Fuel Tank systems on most GA aircraft...
Join Date: Dec 2010
Location: Low desert California
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More to the point, as low wing airplanes typically have noticeable dihedral, the vent space of the tank begins at the outer portion of the tank, and works it's way inboard as fuel is burned. To connect the vent spaces in the tanks at the outer tank end, it would be necessary to have the vent tube come across above the top surface of the wing. For all the high wing aircraft, there is less dihedral, so the vent space can be captured at the inboard end of the tank, and thereafter, connect the two tanks across the spar carrythrough, with a bit of a bend up in the vent line, so it will drain into either tank.
This characteristic is somewhat related to the requirement (by AD, for some Cessnas) for two venting fuel caps, as if the net line is plugged, and one cap does not vent, you cannot use the fuel in that tank. There is more to it than just that, but it gives you an idea. It is noteworthy, that the design standards (generally known as "part 23"), is a lot of hard learned wisdom, we don't always know what the lesson was, but we benefit from the learned outcome! Part 23 makes for an interesting read, if you wonder about aircraft design.]
This characteristic is somewhat related to the requirement (by AD, for some Cessnas) for two venting fuel caps, as if the net line is plugged, and one cap does not vent, you cannot use the fuel in that tank. There is more to it than just that, but it gives you an idea. It is noteworthy, that the design standards (generally known as "part 23"), is a lot of hard learned wisdom, we don't always know what the lesson was, but we benefit from the learned outcome! Part 23 makes for an interesting read, if you wonder about aircraft design.]
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ARRANGEMENT
§ 3.430 Fuel system arrangement. Fuel systems shall be so arranged as to permit any one fuel pump to draw fuel from only one tank at a time. Gravity feed systems shall not supply fuel to any one engine from more than one tank at a time unless the tank air spaces are interconnected in such a manner as to assure that all interconnected tanks will feed equally. (See also§ 3.439.)
§ 3.430 Fuel system arrangement. Fuel systems shall be so arranged as to permit any one fuel pump to draw fuel from only one tank at a time. Gravity feed systems shall not supply fuel to any one engine from more than one tank at a time unless the tank air spaces are interconnected in such a manner as to assure that all interconnected tanks will feed equally. (See also§ 3.439.)
I cannot explain why the Aeronca Sedan fuel system does not conform, I just read what the requirement says. 'Sounds as though you clearly understand the intent of the design requirement.
The certification requirement for GA airplanes states that for there to be a "both" fuel selector position, it is required to interconnect the vent spaces of the left and right tanks with a vent tube with no low spot. The geometry of a low wing aircraft with dihedral makes it impossible to do this, so only "L" and "R" positions are permitted by design arrangement.
