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wing tank wing tank
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----------------tee---------crossfeed valve---------tee---------------
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transfer pump transfer pump
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-------------header-----------------
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boost pump
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engine
What happens when one tank (not completely filled at the last stop) runs dry first?
Terrence, if the crossfeed valve is CLOSED and one wing tank runs dry first, we get a "wing low" light and, if left on, that pump would draw air. No big deal, it doesn't hurt the pump for relatively short periods and the engine doesn't care since it's being fed from the header.
If the crossfeed valve is OPEN the dihedral of the wings and the fact that all of the plumbing and transfer pumps are BELOW the level of the bottom of the wings will cause the fuel to GRAVITY flow so as to equalize. Both wings will run out of fuel at the same time. If one wing has a higher level of fuel, that fuel will flow downhill into the opposite wing until they are equal. The last drop of fuel in the entire wing system will be at the transfer pumps. Only if BOTH wings are empty will either transfer pump input see any air. The only exception to this would be operating in a prolonged slip with the crossfeed open. Even then, it wouldn't effect engine operation since the engine is being fed from the header ONLY.
But, wouldn't the wet fuel-pumping Facet draw the less-resistant air through the open crossover and into the pump and lose its prime?
No. Fuel will flow THROUGH the crossfeed valve from one wing to the other. Only when the level of both wings are equal will the flow stop. (assuming that you're not flying in a prolonged slip). So there will be no air to draw untill there's no fuel left in EITHER tank. Hmmmm, maybe the confusion is from my choice of words. Perhaps I should have labled the valve a crossFLOW valve rather than a crossFEED valve.
Has anyhone tested this?
YES!!!! We have tested this system quite extensively. I have started with full wings and burned both wings empty using only one pump. I have done this test several times with a simulated transfer pump failure. I have also done tests in which I've flown with the crossfeed valve closed and turned off one transfer pump. The idea here was to simulate that a transfer pump failed and I hadn't noticed. I waited until I got a "wing low" light (although the imbalance would be hard to miss as considerable aileron stick force was eventually required). I then opened the crossfeed valve. The wings began to equalize and I was able to burn BOTH wings empty with one pump.
In addition to the redundancy the crossfeed valve provides, we have 2 electrical systems with 2 batteries. The fuel transfer system normally operated on the essential bus. We do however, have a seperate electrical feed from the normal bus that by-passes all of the relays and circuit breakers and powers the transfer pumps directly through a switch. We felt that this level of redundancy was necessary for some of the long overwater legs that we fly where an alternate would be well beyond header tank range. In order for us to be unable to burn ALL wing fuel, we would have to have either a dual pump failure or a dual electrical failure.
Where are you based? I'd be happy to show you the system and demonstrate how it works.
Bill Harrelson
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