747-400 Main Deck Cargo Handling

[QAVION]

A few days ago, whilst in the cockpit of an SQ 747-400 Freighter, I noticed that APU Generator #2 was AVAILable, but not selected whilst main deck cargo was being loaded.

Is it standard practice/a precautionary measure to evenly distribute the electrical load by doing this... or is there actually some kind of airplane logic preventing both the right sync bus and the main deck cargo handling systems being fed by APU gen #2 at the same time? I couldn't find any reference to it in the -400 Maintenance Manual (Doors or Elec Chapters).

Thanks.
Q.

[spannersatcx]

QAVION, cargo handling bus is powered from ext b plugged in and on but not selected on on flt deck, same applies for apu gen 2. It is a standard config for all 747 freighters/combi's.

[QAVION]

"It is a standard config for all 747 freighters/combi's."

Hi, Spanners.
I seem to recall this was true of the Classic Combis, but I couldn't find any reference to it in my notes or in the 400 BMM. It would be nice to see it in writing somewhere. Do the BCU's have the logic for this or is it purely done by external interlocks/relays?

Thanks.
Q.

[Moonbeam Purple]

Q

I'll quote dirctly from the 747 OM:

"The main deck cargo handling bus is powered on the ground when external power 2 or APU generator 2 is available. An interlock prevents either source from powering main electrical busses and the main deck cargo handling bus simultaneously. Pushing the available Power source switch ON deenergizes the main deck cargo handling bus. When both external power 2 and APU generator 2 are available, external power is used. Pushing external power 2 switch ON transfers main deck cargo handling loads to AOU generator 2.

NOTE: When cargo handling is required, selecting only the number one auxiliary power source ON ensures the main deck cargo handling bus remains powered by the number two auxiliary power source."

End quote.

Operating Manual 6.20.6.

Best rgrds

MP

[CargoRat2]

QAVION
Don't know the why's & wherefores but on all our -400Fs, APU GEN #1 ON, #2 AVAIL. Same with EXT PWR (1 ON, 2 AVAIL).
As a note for external power; the -400F needs both ext busses plugged in to the GPU; #1 for lowerdeck cargo handling, #2 for maindeck cargo & maindeck doors.
Now that I've mentioned that, I remember I couldn't open the side-cargo door the other day in ZRH. After a lot of running around, found both #1 & #2 ext pwr "ON". #2 reset to "AVAIL"; as if by magic, the door opens...

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rgds Rat

[QAVION]

Many thanks chaps! More inspiration to help me continue looking for the those elusive words in the "fix-it" manuals.

Cheers.
Q.

[Flight Safety]

Just a question about this issue. Does this electrical bus and power source arrangement mean that the freight handling systems on the -400F (or -400 Combi) depend on external power to operate? In other words are these aircraft dependent on external power to load and offload freight, or is an internal power source available for these functions?

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Safe flying to you...

[QAVION]

Ooops... One more question....

Re "When both External Power 2 and APU Generator 2 are available, external power is used."

Does this autoselection of External Power 2 (with both sources available) mean that you can select APU Gen #2 ON in this situation without affecting cargo handling and doors? i.e. APU Gen #2 powers (or helps power) the aircraft and Ext Pwr #2 powers the cargo systems?

Thanks.
Rgds.
Q.

[Moonbeam Purple]

Flight Safety - NO, as long as you have available lights in either the Ext pwr 2, or the APU GEN 2 switches,then you can operate the cargo handling system! So the aircraft can be loaded with only APU PWR available. Good thing when you think of some of the places we visit from time to time!

QAVION - NO, not as far as I'm concerned(A good question though!).

Hi RAT - I guess it's time for one servezas pretty soon - just to update things in and around LUX! :) I will call you the next time I'll have to spend more than 24 hours in LUX.

Best RGRDS

MP

[CargoRat2]

QAVION
You can select the APU GEN ON while still on external power, assuming the APU is runnning. It does however switch off the cargo system, which is bloody annoying since this involves reseting it.

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rgds Rat

[QAVION]

"You can select the APU GEN ON while still on external power, assuming the APU is running. It does however switch off the cargo system, which is bloody annoying since this involves resetting it."

Thanks... CR2.
So even though the Cargo was being powered by the default power source, selecting APU Gen 2 ON, still affected the cargo. Weird!

BTW, by "resetting", you mean simply pushing the APU Gen 2 button (back to AVAIL)... or is there something else which has to be reset outside the cockpit? (I may as well get the full story here whilst I'm getting all these excellent answers ;-)).

Cheers.
Q.

[CargoRat2]

Sorry QAVION, by reseting I mean the Cargo System (by the Nose Cargo Door) - this usually means the Loadmaster can walk the length of the aircraft, reset it, then walk all the way back again (to the side door).
As for the GEN, makes sense. APU is switched on shortly before departure (loading usually still in progress). APU generator is then selected, ground cart removed when convenient.

Moonbeam; you're right it's been a while since we had a few beers!



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rgds Rat

[Flight Safety]

Thanks Moonbeam.

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Safe flying to you...

[QAVION]

More confusion.....

I was on a Combi 747-400 (KLM) last night... Both APU Generators were selected ON... and there were no problems with loading (or should I say no electrical problems.... the drive system was so powerful, the cargo was punching holes in the sidewall panelling). I guess this aircraft would only stop loading if there was loadshedding (as opposed to some kind of aircraft logic stopping power going to the cargo (at all times) if APU 2 or EXT 2 was selected ON).

Cheers.
Q.