PPRuNe Forums - Fuel Tank systems on most GA aircraft...

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Quote:

Originally Posted by megan (Post 10289020)

T-28

It's amazing the difference a dash makes. T28 follows.

https://cimg4.ibsrv.net/gimg/pprune....620979513e.jpg

T28 Super Heavy Tank in Aberdeen Proving Ground 1945

Of course tanks have some less successful fuel system designs as well...

https://cimg0.ibsrv.net/gimg/pprune....ee711be632.jpg

Quote:

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.]

Pilot Dar, what you said above in a few posts meets the current FAA Part 23, but the old CAR 3 planes which were up to 1956 didn't have those same rules. For instance, the Aeronca Sedan, 15AC, had no interconnect across the fuselage to both tanks. Just a goofy vent system that had two 1/4" vent lines meet under the fuselage which would get filled with bugs and dirt thus leaving no vent. Then the bladders would collapse like a baby nurser bottle, and eventually the engine would quit for lack of fuel. Even though the fuel gauges showed near full as the gauges were a plastic tube connected to the top and bottom of the inboard side of the bladder. Lots of Sedan crashed because of this crazy system.

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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.)

CAR 3.430 of March 13, 1956. The wording of CAR 3.430 of November 01, 1949 is identical. My bold.

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.

Quote:

Originally Posted by Pilot DAR (Post 10288513)

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.

It's not impossible to do in a low wing aircraft. I know of at least one low wing aircraft which operates with the selector effectively set to both (It only has an ON/OFF fuel valve) and has an interconnected vent tube.