VALVE TORQUE MOTOR
>1000 PSI
P
1ST STAGE
SOLENOID
PRESS SW

>1000 PSI ORIFICE
P
2ND STAGE
SERVO SW
AUTOBRAKE PRESSURE CONTROL MODULE
BRAKE PRESSURE
Automatic Pressure Control Module and Hydraulic Schematic
Figure 2

FROM AUTOBRAKE SERVO VLV DRIVE OUTPUT
PRESSURE CONTROL SERVO VALVE
T81914
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A 737-300/400/500MAINTENANCE MANUAL
 7.  Automatic Braking System Control Inputs (Fig. 3)
_______________________________________
 A.  The control inputs to the automatic braking system, furnished by wheel speed switches, air-ground sensing (inboard squat logic) relay, speedbrake arming switch and engine throttle switches, are provided to the microprocessor in the antiskid control unit. These inputs, plus an operational antiskid system (no antiskid system fault detected) input, are condition requirements for the automatic braking system.
 (1)  
Four wheel speed transistor switches, located in the antiskid control unit, also provide the logic path for autobrake application when at least one wheel speed on each main gear is greater than 60 knots. The logic path enables a 28-volt dc output from the antiskid module to energize the R289 (A/B ON) relay.

 (2)  
The air-ground sensing relay R278, used in the landing gear lever latch and safety relay circuit, also provides a ground (inboard squat) signal (airplane in air) or an open circuit signal (airplane on ground) for automatic braking system logic. The relay is located in the landing gear relay module (E-11).

 (3)  
The automatic speedbrake arming switch S276, operated by the speed brake handle (AMM 27-62-00/201), provides a signal to the microprocessor to disarm the automatic braking system upon reading the sequencing of the speed brake lever movement from (or through) the ARM position to "stowed." The switch signal is 28 volt dc when the speedbrake handle is raised, and an open circuit when the speedbrake handle is stowed.

 (4)  
A total of four engine throttle actuated switches, two (S139 and S283) for left engine throttle and two (S133 and S140) for right engine throttle, provide logic signals to the antiskid module for autobrake operation. Airplanes with thrust reverser sync-locks have microswitch pack M1639 (Engine 1) and M1640 (M1640) and not the 4 individual switches. To initiate autobraking during landing, both throttles must be in the retard position (switches closed) to provide four logic highs to the microprocessor in the module.

 8.  Operation (Fig. 3)
_________
 A.  Functional Description - Automatic Braking system
 (1)  When selected by the pilot the automatic braking (autobrake) system operates in conjunction with the antiskid system to obtain automatic (feet off) braking during touchdown. The system maintains a constant deceleration level throughout the landing roll or constant full pressure for refused takeoff roll and provides on-ramp, off ramp and abrupt dropout signals when operating in the automatic braking mode. Touchdown, locked wheel and skid protection are available, as described in 32-42-0, and are independent from the autobrake system.
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A 737-300/400/500MAINTENANCE MANUAL

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A 737-300/400/500MAINTENANCE MANUAL

G38083
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