Intermittent blanking of the Display Units during APU start with Battery power only

After power up of the a/c with battery power only, after 90 seconds 3 display units are powered on. But during the APU start with battery power only there is a possibility of “Display Unit Blanking”.

Cause of intermittent Display Unit Blanking:
Blanking Display Units may occur if the DEU-1 powersupply voltage drops below 18VDC. This causes the DEU to startup again causing the captains DU’s and upper DU to be blank for 90 seconds again.

Normally this problem should not occur because most a/c are modified in where the SCU is switched in AIR-mode (APU Ignition & starting system (WDM 49-41-11)), this means that the SCU prevents that the power does not drop below 20V during a battery APU start.

However a combination of factors, like small losses, SCU tolerances, Display System tolerances etc, may cause a powerdrop thats just triggers the DEU-1 treshold of 18 VDC causing the blanking display during a battery APU start.

NOTE:   When the airplane is in the air, should CDS experience a power-down/power-on recovery, the restart time for CDS is on the order of 10 seconds.  The reason that the DEU restarts more quickly in flight than when on ground is that the CDS in-flight power-up sequence skips or excludes running its self-test process.  Excluding/skipping the self-test process for an in-air power-up recovery is a design feature intended to permit the DEU to begin processing display images as quickly as possible.

Boeing recommends the test of the DEU / CDS and the battery to make sure they are in a healthy condition if the blanking display event occurs.

1.       TASK 24-31-21-710-801, Main Battery Charger Operational Check

2.       SUBTASK 31-62-21-730-001, which performs the Loop Check for the Display Unit

3.       SUBTASK 31-62-21-750-001, which looks for CDS FAULT or DSPLY SOURCE

4.       SUBTASK 31-62-21-860-023, which puts the DEU through the Ground Test

NOTE:   Fault 49-41249 “Start Converter Unit Shows Low Battery Power” will show on the CDU in the APU BITE, if there is not enough power to start the APU on battery.

There is a request at Boeing to put this “blanking display unit” problem into the FIM, with mentioned explanation and solutions / tests.

Source: Peter en Randy, thanks for your input.


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