(4)
The (A/B ON) relay R289 provides switching functions for supplying 28-volt dc autobrake-on power to the antiskid control module and to the solenoid shutoff valve in the autobrake pressure control module (V122). Relay 289 energizes when at least one wheel speed on each main gear is greater than 60 knots.
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3. Autobrake Pressure Control Module (Detail D, Fig. 1)
A. The autobrake pressure control module is connected to system B hydraulic lines (Fig. 2), and is located on forward side of right main gear wheel well aft bulkhead at station 727. The module consists of an electrohydraulic 2-stage pressure control servo valve, a solenoid operated 2-stage shutoff valve, two hydraulic pressure actuated switches (one at the output of the solenoid shutoff valve and the other at the output of the pressure control servo valve), and an inlet pressure fluid filter. The module supplies autobrake pressure to all four main wheel brakes through the four autobrake shuttle valves.
B. The solenoid operated shutoff valve will port hydraulic pressure to the pressure control servo valve when the autobrake system is armed and then activated by autobrake logic (wheel speed greater than 60 knots, throttles retarded, and airplane on ground). The valve port opens to allow pressure to return when not electrically energized.
C. The electrohydraulic 2-stage pressure control servo valve receives pressure from the shutoff valve and modulates the output to the brakes. The valve 1st-stage consists of a dc torque motor, a jet pipe and receiver, and a pressure balance-feedback-piston/spring assembly. The 2nd-stage is a 3-way pressure balance metering spool that controls pressure to the brakes as modulated by the 1st-stage valve.
D. The shutoff valve pressure switch is located at the shutoff valve output and monitors the shutoff valve operation. The switch opens above 1000 psi to provide a logic "high" with autobrake selected. The switch goes to the relaxed position to provide a logic "ground" signal when the autobrake selector switch is in any of the four autobrake selected positions and pressure falls below 1000 psi. Any incorrect logic will disarm the autobrake and illuminate the disarm light. The control servo valve pressure switch monitors the servo valve output pressure to detect control system faults. The switch is connected to the fault monitor logic to indicate high pressure (above 1000 psi) with an open circuit and indicates low pressure (below 1000 psi) with a closed (logic ground) circuit.
4. Autobrake Shuttle Valves (Fig. 1 and 2)
A. Four shuttle valves, located on main gear wheel well ceiling, are installed in the autobrake system, one in each of the four brake pressure lines upstream of the antiskid valves. The valves sense hydraulic pressure from the autobrake pressure control module and from the brake metering valves. The shuttle valve will port pressure to the antiskid valve from the source (autobrake pressure control module or brake metering valve) of highest pressure.
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5. Hydraulic Pressure Switches (Fig. 1, 2, and 3)
A. Three hydraulic pressure switches (S762, S763, S764), located in the main gear wheel well, are installed in the hydraulic brake lines. Two pressure switches (S762 LH, S763 RH), identified as brake pedal pressure switches, sense the brake pressure downstream from the brake metering valves (system A pressure inboard brakes and system B pressure outboard brakes). The switches actuate when the hydraulic pressure is greater than 200 psi and will provide a ground signal to the microprocessor in the antiskid module. The switch (S764), identified as the system pressure switch, senses the pressure downstream of the autobrake pressure control module and the flow limiting hydraulic fuse. The switch S764 actuates when the hydraulic pressure is greater than 200 psi and will provide an open circuit signal to the microprocessor in the antiskid module.
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