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Guest toby01

Fuel tanks and engine start.

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How come it is possible to start the engines even though the fuel tanks pumps are not pushed in? Common sense would tell that the fuel tanks need to activated in order to supply fuel to the engines to operate. Right?

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EDP's and gravity feed will work in most conditions..... The Electrical pumps are there to supply the engines under ANY condition. Even The APU will run with all pumps off as long you have fuel in the tank.Also pls sign your real name.Rob

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Its a requirement that the aircraft MUST be able to run with all the pumps off.

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>Its a requirement that the aircraft MUST be able to run with>all the pumps off.I guess this is another mesure of how safe these birds are.Regards

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>Even The APU will run with all pumps off as>long you have fuel in the tank.>How can the APU (up there) get fuel (down there in the wings or under the fuse) by gravity means ????Jose Luis.

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I never said the APU can get fuel by gravity means. The engines can be supplied by gravity and the EDP (ENGINE DRIVEN PUMP) the APU relies on the AC PUMP if that fails then the DC pump.. I am sure it's more technical then that, I don't fix 747's for a living.Rob

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"Common sense would tell that the fuel tanks need to activated in order to supply fuel to the engines to operate. Right?"If all the engines flamed out in flight causing you to lose all main bus AC electric power (the power source for the fuel pumps), how could you relight the engines? The FAA/CAA/etc would never allow aircraft to fly if this was the case.When the engines are windmilling, pumps on the engine suck fuel out of the wing tanks (i.e. the tank pump fuel feed plumbing is bypassed). This is assisted by gravity (just as it would if you were siphoning fuel out of a car fuel tank... Once you get an initial fuel flow, it is helped along by gravity) Don't feel bad.. this question is asked regularly on this forum. People just forget to use the Search function (or can't find the right keywords).The APU, in theory, could also suck fuel out of the tanks, but I can't say I've tried it in real life to see if it works. The fuel has to go a long way uphill, so you'd probably get cavitation.. and the APU would cough and splutter or perhaps not start at all.The APU is normally fed with one or more main tank pumps (depending on the configuration of the crossfeed valves), but if low fuel pressure is sensed in the APU feed line, a dedicated APU DC-powered fuel starts up (this can be run from the battery).Cheers.Q> Ян

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