(m)
ADC maintenance words include BITE test fail/OK data.
(11) The ADC also provides an overspeed discrete when computed airspeed or MACH is greater than the maximum allowed.
B. BITE
(1)
The functional test switch on the front panel initiates a self-test of the internal circuits of the respective ADC. The test switch is also used to reset the ADC faultball once the ADC fault has been corrected.
(2)
The external sensor fault LED display is only operational during the system self-test. The display identifies input sensor failures. It displays a 0-9 single digit number corresponding to specific failures on input sensors. When the test switch is pressed, a dash (-) appearing in the LED indicates no external sensor faults.
(3)
The ADC fail annunciator indicates whether or not the ADC had a failure during the last flight. The annunciator shows black for no failures and yellow for a failure.
(4)
BIT Fault Monitoring
(a)
The ADC built-in-test (BIT) equipment uses software tests in combination with BIT hardware to detect ADC and input sensor failures. BIT tests are performed during the operational program.
(b)
The BIT checks all input data, internal circuits, CPU operations and memory. It uses wrap-around tests to check the transmitter output. If the latter test fails for more than 2 seconds, ARINC transmissions are halted. After a failure and halt, one ARINC transmission is made every 5 seconds thereafter to do another wrap-around test. If any of these tests pass, ARINC transmissions are resumed.
(c)
The results of the BIT tests are reported in maintenance words on the ADC output data bus. The results also set the sign status matrix in the binary outputs and inhibit the BCD output when a failure occurs. For detected ADC internal failures, the fault ball on the front of the ADC changes from black to yellow. In addition, the BIT logic will store in-flight failures into a fault memory. This information is used for bench maintenance.
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747-400 MAINTENANCE MANUAL
(d)
The BIT also checks that the operational program has executed all of the program that it should. If the program is not operating properly, the fault ball on the front of the ADC will turn to yellow. This fault will fail all outputs from the ADC.
(e)
Once the fault ball is set, it cannot be reset until the fault causing the set condition has been corrected. Once that fault is cleared, the fault ball is reset by pressing the test button on the ADC front panel.
(5) Self-Test
(a)
The self-test provides a confidence check of the ADC. The test can be started by pressing the test switch on the ADC or through the central maintenance computer system. To prevent bad test results, the self-test function is prevented in flight by the air/ground logic.
(b)
The self-test indicates faults and sends canned outputs to the flight deck displays. For the first two seconds of the test, the master warning lights on the lightshield illuminate, a siren is heard from the flight deck aural warning speaker, and the >OVERSPEED warning message is displayed on the main EICAS display. The ADC front panel indicates external sensor faults.
(c)
The following indications occur on PFDs during the ADC self-test. 1) For the first seven seconds, the altimeter, airspeed, and
MACH indications start to drive up scale.
(d)
During the ADC self-test, a numeric display on the ADC front panel indicates external sensor failures, which do not set the ADC fault ball. It is activated when the functional test button on the ADC front panel is pushed and remains on as long as the test button is pressed in. The readout indicates a BIT detected failure of external sensor data. Each digit represents a failure as shown.
0 - Spare 5 - TAT probe input 1 - No . 1-AOA input 6 - No. 1 Baro-correction input 2 - No. 2-AOA input 7 - No. 2 Baro-correction input 3 - SDI parity 8 - No. 3 Baro-correction input 4 - Spare 9 - Aircraft type program
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