(2)
Altitude Hold (ALT HOLD) - AFDS provides single channel AP/FD commands to aquire and hold the current altitude when ALT HOLD is selected or, automatically transition to ALT HOLD from other pitch modes. The point of transition to ALT HOLD depends on the vertical speed, airspeed, and the distance from the MCP selected altitude.
(3)
Vertical Navigation (VNAV) - AFDS provides single channel AP/FD control based on steering command inputs from the Flight Management Computer (FMC).
(4)
Approach (APP) - AFDS provides single channel AP/FD control to capture and track localizer and glideslope. Also, if multi-channel engaged and during approach, AFDS provides dual or triple channel AP/FD control until touchdown.
(5)
Flight Level Change (FL CH) - AFDS provides single channel AP/FD commands to aquire and hold the airspeed or mach number selected on the MCP as the airplane transitions to a new altitude.
(6)
Takeoff (TO) - AFDS provides FD pitch steering commands from takeoff rotation through cruise mode engagement.
(7)
Go-Around (G/A) - AFDS provides AP/FD commands for a positive climb-out following an aborted approach.
G. Each FCC is hardwired to one, and only one, EAS, thus providing for three independant and redundant channels. Each FCC provides two discretes and one dc analog signal which completely control its respective autopilot servo. The two discrete 28 volt dc signals control the arm solenoid and engage solenoid on the EAS. The dc analog signal is the command signal for the electrohydraulic servovalve. The FCCs also provide ac excitation for the two linearly variable differential transducers (LVDTs) on each servo. The LVDTs provide servo position and surface position feedback to the FCCs. Servo position refers to the position of the autopilot actuator within the servo as sensed by the attached LVDT. Surface position refers to the position of the output crank of the servo as sensed by the attached LVDT.
2. Elevator Autopilot Servo (Fig. 4)
________________________
A. The three elevator autopilot servos (EASs) are mounted on the aft side of the stabilizer hinge bulkhead and are held in place with four mounting bolts. Looking down on the aft quadrant torque tube. The Center EAS is on the left, the Right EAS is the center servo (left side of torque tube) and the Left EAS is on the far right. Each EAS is linked by an adjustable control rod from its output crank arm to the aft quadrant torque tube.
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A
747-400
MAINTENANCE MANUAL
ENGAGE SOLENOID
ELECTROHYDRAULIC VALVE
SERVOVALVE (EHSV)
ARM AUTOPILOTSOLENOID LVDTVALVE
OUTPUT
(SURFACE) OUTPUT
LVDT CRANK
MOUNT
POINTS (4)
ELEVATOR AUTOPILOT SERVO
ELECTROHYDRAULIC
SERVOVALVE
RETURN PORT PRESSURE
REGULATOR
ENGAGE
SOLENOID
VALVE
R
C
P
ARM
SOLENOID
VALVE R
C
P
PRESURE
PORT ENGAGE
PISTONS
FILTER
OUTPUT
ARM
OUTPUT
CRANK
Elevator Autopilot Servo - Component Details
Figure 4
AUTOPILOT LVTD
AUTOPILOT ACTUATOR PISTON
OUTPUT (SURFACE) LVDT
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MAINTENANCE MANUAL
B. The Left EAS is fed by hydraulic system No. 3, the Center EAS is fed by hydraulic system No. 1, and the Right EAS is fed by hydraulic system No. 2 (Ref 22-00-01/001).
C. The basic function of the servo is to position the airplane control linkage in response to electrical command signals provided by AFDS.
D. The EAS is an electrohydraulic actuator driven by an electrohydraulic servovalve. The actuator output force is controlled by a regulated pressure that is supplied to two concentric detent pistons. Two solenoid valves allow the servovalve to be pressurized separately from the detent piston, for arming and engaging respectively. The output displacement of the detent pistons is transmitted to the control linkage through an internal crank arm. The positions of the actuator and output arm are monitored by identical LVDT type position sensors.
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