(2)
Under most conditions antiskid protection for each wheel is controlled independently. The wheel speed is monitored for sudden changes that indicate a skid, and the antiskid valve for that wheel is operated to release brake pressure as necessary. Antiskid brake release signals do not occur at speeds of less than approximately 8 knots.
(3)
The antiskid system is divided into a normal and alternate system. The normal system operates when the brake system is pressurized by hydraulic system 4. The alternate system operates when the brake system is pressurized by hydraulic system 1 or 2. The normal system controls each wheel independently as described above. The alternate system controls skids in axle wheel pairs. The alternate system also supplies locked wheel and hydroplane protection.
(4)
Eight circuit breakers on the P180 DC Power Distribution Panel supplies power to the antiskid system. Each circuit breaker supplies power to one antiskid card in the brake control unit. Each antiskid card controls two wheels independently.
(5)
Antiskid indication and fault messages for the flight crew are on the Engine Indication and Crew Alterting System (EICAS). Antiskid system malfunctions can be isolated to the control unit or any other Line Replaceable Unit (LRU) using the Central Maintenance Computer (CMC).
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
747-400
MAINTENANCE MANUAL
(6) The antiskid system contains these components:
-8 Wheel Speed Circuit Cards (in the Brake System Control Unit)
-16 wheel speed transducers (one in the end of each main gear axle)
-
4 Normal Antiskid Module Assemblies (one for each gear)
-
4 Alternate Antiskid Module Assemblies (one for each gear).
(7) The antiskid system has airplane interface inputs as follows:
-Parking brake valve position signal (AMM 32-44-00/001)
-Landing gear lever position (down or not down) signal (AMM 32-61-00/001)
-Inertial Reference System (IRS) speed data (AMM 34-21-00/001)
-
Central Maintenance Computer (CMC) for test and trouble shooting (AMM 45-10-00/201).
C.
Autobrake System
(1)
This system operates the brakes automatically at touchdown when the average wheel speed increases to 60 knots or more. A deceleration selection is made before touchdown and the system supplies a constant deceleration during the landing roll. The antiskid system must be operating normally during autobrake operation. The flight crew can disarm automatic braking by manually operating the brakes.
(2)
A deceleration rate selection is made with a selector switch on the autobrake control module installed on the flight deck control stand. Five deceleration levels (1, 2, 3, 4, and MAX AUTO) are available. Before a takeoff roll the selector switch can be set to RTO for refuse takeoff braking.
(3)
The system uses the average speed of the forward wheels on each truck to calculate the 60 knot spinup for autobrake operation. Airplane IRS longitudinal deceleration data is used for deceleration control. The system compares IRS deceleration data with the deceleration selection to calculate a brake pressure command signal. The signal operates a servo valve that controls pressure to the brakes.
(4)
When an RTO selection is made the system does a self test and arms if there are no malfunctions found in the antiskid system or the autobrake system. When the thrust levers are moved to idle during a takeoff roll and an 85 Knot spinup is measured, the system will put 3000 psi pressure to the brakes. A change to manual braking is done by pushing the brake the pedals until the system disables.
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