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A 747-400 MAINTENANCE MANUAL
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SEE A
BRAKE INDICATION DISABLE SWITCH
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BRAKE SYSTEM CONTROL UNIT
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DATE MFDPART NO. 42-747 BOEING PN S283U002-SERNO BRAKE CONTROL UNIT HYDRO-AIRE DIVISIONCRANE BURBANK, CALIFORNIA 91510 DISABLE BRAKE INDICATION 12 11 10 987 6 NORM 14 15 16 13 5 4 3 2 1 :
3 1 4 2 7 5 8 6 DISABLE BRAKE INDICATION 14 15 16 13 11 12 10 987 6 5 4 3 2 1 NORM 11 9 12 10 15 13 16 14
A-A H/A445-10
Brake System Control Unit Location Figure 1
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A
747-400
MAINTENANCE MANUAL
D. Main Wheel Card
(1)
Power for the main wheel circuit cards is supplied by DC bus 1, 2, 3, or 4. Power for each card is supplied separately through a circuit breaker on the P180 DC Power Distribution Panel in the electrical and electronics compartment. Each card contains circuits to operate antiskid for two wheels independently. Thus each circuit breaker supplies power for two wheels as shown below:
~.............................................................. | CIRCUIT BKR | WHEEL | BUS | ................................................................ | C369 | 1 & 3 | 28 vdc Bus 1 | ................................................................ | C370 | 14 & 16 | 28 vdc Bus 1 | ................................................................ | C664 | 2 & 4 | 28 vdc Bus 2 | ................................................................ | C665 | 13 & 15 | 28 vdc Bus 2 | ................................................................ | C10078 | 5 & 7 | 28 vdc Bus 3 | ................................................................ | C10079 | 10 & 12 | 28 vdc Bus 3 | ................................................................ | C10080 | 6 & 8 | 28 vdc Bus 4 | ................................................................ | C10081 | 9 & 11 | 28 vdc Bus 4 | 2......................….................….....................1
(2)
The main wheel card monitors wheel rotational speed and airplane lateral velocity and modulates the brake pressure to prevent skids or locked wheels during runway and taxi operations.
(3)
The rotational speed of each wheel is received from a wheel speed transducer installed in the end of each main gear axle. The wheel speed signal is a sine wave with a frequency directly proportional to the speed at which the wheel turns. The signal from each wheel is used to calculate a reference velocity signal. This reference signal is then compared to the actual velocity signal of each wheel to obtain an error signal that is used for wheel speed control.
(4)
The velocity of each wheel is also compared to the velocity of its tandem wheel to supply locked wheel protection. Whenever a wheel speed drops below 30% of the speed of the tandem wheel, the brake on the slow wheel will be released until its speed signal increases to that of the tandem wheel.
(5)
Touchdown/hydroplane protection is supplied directly to each of the aft wheel pairs. If an aft wheel reference velocity is 50 Knots below the IRS ground speed, the brake pressure to the aft wheel is released. This allows the speed of the aft wheel to increase to ground speed if brake pressure is applied prior to touchdown, or under hydroplane conditions.
(6)
The forward wheels are supplied with touchdown/hydroplane protection through locked wheel protection.
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