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9. Operation
A. Functional Description - Automatic Braking System (Fig. 3)
(1)
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 or takeoff roll and provides on-ramp, off ramp and abrupt dropout signals to operate the antiskid system in the automatic braking mode. Touchdown, locked wheel and skid protection are available, as described in 32-42-0, and override any inputs from the autobrake system.
(2)
To obtain autobrake system operation, the following conditions are required:
(a)
Autobrake power is available (AUTOBRAKE CONTR circuit breaker on P6-3 panel closed).
(b)
INBD and OUTBD ANTISKID system switches are ON and antiskid system is operational.
(c)
AUTOBRAKE selector switch on P2-2 panel is at MIN, MED, or MAX.
(3)
Operation for landing mode
(a) The system is turned on when the AUTOBRAKE selector switch is placed at MIN, MED or MAX decel position. The AUTOBRAKE DISARM light will illuminate and remain illuminated only if the turn-on self-check (TOSC) detects a system malfunction. Following are conditions required to pass the self-test: 1) Both brake pedal pressure switches (S762 LH, S763 RH) open, indicating less
than 200 psi pressure. 2) System pressure switch (S764) closes, indicating less than 200 psi pressure. 3) Both solenoid valve pressure and servo valve pressure of the autobrake
pressure control module (V122) are less than 1000 psi. 4) Wheel speed is less than 60 knots. 5) Airplane is in flight mode.
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(b)
Upon successful completion of a TOSC check the system remains in a waiting state until touchdown. At touchdown, autobrake activation (autobraking) will start when following conditions are met: 1) At least one wheel on each gear is greater than 60 knots (based on wheel
speed relay logic).
2) Wheel velocity is stabilized.
3) Both throttles are retarded.
(c)
During autobraking, 28 volts dc is sent to the solenoid valve of the autobrake pressure control module (V122), admitting 3000 psi to the servo valve. Autobrake control immediately schedules a sharp buildup of initial on-ramp pressure to 200 psi, then at 200 psi/second for 1.5 seconds to the end of the first period. A second period with a rate of pressure buildup proportional to the deceleration selected then begins which will achieve the selected deceleration within 2.5 seconds of autobrake actuation.
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(4)
Brake application
(a)
The autobrake pressure proportional to the deceleration selected passes from the servo valve to the autobrake shuttle valves. The four shuttle valves connected to both metered pressure and autobrake pressure lines will pass the hydraulic pressure of the line with the higher pressure and block the other input. This pressure is applied through the four corresponding antiskid valves thus applying the brakes.
(b)
During normal braking, the autobrake shuttle valves will pass pilots' metered pressure since the autobrake pressure is zero. When autobrake pressure is applied, the valve will move and pass the autobrake pressure until brakes are manually applied. At this time, the shuttle moves over after the autobrake pressure is exceeded by the metered pressure. When the metered pressure exceeds 200 psi, the pressure switch actuates causing the autobrake system to disarm, and the solenoid shutoff valve and pressure control servo valve are vented to return.
(5)
Autobrake disarm
(a) Disarming can be either pilot or failure initiated. Disarming causes the disarm relay R290 to set to disarm mode and remove 28 volts dc power to the solenoid valve of the autobrake pressure control module.
Various causes for autobrake disarming during landing and the illumination of the AUTOBRAKE DISARM light are described in Fig. 4.
B. Control
(1) Arm autobrake system during landing
(a)
Place AUTOBRAKE selector switch to MIN, MED or MAX position to turn on system.
(b)
Check that AUTOBRAKE DISARM light remains off indicating the self-test is completed and the autobrake system is armed. Observe for approximately 3 seconds.
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