757 AMM 飞机维护手册 自动飞行 AUTOFLIGHT 1(123)

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 2)  The resulting elevator command signal is applied to the pitch servo amplifier to drive the electrohydraulic servovalve. With elevator servo hydraulic arm logic applied, the electrohydraulic servovalve moves to center the actuator piston (servo actuator) about the output crank linkage. The servo position LVDT provides feedback to null the elevator command signal and stop the servo actuator. At A/P engage, elevator servo detent engage logic is applied to the servo. Servo detent pistons then clamp the actuator piston to the output crank linkage without generating a transient. The A/P engage logic transfers pitch steering commands to the elevator trim integrator for A/P engaged operation.
 (b)  Flight Director Pitch Command 1) Flight director pitch commands are computed continuously whether or not the A/P is engaged or the F/D is on.
 2)  The F/D pitch commands are computed from selected pitch attitude commands, present pitch attitude, pitch attitude rate, and versine. Computed F/D pitch commands are summed with pitch rate. If in takeoff (T/O) mode, a reference +8 degrees (theta) pitch signal is summed with pitch attitude and then summed with the F/D pitch commands. Computed F/D pitch commands are applied to the signal generators and EADIs of the EFIS.
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MAINTENANCE MANUAL

AUTOPILOT PITCH CONTROL SERVOFLIGHT CONTROL COMPUTER
Output Signal SynchronizationFigure 8
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757
MAINTENANCE MANUAL

 3)  A F/D ON discrete causes the F/D command bars to be in view and the F/D pitch command displayed. With the F/D switches off, the F/D ON discrete is not provided and the EFIS inhibits the F/D displays. When the airplane radio altitude is greater than 5 feet and the A/P is CMD engaged, a fixed bias is sent to the EFIS which biases the F/D bars out of view. The airplane is then under A/P CMD ENGAGE control to selected modes.
 (4)  Flight Director - Pitch Channel (Fig. 9)
 (a)  
Flight director pitch commands are computed continuously regardless of whether or not the A/P is engaged or the F/D is on. Positioning the F/D switches ON enables flight director displays for all available modes.

 (b)  
The VNAV, V/S, ALT SEL, ALT HOLD, FLCH and APP modes are available as F/D modes. The IAS/MACH speed selection is available for the FLCH mode. The pilot controls the airplane in response to F/D commands on the EADI directly with the control column/wheel.

 (c)  
The F/D pitch commands are computed from selected pitch attitude commands, present pitch attitude, pitch attitude rate, and versine. Computed F/D pitch commands are summed with pitch rate. If in takeoff (T/O) mode, a reference +8 degrees (theta) pitch signal is summed with pitch attitude and then summed with the F/D pitch commands. Computed F/D pitch commands are applied to the symbol generators and EADIs of the EFIS.

 (d)  
A F/D on discrete causes the F/D command bars to be in view and the F/D pitch command displayed. With the F/D switches off, the F/D ON discrete is not provided and the EFIS inhibits the F/D displays. When the airplane radio altitude is greater than 5 feet and the A/P is CMD engaged, a fixed bias is sent to the EFIS which biases the F/D bars out of view. The airplane is then under A/P CMD ENGAGE control to selected modes.

 (e)  
The flight director switches on the MCP cause the flight director bars to appear on the EADI for that side selected. The flight director bars provide the pilots information for manually flying a preselected flight profile. If the F/D is switched ON while the aircraft is on the ground, takeoff mode will automatically engage. If the F/D is switched ON in flight and no other mode is selected, the vertical speed mode automatically engages.

 (f)  
The flight director EADI indications are controlled by the FCC selected with the FLT DIR instrument select switch. The FCC controls the flight director or the autopilot in CMD, but not both at the same time. If the flight directors source FCC is also engaged in CMD, the flight director bars are biased out of view.
　
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