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Guest tompeel

Tank to Engine- when to switch

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I'm new to the PMDG 747 so bear with me if there's a simple answer. I just ran into the tank-to-engine switch issue on a flight which left me doing an emergency landing with 15000 lb in each of #1 and #4 and only 5000 in #2 and #3. I realize I should have switched the valves earlier, but my question is, is there a simple way to predict when to do this, or do I just have to sit and stare at the EICAS display? Why doesn't it switch automatically?T.

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The Fuel tank to engine change generally takes place between 50 & 60 tonnes in tanks so bank on it happening sometime around that time. Obviously the EICAS will tell you when anyway. If you are left with a situation where you have too much fuel in a couple and not enough in the others then all you need to do is change the pumps so that the engines run off the two tanks with the most in until the levels equalise again and then you can go back to the normal configuration. Tom P

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"Why doesn't it switch automatically?"Combinations of manual and automatic control provide the best flexibility should malfunctions occur (fuel leaks, pump failures, etc). No doubt Boeing spent a lot of money figuring out the best combination. Also, having the pilot participate in the process keeps his/her mind active and gives him/her a better understanding of the system. An automatic system would have to juggle up to 9 fuel tanks (in normal and abnormal situations) on a 744. No easy task.For hundreds of thousands of dollars a year, I'd gladly sit there are watch the screens for the pilots :(Cheers.Q>

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>I realize I>should have switched the valves earlier, but my question is,>is there a simple way to predict when to do this, or do I just>have to sit and stare at the EICAS display? I think the EICAS also gives out an aural warning when Tank-to-EGN occurs. Usually, whenever I hear some sort of noise other than the engines, some things up. Most of the time it's just Tank-to-EGN; most of the time...Ryan GamurotLucky to live Hawai'ihttp://www.virtualpilots.org/signatures/vpa296.png

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Thanks for the replies. The Ginge writes that I can just switch the pumps round so that the engines get fed with the tanks with the most fuel. Maybe I'm missing something about how the pumps work. When #2and #3 tanks have more than #1 and #4, then all the engines run off #2 and #3, right? Then when #2 and #3 drop below the #1 and #4 levels, you switch tank-to-engine. However, I can't figure out how you can feed all the engines off #1 and #4 only. If you don't switch, you can wind up being above MLW and have a fuel emergency, right?I guess I can figure out when the switch is necessary by monitoring the fuel flow rate and doing some arithmetic, however considering all the other fine things the FMC calculates like TOC, TOD and step climbs, it wouldn't seem that predicting the fuel switch point would be too much extra, so I could go and get a coffee from time to time.;-) T.

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>Thanks for the replies. The Ginge writes that I can just>switch the pumps round so that the engines get fed with the>tanks with the most fuel. Maybe I'm missing something about>how the pumps work. When #2and #3 tanks have more than #1 and>#4, then all the engines run off #2 and #3, right? Then when>#2 and #3 drop below the #1 and #4 levels, you switch>tank-to-engine. However, I can't figure out how you can feed>all the engines off #1 and #4 only.> If you don't switch, you can wind up being above MLW and have>a fuel emergency, right?>I guess I can figure out when the switch is necessary by>monitoring the fuel flow rate and doing some arithmetic,>however considering all the other fine things the FMC>calculates like TOC, TOD and step climbs, it wouldn't seem>that predicting the fuel switch point would be too much extra,>so I could go and get a coffee from time to time.;-) When tank 2 and 3 have more fuel, you use them to feed all four engines because they have what are called Override Pumps. These pumps are powerful enough to feed all four engines off just 2 fuel tanks. When you switch to tank-to-engine, you actually have each engine feeding off it's own tank, hence, tank-to-engine. You see, when all four main wing tanks have the same level of fuel, you are closing crossfeed valves 1 and 4. This causes engine 1 to feed only from tank 1 and the same with engine and tank 4. With tanks 2 and 3, you shut off the override pumps since the extra power isn't needed anymore, and although crossfeed 2 and 3 are open, tank 2 and 3 is only pumping enough fuel to supply their respective engine. This also helps to keep the weight evenly distributed throughout the wing.If you don't switch, you don't really have a problem. If you leave the tanks configured in it's pre-tank-to-engine state, that is with overrides on and crossfeeds open, tanks 2 and 3 will continue to supply fuel to all four engines until they reach 7,000 pounds each. Then the override pumps automatically cut out from the lack of sufficient fuel and then all four tanks automatically feed the engines. The only catch is that now you have heavy wingtips. Ryan GamurotLucky to live Hawai'ihttp://www.virtualpilots.org/signatures/vpa296.png

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If you don't switch, you don't really have a problem. If you leave the tanks configured in it's pre-tank-to-engine state, that is with overrides on and crossfeeds open, tanks 2 and 3 will continue to supply fuel to all four engines until they reach 7,000 pounds each. Then the override pumps automatically cut out from the lack of sufficient fuel and then all four tanks automatically feed the engines. The only catch is that now you have heavy wingtips. That's something I didn't know. I thought I had seen the fuel levels in #2 and #3 go below 7,000, though. I tried reloading the situation later - I have Autosave installed - and the reloaded flight had all the tanks set at exactly the same level.T.

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Just adding to what Qavion and rgamurot have said.Each Override/Jettison pump can supply two engines at takeoff and cruise power. Each normal pump provides enough fuel for one engine at takeoff thrust and two engines at cruise thrust. Therefore, a person can use a single main tank pump(non-override) to feed two engines in cruise if needed.Cheers,JohnBoeing 727/737 & Lockheed C-130/L-100 Mechanichttp://www.sstsim.com/images/team/JR.jpgwww.SSTSIM.com

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Think the 747 fuel system is complex? Try controlling the fuel manually in SSTSim's Concorde - makes the 747's fuel system a doddle!

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>Think the 747 fuel system is complex? Try controlling the>fuel manually in SSTSim's Concorde - makes the 747's fuel>system a doddle!Very true! That's why I always fly Concorde with the VFE. Then again, even he has my CG alarm going off at all stages of the flight.Ryan GamurotLucky to live Hawai'ihttp://www.virtualpilots.org/signatures/vpa296.png

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