(744s) Fuel Tank question or issue

[Arnaud 0]

Gentlemen,I have recently noticed something strange on the 744F: when I load an important amount of fuel (always using the PMDG menu option to do that, of course), before start-up, the main 1 and 4 tanks would always and normally read their max capacity (13.600 kgs each). This is normal.Then the center tanks are filled etc...What I have just noticed, and twice, is that few hours after take-off,and long before the tank to engine procedure, the main 1 and 4 suddenly read only 12.700 kgs, when I never touched anything.As you see on that screenie:http://aiarnaud.free.fr/pics/fuel.jpgIs this a known fact? And, is this normal? I think not, in fact, as I havent read anything about it anywhere.I made quite lots of flights with both 744 and 744F and I remember well the amount of fuel in main 1 and 4 just before the tank-to-engine switch was always 13.600 kgs.Any help would be appreciate here. I have no clue why this suddenly happens (I suspect it's linked somehow to the high value of fuel I boarded on my latest flights, because usually I cant do flights longer than five or six hours, and now I have the opportunity to do some ten-eleven hours long flights).I am planning on making some tests and see what happens on the next flight, like keeping an eye non-stop to the fuel EICAS page...Since 'loosing' 1.300 kgs of fuel for no reason annoys me.Regards,

[cowpatz 243]

Cant say I have noticed this one before. Have you looked at Progress page 2 in the FMC to compare the calculated versus totaliser quantities?CheersSteve

[rgamurot 1]

I operate in lbs. but I frequently see this occur. I remember looking at the fuel page before takeoff with my flaps deployed and my two inboard cross feed valves were closed. This automatically happens with the flaps extended. (pg. 46, Chapter 11 in ops manual)This cuts off fuel supply from tanks 2 and 3 to engines 1 and 4. If you have a good amount of fuel in your centre tank, then this normally supplies EGN 1 and 4. However, sometimes those two engines need more fuel so tank 1 and 4 will supply the extra fuel (or all the fuel if the centre tank is dry).Ryan Gamurothttp://www.precisionmanuals.com/images/forum/supporter.jpg

[Qavion 19]

Did you carry out any violent manoeuvres after takeoff? :-P Perhaps the fuel is spilling out of the wingtank vents :)It IS normal for the fuel system to start using small quantities of fuel from the main tanks prior to reconfiguring pumps. E.g. When the Centre tank qty values are a little low, the CWT pump output pressure is not as strong as it normally is. Because of this, wing boost pump check valves start to open (and fuel will be pumped from the main tanks). I vaguely recall that this behaviour was programmed into the sim.Cheers.Q>

[Arnaud 0]

Thank you guys,I understand that my knowledge of the 744 fuel management is poorer than I thought :-) , and understanding this is a good thing!Following Ryan's indictaions, I've read all Chapter 11 of the ops manual, page 46, and they indeed clearly mention it.However, I'll have to read it again, because it's not still clear to me.The fact that fuel from main 1 and 4 is being burnt right before and during take-off when the Center tank is filled is something I didnt know.As I said, I hardly need to fill the center tank since I usually do only shorts flights, that's probably the reason why I had never witnessed this before.Thank you again for your replies.

[Arnaud 0]

Gentlemen,Here I am again. I am actually flying, notably for test purpose (and also in order to complete my endless 'world tour'), from Sydney Australia to Singapore with the 744F.Took-off at max take-off weight, and loaded 137.000 kgs of fuel.I noticed that when flaps are set to 20 prior to take-off, the fuel tanks, as a matter of fact, switch from the main 3&4 for engines 1,2,3,4 to center tank feeding engines 1,4. As in the manual.Main tanks 1 and 4 quantity on the other hand, remained unchanged (their max capacity 13.6 tons) untill the center tank read less than 2.000 kgs. Once the center tank went below 2.000 kgs, the main 1 and 4 fuel tank quantity start to decrease, while no transfer is indicated no-where, and I cant see anywhere where this fuel goes as engine 1 and 4 are still feeded by the emptying center tank.http://aiarnaud.free.fr/pics/fuel_b.jpgSo here's my question again: where does that fuel from main 1&4 going?I shall repeat again that, during all my previous flights, with smaller amount of boarded fuel, this never happened before.Hope you will throw some light here.Regards,

[Qavion 19]

