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Guest cloudbase

PMDG Fuel pumps/tanks

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Guest cloudbase

I loaded and fueled up my 737-700 and took off and all was well until halfway through my flight both my engines failed, I quickly scrambled for the abnormal procedures manual and ran through the appropriate failed engine drill and looked for the nearest field, after loosing 10,000. I looked at the fuel guages and both my wing tanks were empty but my central tanks were full, The problem was that I hadnt turned on the central fuel tank pumps.My question now is can I transfer fuel from wing tanks to central tanks and vice versa, I assume you can transfer between wing tanks such as if one engine failed. There are 6 fuel pumps for 6 individual tanks on the 737-700, left and right wings have aft and forward tanks and the central tank seems to have a left and right.Do the pumps only pump to the engine or can the fuel flow be directed to another tank, if so what knobs/switches do I press.there is a cross feed switch, what does this do and when should it be used.Also what is common procedure in real life to use the wing tanks or central tanks first, I think I read the wing tanks should be used first to reduce flying/landing stress on the wings?? PS I thought the 737-700 glides quite well but not as good as the eta which has a similar wingspanhttp://www.jaxida.dk/images/allweather/ETA..._album_ETA1.htmDavid

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Guest teeloo

I am not an expert on the 737.As far as I know you could transfere anyway you like by switching pumps on and off. However I don't know if you can physically add fuel to other tanks. I am not into the fuel system of the NG. Leave that to an expert to say (anyone out there?).I do know, though, that center tanks are always emptied first!Reason for this is to reduce stress on the wings on the fuselage side.So it really is the opposite as you stated.Mind you the wings produce the lift, so any weight right 'under' it (ie wing fuel tanks) are directly connected so not much stress there.However the fuselage is a have lump that needs to be as light as possible for least amount of stress on the wingroots.btw: you can compare it to your gliding: A fat guy bends the wings more then a light guy!Flying with water in the wings doesn't 'bend' the wings as much as a fat guy in the cockpit!;-)Teeloo

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Guest cloudbase

thanks for the reply,I guess when you land its best to have the remaining fuel in the wing with the weight over the wheel than to have it centrally.hey, do you know what is an acceptable rate of descent when you hit the tarmac in the B737David

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Guest PittsburghII

Hi David, I too am no RL driver of the NG, so I may be wrong in my statements here. The NG suction or gravity feeds from the wing tanks (of which I believe there are only two, but fore and aft pumps on each totaling in four switches as can be seen on the overhead -- could be wrong here). I don't think it is possible to suction or gravity feed from the center tank hence your flame out. I think the easiest (knowing that an inflight flame out is never easy) way to deal with this would be to turn on the center tanks pumps and try to start the engines in flight. I don't think it is actually possible to transfer fuel from one tank to another. I only believe the Concorde had that capability. The Concorde did it to ensure the center of gravity was right when entering supersonic (or, so I believe). I know, from talking to an old 767 driver, that sometimes on long flights there would be a fuel imbalance between the two wing tanks (after having depleted the center tank). The only way to remedy this was to turn on cross feed and turn off the pumps on the tank containing the most fuel allowing it to feed two engines. Only point is to remember to turn on the pumps on the other tank once the level has equalized. So, I belive the cross feed is only helpfull when you want to feed two engines from one tank, for instance if you have a leak in your system. In that case, one just have to be carefull not to do like that Canadian crew that inadvertently cross fed the wrong way and effectively dumped a load of fuel in a matter of a few minutes.Hope this helps,BoazEKCH

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Hi David,>>My question now is can I transfer fuel from wing tanks to>central tanks and vice versa, I assume you can transfer>between wing tanks such as if one engine failed. The 737 has no tank to tank transfer capability.>>There are 6 fuel pumps for 6 individual tanks on the 737-700,>left and right wings have aft and forward tanks and the>central tank seems to have a left and right.There are only three fuel tanks with two pumps in each tank. The No.1 and 2 tanks are the main engine feed tanks. The centre tank has higher pressure pumps so when they are on, the engines will be fed from the centre tank automatically.>>Do the pumps only pump to the engine or can the fuel flow be>directed to another tank, if so what knobs/switches do I>press.The feed is only from tanks to engines and not tank to tank.>>there is a cross feed switch, what does this do and when>should it be used.This should be used to balance fuel between #1 and #2 tanks or for abnormal conditions. Say you have 6500 LBS in #1 and 5500 LBS in #2. Open your crossfeed valve and turn the pumps off in #2 tank. This allows #1 tank to feed both engines. When the levels are equal just turn on the #2 pumps and close the crossfeed. The same can be done for the opposite situation.>>Also what is common procedure in real life to use the wing>tanks or central tanks first, I think I read the wing tanks>should be used first to reduce flying/landing stress on the>wings?? Centre tank fuel is always used first. This allows the wings to be 'span loaded' with the fuel in #1 and #2 tanks. In the real world 1000 lbs is left in the centre tank due to an FAA Airworthiness Directive concerning centre tank explosions.Here is a good fuel schematic.http://www.telusplanet.net/public/jrberg/photos/ngfuel.JPGCheers,JohnBoeing 727/737 & Lockheed C-130/L-100 Mechanichttp://www.precisionmanuals.com/images/forum/ng_driver.jpg

