A 747-400 MAINTENANCE MANUAL

 32-42-00
 ALL  ú ú 05 Page 53 ú Jun 10/94
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
747-400 MAINTENANCE MANUAL
 (2)  
The body gear steering is armed when these conditions occur:

 (a)  
Both AIR/GROUND signals indicate the ground mode for at least 10 seconds (or at least 1 second following a power interruption).

 (b)  
The fourth highest of the 8 wing gear wheel speeds is less than 20 knots on increasing wheel speed (disarm when accelerating). The fourth highest of the 8 wing gear wheel speeds is less than 15 knots on decreasing wheel speed (arming when decelerating).

 (c)  
If the left and the right IRS ground speed are less than 40 knots.

 E.
Brake control system BITE.

 (1)  
The brake control system BITE contains the circuits necessary for a complete fault isolation and test of the brake control system. All testing and trouble shooting of the brake control system is done using the central maintenance computer system. All brake system malfunctions can be isolated to line replaceable units (LRUs). Fault information on LRUs is stored as intermittent or continuous faults in the CMC. The faults can be recalled and corrected during maintenance operations.

 (2)  
The major functions of the CMC that are used in the maintenance of the brake control system are:

 -Present leg faults recall
 -Existing faults recall
 -Brake System BITE Test
 -Antiskid Brake Release Test
 -Autobrake Application test
 -Torque Sensor Zeroing test
 (3)  
The central maintenance computer is operated using a Control Display Unit (CDU) (Ref 45-10-00/001). Line select keys permit the maintenance person to do any test or maintenance operation specified on the screen adjacent to the key. The maintenance person can get a report on faults by pushing the line select key adjacent to the PRESENT LEG FAULTS or EXISTING FAULTS message on the CDU.

 (4)  
Interactive tests are tests in which a maintenance person must do specific steps at specified intervals in the test.

 (a)  
The Brake System BITE tests the Brake System Control Unit (BSCU), the Antiskid System, the Autobrake System, the Brake Torque Limiting System, the BITE cards, and all LRU's. When the test has completed a "PASS" or "FAIL" will be show on the CDU. If a failure occurs, the line select key (LSK) may to used to show the failure. Once the failure is corrected, the test should be repeated until a "PASS" is obtained. This test should be the first test run when doing maintenance on the brake control system and will supply the most complete system condition.

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 32-42-00
 ALL  ú ú 04 Page 54 ú Jun 10/94
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
747-400 MAINTENANCE MANUAL
 (b)  
The Antiskid Brake Release test fully releases all brakes. Each brake can be selected one at a time on the CDU. A pressure gage must be installed on each brake to make sure the antiskid system operates correctly. To do a manual test of the antiskid system, remove each hubcap and turn the antiskid transducer shaft. This transmits a signal to the BSCU which releases the brakes on both tandem wheels. During these tests, a maintenance person must look for correct antiskid operation.

 (c)  
During the Autobrake Application test, a pressure gage must be installed on one of the brakes so that brake pressure can be monitored. The autobrake selector switch must be armed in any position for the test. The test is started through the CMC. Each autobrake selector switch position will show a different pressure.

 (d)  
The Torque Sensor Zero test sets the offset of each torque sensor to zero. The parking brake must be released. Each sensor can be selected using the CDU. This test should be done after a brake torque rod is replaced. IF the torque sensor offset is greater than 5000 pound-feet, the offset cannot be set to zero, and a "Brake Limiter" EICAS and CMC message will be set. An autozero occurs for all wheels two minutes after takeoff.

 (e)  
A Torque Limiter Release test can be done by disconnecting each brake torque rod connector and inserting a test tool in place of the sensor. A high torque signal is transmitted by the test tool and the BSCU releases the brakes. A presure gage must be installed in the brake so a maintenance person can look for correct brake operation.
　
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