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justme

Fuel Pumps on 747-400

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Hi,I just want to ask if it is normal that in flight or on the ground after shutting down all fuel pumps then the engines continue to work and never stop due to lack of fuel. Engines able to pull out fuel from the tanks without the fuel pumps? Then why we need them? Please clarify someone.Matyas Majzik

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There is an "unlimited fuel" option or something like that in FS9 settings, (I think it is in the "realism" section).Just uncheck that option.Jose Luis.


signed: José Luis

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I does not mean that. I am not speaking of amount of fuel. It does not matter. Of course unlimited fuel is not turned on.I mean that during flight or staying on the ground with all four engines running when I shut down all fuel pumps on the overhead panel then this does not affect engine performance and the engines without working fuel pumps still running perfectly. This is odd. I do not understand why.I thought that if I shut down all fuel pumps then the engines will stop working without fuel transfer from the tanks.Matyas Majzik

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One question, are you running the APU ?.If the APU is running, main-2-aft pump will be automatically running too and you will not be able to turn it off unless you switch off the APU.So, if that pump is running and the X-feed valves are open, you will be feeding the 4 engines with that pump, then, the engines will continue running. The only way to stop them should be cutting off the four fuel control switches in the pedestal or stopping the APU.Well, this is only my theory...Jose Luis.


signed: José Luis

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Matyas,The engines still draw fuel from the tanks without fuel pumps being on by suction feed. That is the engines suck fuel from the tanks when running.The reason we need pumps is the suction feed is not reliable at higher altitudes or at high thrust settings,so we use pumps to add pressure to the fuel line.I hope that answers your questioncheersJon

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Thanks, that was my question. Is this simulated that suction feed is not enough? For example if I am flying at FL360 at maximum trust (mach 0.89) without fuel pumps. I am doing hard turnings but the engines are still at perfect fuel feed.Matyas Majzik

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Probably not simulated then. The manual in the real one says you MAY get "cavitation" which is pockets of air in the fuel at high altitude.The engines would probably run fine with no pumps, but in aviation probably isn`t good enough!cheersJon

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"I am doing hard turnings but the engines are still at perfect fuel feed."Are they coordinated turns? Or are you slipping? If coordinated, the fuel will remain relatively flat in the tank (and the fuel pickup lines are still going to get fuel). Internal baffles in the tank should keep the fuel from sloshing about too much. Also, NACA (air) scoops under the wings keep a positive pressure on top of the fuel in the tanks to assist fuel flow.The tank pumps are there to assist the engine pumps, especially when the CWT/HST tanks are being used. Fuel will not gravity/suction feed from the CWT or HST. Hope this helpsCheers.Q>

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Thanks. I am always interested in technical questions :)I always like to fly from a technical point of view :)Matyas Majzik

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Q,Can you elaborate on these NACA scoops and their designed role? Why a positive pressure in the tank? (That is a low pressure?). Are they there purely to assist fuel flow or is that just a biproduct of another function?Thanks, always interested in your posts! :()Subs

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Yes, it's only a low positive pressure (a few psi). Unfortunately, I can't find any written information on the function of the scoops, but someone may have told me in the dim, dark past, that it has a few different functions, but what its primary function is, I'm not sure (suppression of fuel vapour, prevention of cavitation, assitance to fuel feed, etc). If you can imagine, if the tanks were airtight.. and you used fuel from them, you would create a vacuum in the tanks (your pumps would be fighting a vacuum). Also, if the fuel tanks were airtight, you wouldn't be able to get fuel into them!Boeings use bleed air pressure to do the same thing for the hydraulic system (A low-ish positive pressure is kept on the hydraulic fluid reservoirs to assist fuel feed).Hope this helps.Cheers.Q>

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