electric motor to position a stop which limits the relative motion between the captain's and first officer's aft quadrants. The motor operates in response to an extend or retract discrete from the stabilizer/elevator asymmetry limiter module (SAM). Switches on the motor output provide signals back to the SAM to indicate if the motor is in the extended or retracted position. The system is operational full time. Asymmetry is limited to 24 degrees at low speeds and 8 degrees at high speeds.

 (7)  
Three identical autopilot servos are provided to drive the elevator control system in parallel with the captain's and first officer's manual system. The servos provide inputs to the control system at the aft quadrants. Each servo is designed to operate either as a single-channel, force-limited servo during climb, cruise, and descent operation or as a three-channel, force-summed system during automatic landings. (See 22-10-00, Autopilot/Flight Director Pitch Channel - Description and Operation).

 B.
Fault Indication (Fig. 9)

 (1)  
Elevator control system fault monitoring is provided by the SAM and EICAS system. The SAM provides fault monitoring of the elevator/asymmetry limiter actuator. The EICAS system elevator fault monitoring can be obtained by pushing the STATUS button or one of the maintenance buttons on the EICAS MAINT panel.

 (2)  
A PCU monitor system is installed to monitor hydraulic pressure in the power control actuators (PCAs) to detect and warn of an excessive pressure differential (above 1140-1350 psi) between the PCAs (PCA force-fight). (Ref 27-23-00 for PCU Monitor System Description and Operation.)

 C.
Control

 (1)  
Manual Operation

 (a)  
Provide electrical power (Ref 24-22-00).

 (b)  
　
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