[Ernie Martin]

Thanks to Fred for taking the time to post the diagram and to Frank for his suggestion.

Looking at the diagram my conclusions are as follows:

1. The auxiliary fuel pumps will draw fuel from whichever tank you have selected. In short, each pump is associated with an engine, not with a tank. This is the case for my current 337G, but was not the case for my previous 337D (which is covered in the paper cited*).

2. There is return fuel going back to the tanks, as you can see from the code legend at the bottom left of the diagram. So, an engine receives more fuel than it needs and the excess gets sent back to the tanks. From the diagram alone I cannot determine whether the excess fuel is roughly the same as the burned fuel (i.e., the engine receives about twice as much fuel as it needs), as is the case for the non-turbo 337D.

3. It appears from the diagram that excess fuel from the front engine goes to both the main and auxiliary tanks of the left side, and excess fuel from the rear engine goes to the tanks on the right side. So this is different from the 337D, where the excess fuel goes only to the main tank. Frank's suggestion to see if your POH says to burn 1 hr fuel from the mains before selecting aux would confirm this. From the diagram, I'm concluding that your POH will not say that, because fuel returns to both. One unanswered question is how much to the main and how much to the aux.

4. If you are cross-feeding (which can only be from the opposite main, never from the opposite aux), excess fuel does not return to the tank from which you are feeding. If the rear engine is being fed from the left main, excess fuel goes to the right main and right aux.

From the above, you can infer operational scenarios. For instance, if you use all the fuel from both left tanks, cross-feed the front engine to draw from the right main and the right main eventually runs out of fuel, where will you find fuel? In both left tanks (the excess that came back from the front engine while it was feeding from the right main has partially refilled these tanks), and possibly in the right aux. You also recognize that the fuel in the aux left tank is not available to the rear engine and the right aux fuel is not available to the front engine; in a very long flight over inhospitable terrain (like Brian von Herzen's Skymaster flight to Europe via Greenland and Iceland) it would be advantageous to use the aux fuel as early as possible in the flight, so that if an engine fails later, the remaining engine can have access to all of the remaining fuel.

Ernie

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* In early Skymasters, including the 337D, there is a pump for the left main and one for the right main. If the rear engine is cross-connected and being fed from the left main, you would turn on the left pump, and if you were connected to an auxiliary tank, neither of the pumps would work.