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Guest cloudbase

Wow, thanks for that great diagram, it all starts to make sense now, I quess that there are 2 pumps per tank is just a redundancy thing in case a pumps fails then there is always a backup.In the diagram I understand the fuel lines with the pumps with 2 per tank It would appear these pumps generate a positive pressure in the main spar fuel lines to the engine, but there seem to be fuel lines with only a one way valve connected to the main central spar fuel line feed for the engines, I assume the fuel pumps in the engines themselves are able to generate a negative pressure and 'suck' fuel directly from the tanks, once again I assume this is a redundancy design in case of pump failure in that fuel will get to the engine under positive 'push' or negative 'suck' pressure in case of engine fuel pump or fuel tank pump failure.Another question now that I have is what the pack switches do, my best guess is it has something to do with air flow control.Thanks againDavid

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Guest teeloo

John!You are the expert I was hoping for!:-)Great lecture!I think I'll open up another topic on techical stuff.Hope you'll explain then too ;-)Teeloo

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Hi David,The valves you are talking about are the Engine Fuel Suction Bypass valves. They are there to allow gravity/suction feed of #1 and #2 tanks in the event of electrical pump failure. The combination of the engines being lower than the fuel tanks, the suction of the engine driven pumps as well as the pressurizing effect of ram air on the tank vent system makes the gravity/suction feed work. As for your last question, the pack switches control the airflow to the air conditioning packs. In effect an ON/OFF switch for the air conditioning.Cheers,JohnBoeing 727/737 & Lockheed C-130/L-100 Mechanichttp://www.precisionmanuals.com/images/forum/ng_driver.jpg

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Guest teeloo

John, You're the guy in this!I have some more questions on this if you don't mind me asking, but I take my chances with an expert in the house ;-)So when I look at it, it seems all pumps are actually (located)inside the Center Tank. Except the engine driven pumps.Is this also done for cooling (fuel cooled pumps). I am not sure since if the center tank is empty, there is no longer a medium to cool with. Wild guess here.Those 'funny' endings on the fuel lines in the Center Tank are presumably pressure sensors, right?Is the crossfeed valve a fail-open or fail-closed valve (not that this is simulated in the PMDG version)?Is it right that I don't see any heat exchangers in the system (oil cooling etc)?Thanks!Teeloo

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Hi Teeloo>John, >>You're the guy in this!Thanks! :-)>So when I look at it, it seems all pumps are actually>(located)inside the Center Tank. Except the engine driven>pumps.>Is this also done for cooling (fuel cooled pumps). I am not>sure since if the center tank is empty, there is no longer a>medium to cool with. Wild guess here.All the electric pumps are located within the centre tank. This is mainly due to maintenance accesibility. The centre tank pumps are accessed in the main wheel well and the wing tanks are through access panels on the lower surface of the wings inboard of the engines but within the centre tank area. The pumps are internally cooled with the fuel it's pumping so it doesn't require full submersion.>>Those 'funny' endings on the fuel lines in the Center Tank are>presumably pressure sensors, right?They are the low pressure switches.>>Is the crossfeed valve a fail-open or fail-closed valve (not>that this is simulated in the PMDG version)?The crossfeed valve maintains last selected position if internally failed. There are abnormal procedures for a valve failed open or closed.>>Is it right that I don't see any heat exchangers in the system>(oil cooling etc)?Since the diagram only shows the fuel system the hydraulic heat exchangers are not shown. A system heat exchanger is in #1 tank and B is in #2 tank. The engine Fuel/Oil heat exchangers are located within the engine fuel system so aren't shown in the diagram as well.Cheers,JohnBoeing 727/737 & Lockheed C-130/L-100 Mechanichttp://www.precisionmanuals.com/images/forum/ng_driver.jpg

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Guest teeloo

Ah nice, nice, nice John!Thanks a lot ones more ;-)

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