34-55-00
ALL ú ú 04 Page 17 ú Feb 15/98
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
747-400 MAINTENANCE MANUAL
(l)
Coaxial suppression lines interconnect the ATC, DME and TCAS (if installed) L-band systems on the airplane. A suppression pulse is transmitted by a system whenever it is transmitting. The pulse goes to the other three systems, keeping then from transmitting. This prevents interference between L-band systems. When the DME receives a suppression pulse, it detunes its front end. This is done to protect the receiver section from close range, high power transient signals from the other L-band systems.
(m)
Suppression pulses are sent to the DME by other onboard L-band systems which are about to transmit a high energy signal. The interrogator protects the front end of the receiver by switching in an attenuator when this signal is received. The interrogator also sends out a suppression pulse.
C. BITE
(1)
The DME contains self-test and monitoring features that check for proper operation of the unit. During in-flight modes, the BITE network continuously checks functions that affect the accuracy of the DME. It checks synthesizer lock, range clock accuracy, CPU program execution, and ARINC 429 tuning data and polarity. Under normal conditions, the self-test is performed every 10 seconds. Once a failure is detected, the self-test increases its test rate to every 3 or 4 seconds.
(2)
If a failure safety related (i.e. incorrect distance, wrong channel, etc.) and detected while the airplane is in flight will result the denial of DME information to the FMC and flight deck displays. A detected failure causes the DME to output a fail warn data word and the DME displays to blank. If the failure is non-safety related, the DME attempts to opperate normally until the airplane is on the ground with displays showing a NCD display. All faults are stored in nonviolatile memory.
(3)
Other functions are checked at intermittent times. These include receiver sensitivity, transmitter power level, data bus operation, AGC tracking and lock on, CPU RAM bit check, and track of the self-test distance signal. Checks are run on these functions at power turn-on and when a mode change command is made from the data bus. They are also run when the DME fails to lock on a signal and when the manual self-test switch is pressed.
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34-55-00
ALL ú ú 04 Page 18 ú Feb 18/00
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
747-400
MAINTENANCE MANUAL
(4)
All faults detected during the intermittent and continous monitoring self-tests are stored in a nonvolatile fault memory. The faults are stored by flight segments for subsequent evaluation in the shop. Each flight segment starts when the airplane takes off as detected by the air to ground relay. The BITE memory can store 5 failures per each flight segment and can store failures for 49 flight segments. Fault data history can be recalled later and used as an aid to shop personnel when trouble shooting the interrogator on the bench via the interrogator ATE connector.
(5)
The LCD screen on the front panel of the DME interrogator provides status of the R/T BITE self-test. The self-test does a seft check for existing internal and external faults.
(a) A functional self test of the DME system is initiated by push and release either one of the test switches on the front panel of the LCD screen. When this happens, the TEST IN PROGRESS display is shown on the LCD screen of the DME system goes through a self test. The test continue for approximatly 5 seconds. The TEST COMPLETE NO FAILURES display is shown for a test pass condition. The TEST COMPLETE FAILURES display is shown for a test fail condition. Push and release the RETURN switch shows the P/N and S/W display. Push and release the WHY switch shows the failure message if a test fail condition has occurred.
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