Arnaud....To refresh:There is no electronic system logic which stops the Fuel Boost pumps in the wing tanks supplying fuel to the engines. Each boost pump has a mechanical "check" valves at its output which only allows fuel to flow in one direction (tank to engine). Note: there should be no transfer of fuel from tank to tank.Since the CWT pumps are normally operating at a much higher pressure than the wing boost pumps, the wing boost pump check valves are forced closed by differential pressure. A large differential pressure closes the boost pump check valves completely and stops the boost pumps feeding the engines (e.g. when the CTW pumps have high pressure). However, as the CWT pumps pressure decreases due to low CWT fuel quantity, the check valves start to open a little bit (less differential pressure). Perhaps the slight opening of the wing tank pump check valves is not enough to activate the logic for the green fuel flow bars to appear on the Fuel Synoptic (from boost pumps to engines), but enough to allow small quantities of fuel to flow from the main tanks to the engines.Perhaps the RW pilots can tell us if flow bars actually do appear at this point? Unfortunately, my manuals don't tell me what logic is behind the flow bars (check valve position or some kind of pressure sensor at a strategic point)Hope this makes sense.Cheers.Q>

[Arnaud 0]

Q,Thank you: this makes sense to me indeed.Yet, my question still remains: where does that fuel go from main 1&4? Because, as you say, the fuel synopsis doesnt show it feeding any engine.

[Qavion 19]

Arnaud...The synoptic display on the real 400 is not super intelligent. I was talking to another engineer today and he thinks the green flow bars for wing tank-to-engine are generated by the *CWT* pump output fuel pressure switches (and other things like crossfeed valve positions).Since there is still SOME pressure at the CWT pump outlet, the flow bars are not being generated for tank to engine. However, the output pressure from the CWT is low enough to allow fuel to flow in small quantities from the wing tanks to the engines. Just because you don't see the flow, it doesn't mean it isn't happening. There are just not enough sensors in the fuel system to show a "full picture".PMDG appears to be doing what the real aircraft would do.Cheers.Q>

[Arnaud 0]

Well,Q, again, thank you :-) I assume that's the end of the thread then, as I also believe PMDG did model that Fuel Synopsis page 'weakness'.I feel more confortable about this now that I am about to proceed to my last World Tour leg (Shenzhen, China, back to London-Stansted).(The trip was EGSS-KMIA-MMMX-MMAA-SAWH-YSSY-WSSS-RPLL-RCTP-VHHH-SGSZ-EGSS aiming to fly over the biggest mountains on the planet).The PMDG 744s is really an outstanding product, and coupled with Walk-and-Follow and a nice personnal worldwide AI set-up, it really gives something very cool.Best regards,

[emvaos 13]

Arnaud,Robert is the man to ask on this since he directly worked (and worked and worked...)on fuel system logic. However, I do remeber him being pretty proud of modelling a specific fuel system feature he called "fuel mixing". Manifold pressure is being continuously checked (essentially manifold fuel level) and in case of very high demands fuel is "secretly" and in very small amounts supllied by all tanks connected to the manifold to prevent the latter running dry and thus fuel starvation to occur. Q's comments above explain the physical mechanism (pressure variations etc.) and the circumstances under which this specific feature might be observed.Best,Vangelis===================================== E. M. Vaos Precision Manuals Development Group www.precisionmanuals.com=====================================

[Check_Airman 3]

I could be confising it with another plane, but doesn't the system automatically go to tank to engine once takeoff flaps are selected, then back to "normal" config once the flaps are up? That might explain the usage form tanks 1 and 4.Am I right?Paul

[Arnaud 0]

Vangelis-Thanks :-) Perhaps Robert will see this thread and enlighten it by confirming your sayings.As the phenomena is consistent (been observing it more than once, and only when the tanks are fully filled), I conclude it's as on the real bird.Paul,Nope, the switch you're talking about indeed takes place when flaps are set to more than 10 before take-off, for some known reason.What I am talking about happens only 6-7 hours after take-off when the main center tank is almost empty (less than 2.000 kgs) and when the remaining of its fuel is transfered to main 2 tank. It's anyway only a small amount of main 1 and 4 tankers.Thank you all...Regards,

[Qavion 19]

Now the question remains of why the fuel doesn't come out of the #2 and #3 tanks when the CWT fuel qty gets low :( Or does it?Cheers.Q>