Esteemed RW B-747-400 Pilots out there, after many hours flying the PMDG version of your aircraft, I have a few Fuel System questions for you:1. How long does it take to fuel a bone-dry B-747-400 up to the gills?2. When reaching the "Tank-to-Engine" point during cruise, why must the inboard crossfeed valves remain open? I would have thought either all or none ought to remain open or closed, so there must be a reason I'm missing.3. Shoudn't the fuel in the stabilizer be burned last (or at least well into the cruise) to keep a rearward CG to turn stabilizer negative lift into positive lift to reduce the fuel burn?4. That center square in the middle of the cabin right between the two aisles and where the wings meet the fuselage I assume is the center fuel tank. How is the problem of air pressure (cabin vs. fuel system vs. outside) managed? Is the air in a partially-filled center tank pressurized at cabin altitude? If yes, wouldn't the tank explode if the cabin lost pressure for some reason? If no, how do they keep the tank walls from imploding as the fuel drains? Note a bladder woudn't solve either of these problems, while throwing aluminum at the problem to strengthen the tank walls would be horrible for the fuel burn. Also if I'm mistaken and the center fuel tank indeed is all below deck in the cargo hold, the hold is also pressurized, so the engineering design issues persist.)I look forward to your answers and explanations! Many thanks!Cheers,- jahman.

Esteemed RW B-747-400 Pilots out there, after many hours flying the PMDG version of your aircraft, I have a few Fuel System questions for you:1. How long does it take to fuel a bone-dry B-747-400 up to the gills?2. When reaching the "Tank-to-Engine" point during cruise, why must the inboard crossfeed valves remain open? I would have thought either all or none ought to remain open or closed, so there must be a reason I'm missing.3. Shoudn't the fuel in the stabilizer be burned last (or at least well into the cruise) to keep a rearward CG to turn stabilizer negative lift into positive lift to reduce the fuel burn?4. That center square in the middle of the cabin right between the two aisles and where the wings meet the fuselage I assume is the center fuel tank. How is the problem of air pressure (cabin vs. fuel system vs. outside) managed? Is the air in a partially-filled center tank pressurized at cabin altitude? If yes, wouldn't the tank explode if the cabin lost pressure for some reason? If no, how do they keep the tank walls from imploding as the fuel drains? Note a bladder woudn't solve either of these problems, while throwing aluminum at the problem to strengthen the tank walls would be horrible for the fuel burn. Also if I'm mistaken and the center fuel tank indeed is all below deck in the cargo hold, the hold is also pressurized, so the engineering design issues persist.)I look forward to your answers and explanations! Many thanks!Cheers,- jahman.

You may want to ask this question over at the PMDG (General) forum, as the owner Robert Randazzo is a United 744 pilot, and may see it and answer it for you!!!

You may want to ask this question over at the PMDG (General) forum, as the owner Robert Randazzo is a United 744 pilot, and may see it and answer it for you!!!

Not sure where you got that from? Robert was type rated on the 744 but as far as I'm aware he's never flown the thing for an airline. Maybe your getting confused with Ryan (Tabs) his father is a United 777 Capt. Anyway I could be wrong but thats what I remember Robert saying.Regarding the crossfeed during tank to engine - Dispite gravity feed and multiple pumps the 2 & 3 crossfeed is still used as a back up in case a tank fails in.

As for question #1 I can help a little bit with a different aircraft. When we fuel from a truck to a KC-135 we usually flow at 250 gallons a minute. With a 10,000 gallon truck which is about 66,000 lbs of fuel depending on temperature takes about 45 minutes to hook-up, fuel and disconnect. When we do the larger 80,000 lbs loads it requires two trucks and usually takes about an hour and fifteen minutes. The KC-135 is nice to fuel with it's two single points which save time so two trucks can flow at the same time but they go a little bit slower for the pressure so it usually flows around 175 gallons a minute for each truck.So you can compare this to what you load your 747 to.As for question #4 with the fuel tanks they have vent lines so they don't create a vacuum when the fuel is drawn out. I've flown an aircraft that had plugged vent lines and it doesn't crush the tank. Instead the pumps are not strong enough to pull the fuel from the vacuum.

...Regarding the crossfeed during tank to engine - Dispite gravity feed and multiple pumps the 2 & 3 crossfeed is still used as a back up in case a tank fails in.

Fair enough, but then why no also keep the 1 & 4 crossfeeds open as well? (If I do, I get an EICAS message...)

As for question #1 I can help a little bit with a different aircraft. When we fuel from a truck to a KC-135 we usually flow at 250 gallons a minute. With a 10,000 gallon truck which is about 66,000 lbs of fuel depending on temperature takes about 45 minutes to hook-up, fuel and disconnect. When we do the larger 80,000 lbs loads it requires two trucks and usually takes about an hour and fifteen minutes. The KC-135 is nice to fuel with it's two single points which save time so two trucks can flow at the same time but they go a little bit slower for the pressure so it usually flows around 175 gallons a minute for each truck.So you can compare this to what you load your 747 to.

Thanks, Chris! The B-744 has 57,285 USG capacity with twin underwing fuel ports, so fueling time would be 57,285 / (2 x 250) = 114 minutes, or about 2 hours. But my gut tells me a 45 minute turnaround is what aircraft are designed for? But perhaps the turnaround could be longer if refueling for max range flights. Also turnaround design time could be longer for large aircraft given the logistics...Love flying the CS-130, BTW!

