B747-8: Hydraulic Demand Pumps and Main tank fuel pumps powered by DC?

[RandomPerson8008]

Hello,

In the 747-8 flight training device (FTD), a DC bus 2 failure induces the loss of the #2 electrically driven hydraulic demand pump and main tank fuel pumps #1 fwd and #4 fwd, among other problems more typically expected from a DC failure. We are training this failure because it occurred in a real aircraft a few years ago.

I was under the impression that motor-pumps were typically AC powered when possible due to the inherent benefits of using AC for high output applications (weight, size).

Without access to an AMM, does anyone have a list of items powered by the DC electrical system? I realize it would be quite a long list, but I am surprised to see this failure resulting in the loss of a demand pump and fuel pumps. There is also a #2 utility bus failure associated with the loss of the #2 DC bus in the FTD, which also surprises me since I thought the utility buses and ELCU's were part of the AC electrical system.

Could these fuel pumps and hyd dem, pump be, in fact DC powered, or is there some other explanation?

To summarize:

Are the #2 & #3 hydrualic demand pumps and some main tank fuel pumps (aside from the dedicated APU DC pump) powered by the DC electrical system?
If not, why would these items fail during a DC bus failure?


Thanks!

[CV880]

I don't have any info on the 747-8 but on the 744 control of the Hyd Demand Pumps is via DC while the pump motors themselves are 3 phase AC powered. So if you lose DC Bus 2 you lose control of the Demand Pump 2. Expect the 747-8 to be similar.

[RandomPerson8008]

Ok, thank you very much CV880. That makes a lot of sense, I suspected something like that was the case, but don't have a way to confirm it.

On a related note, I have read on the 777 that the cockpit flight displays are normally DC powered. I suppose this simplifies standby power scenarios by eliminating the need for a standby inverter? On the 744/748 the displays are AC powered (standby or transfer buses).

Do the individual displays in the 744/748 have inverters built into them (like a household LCD TV would) that take the 115V 400hz AC and convert it to DC that's useable by the internal components of the LCD unit? Maybe there is a technical reason for this change on the 777, given that the 747-400 originally came with CRT displays (maybe it was easier in the 80's to use CRT displays with AC power, even though, as I understand it, CRT's internally convert AC to DC anyway). Maybe there is some minor weight savings and elimination of complexity by supplying the displays in the 777 directly with DC power?