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mr.crocodile

Lack of fuel and Engine fail

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Hi, I'm new user of the AVSIM forum. My name is Andrea Nardi.I noticed that there are some strange behaviours reguarding the fuel quantity related to the eventual engine failure.PMDG 737-700. I wanted to see the behaviour of the engines in case one wing tank became empty and the other one still had enough fuel in it. I tried to set a certein quantity of fuel (2000kg) in one wing tank and I left the other with just a few kgs of fuel (let say 20kg) and the center tank empty with its pumps off.The pumps of the wing tanks are obviously ON but the CROSS FEED is closed. After a few seconds (when the tank with a very low quantity of fuel is completely empty) both the engines stops. I don't understand why this happens.The other strange thing is that in normal conditions (wing tank with fuel) it seems that also with the wing tank fuel pumps off the engines starts and continue to work regularly. Is that a correct behaviour or not?Thanks

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Andrea,As to your first question, no that doesn't sound correct as far as how the plane should operate. The #2 engine should still feed from the fuel from the Right Main Tank.As to your second question:

Suction FeedWhen main tank fuel pump pressure is low, each engine can draw fuel from itscorresponding main tank through a suction feed line that bypasses the pumps. Asthe airplane climbs, dissolved air is released from the fuel in the tank due to thedecrease in air pressure. This air may collect in the suction feed line and restrictfuel flow. At high altitude, thrust deterioration or engine flameout may occur as aresult of the fuel flow reduction.The dissolved air in the fuel tank will eventually deplete after reaching cruisealtitude. The depletion time is dependent upon airplane altitude, fuel temperature,and type of fuel. Once the dissolved air is depleted, the engine may be capable ofsuction feed operation at cruise power.

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Andrea,As to your first question, no that doesn't sound correct as far as how the plane should operate. The #2 engine should still feed from the fuel from the Right Main Tank.
Yes, I understand it. So is it a bug of the PMDG or can I do something to solve it?
As to your second question:
Suction FeedWhen main tank fuel pump pressure is low, each engine can draw fuel from itscorresponding main tank through a suction feed line that bypasses the pumps. Asthe airplane climbs, dissolved air is released from the fuel in the tank due to thedecrease in air pressure. This air may collect in the suction feed line and restrictfuel flow. At high altitude, thrust deterioration or engine flameout may occur as aresult of the fuel flow reduction.The dissolved air in the fuel tank will eventually deplete after reaching cruisealtitude. The depletion time is dependent upon airplane altitude, fuel temperature,and type of fuel. Once the dissolved air is depleted, the engine may be capable ofsuction feed operation at cruise power.
Thanks for this article that explains a bit more. But this behaviour is not the same as for the main central tank.Infact I made other tests. Wing tanks completely empty and fuel only in the main central tank.If I try to start an engine without turning the corresponding central tank fuel pump on the engine does not start.The engine starts also if I turn on the central tank fuel pump of the other engine and opening the cross feed. And till there I think that it's normal.The strange behaviour is this:left "center tank" pump ON, right "center tank" pump OFF, cross feed ON. All engine start regularly. If I close the cross feed the right engine continues to run. If I switch off also the left pump both the engines turn off. (the same is with left pump off, right on).What do you think about this?

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Both issues sound like sim problems, and the plane shouldn't operate like this. (As far as I can tell from looking at the schematics.)But then again, you shouldn't operate the plane like this. I think the sim is modeled after following Boeing SOPs. When would you be flying with fuel in the center tank and none in the mains? If you were in a low fuel situation, the crossfeed valve would be open.But, you're right - this is not how the sim should react.

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Both issues sound like sim problems, and the plane shouldn't operate like this. (As far as I can tell from looking at the schematics.)But then again, you shouldn't operate the plane like this. I think the sim is modeled after following Boeing SOPs. When would you be flying with fuel in the center tank and none in the mains? If you were in a low fuel situation, the crossfeed valve would be open.But, you're right - this is not how the sim should react.
Yes, I definitely agree with you.Thanks a lot.

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