As for question #4 with the fuel tanks they have vent lines so they don't create a vacuum when the fuel is drawn out. I've flown an aircraft that had plugged vent lines and it doesn't crush the tank. Instead the pumps are not strong enough to pull the fuel from the vacuum.

Agreed, but fuel venting only works when the tanks are exposed to outside air. But what if they're inside the cabin/cargo hold with a 5 to 7 psi pressure differential?Cheers,- jahman.

Fair enough, but then why no also keep the 1 & 4 crossfeeds open as well? (If I do, I get an EICAS message...)Thanks, Chris! The B-744 has 57,285 USG capacity with twin underwing fuel ports, so fueling time would be 57,285 / (2 x 250) = 114 minutes, or about 2 hours. But my gut tells me a 45 minute turnaround is what aircraft are designed for? But perhaps the turnaround could be longer if refueling for max range flights. Also turnaround design time could be longer for large aircraft given the logistics...That sounds about correct. Since it is a newer aircraft as well it probably has better fuel tanks and lines to fuel quicker. Also remember too that you will have fuel reserve and more left over from the previous flight plus they rarely go to max fuel load as well. That is why the 45 minute turns work.Agreed, but fuel venting only works when the tanks are exposed to outside air. But what if they're inside the cabin/cargo hold with a 5 to 7 psi pressure differential?I've never seen a fuel tank that is inside the pressure vessel of the aircraft. I am not familiar with these larger aircraft but I have a feeling they are not apart of the pressure vessel.

That's what's so intriguing: The 744 has this giant 20,000 USG center tank inside the fuselage, and the entire fuselage is pressurized (with the pressure differential needed to "inflate" the fuselage skin for rigidity.)So either the tank is also pressurized (likely), but then it would burst if the cabin depressurized at altitude, or it is not pressurized (unlikely) and there's plenty aluminum there to stiffen the tank walls to resist the pressure differential.BTW the 777-200LR has three optional 5,625 USG tanks (total) in the aft cargo hold, so the same questions apply.Cheers,- jahman.

That's what's so intriguing: The 744 has this giant 20,000 USG center tank inside the fuselage, and the entire fuselage is pressurized (with the pressure differential needed to "inflate" the fuselage skin for rigidity.)So either the tank is also pressurized (likely), but then it would burst if the cabin depressurized at altitude, or it is not pressurized (unlikely) and there's plenty aluminum there to stiffen the tank walls to resist the pressure differential.BTW the 777-200LR has three optional 5,625 USG tanks (total) in the aft cargo hold, so the same questions apply.Cheers,- jahman.

From what I have seen with these additional tanks is that they build the pressure vessel around them.

That's what's so intriguing: The 744 has this giant 20,000 USG center tank inside the fuselage, and the entire fuselage is pressurized (with the pressure differential needed to "inflate" the fuselage skin for rigidity.)So either the tank is also pressurized (likely), but then it would burst if the cabin depressurized at altitude, or it is not pressurized (unlikely) and there's plenty aluminum there to stiffen the tank walls to resist the pressure differential.BTW the 777-200LR has three optional 5,625 USG tanks (total) in the aft cargo hold, so the same questions apply.Cheers,- jahman.

The center fuel tank on the 747 is not contained within the pressure vessel... it is beneath the floor of the lower cargo hold, but outside of the pressurized fuselage structure. No part of the fuel storage system on a commercial aircraft is contained within the same space occupied by passengers.All fuel tanks are vented to the external atmosphere, though the actual pressure inside the tank will usually be a little higher than ambient atmospheric pressure due to the fuel vapors.Jim Barrett

OK, thanks for the info about the 744 center tank.Found a photo of the 772LR optional fuel tanks I seem to recall downloading from the Boeing web, showing the tanks inside the (pressurized, I presume) cargo hold:So at least for the 772LR, the question remains: At what pressure is the fuel carried inside? If at cargo hold pressure, loss of pressuriztion could explode the tanks (unless a reinforcing structure is built into the tanks), while if at outside pressure, how do they keep the tanks from imploding (a way more difficult task) without adding a large amount of weight to the tanks?Cheers,- jahman.

OK, thanks for the info about the 744 center tank.Found a photo of the 772LR optional fuel tanks I seem to recall downloading from the Boeing web, showing the tanks inside the (pressurized, I presume) cargo hold:So at least for the 772LR, the question remains: At what pressure is the fuel carried inside? If at cargo hold pressure, loss of pressuriztion could explode the tanks (unless a reinforcing structure is built into the tanks), while if at outside pressure, how do they keep the tanks from imploding (a way more difficult task) without adding a large amount of weight to the tanks?Cheers,- jahman.

Fuel tanks by their nature are already built to very rugged, high-strength structural standards - they have to be, to contain the outward pressure of thousands of gallons of liquid fuel when they full. If the fuselage itself can be designed to contain the normal maximum differential pressure (between inside and outside the airframe) of approximately 9 p.s.i., no reason why the fuel tanks cannot do likewise. (in either direction). I believe that the optional cargo-hold aux tanks on an Airbus 320 actually use internal cabin pressure to "push" the fuel out of the tank. On that aircraft, the aux tanks feed into the normal center tank, from whence it is pumped to the engines - just as the stab tank on a 747-400 feeds its contents into the center tank. Perhaps the 777 is similar, but I'm not familiar with the specifics on that aircraft.Jim Barrett

Thanks for your replies, Chris and Jim!Cheers,- jahamn.