National Aeronca Association Bulletin Board

I'm replacing the fuel vent line during my rebuild and have a question.

Image "A" is the way it was, I would like to eliminate the coupling, (less fittings, less chance of leaks),
problem is once you bend the (2) 90's and then slide the fuel line into place thru the 2 wing ribs inboard,
I can't get the 45 up the neck.
If I eliminate the coupling, how do I get the 45 degree bend that goes up into the filler neck?

couldn't I just do image "B" without the 45 ...??
I mean it's all just fuel & air anyway, ...is'nt it? , not sure I see the Pros & Cons of either A or B.
enlighten me, please. Jim

Dont you need to have the end of the vent line as high as possible in the tank to prevent a hydraulic-lock situation ? (I dont know either, just guessing...)

You might want to consider keeping that coupling, because what if you ever had to remove one of the tanks ? Would "B" make that a much harder task ?

Hasta ~

Bob K.
Anchorage, AK

Jim it's a notable idea to eliminate as many joints as possible, but think Bob's spot on about years later when doing maintenance. With that extra coupling, you can disconnect the tank and slide it out, but without the coupling, you'd have to snake the vent line out of the tank while trying to remove it. And that would be a pain in the ass!

What kind of tanks to you have? 5, 6, or 13gal ?

The other thing peculiar is the vent line location. Usually you want the vent line higher than the fuel tank so it doesn't trap fuel. If the line fills with fuel, then the pressure won't equalize between the tanks and the whole idea of inter-tank venting is moot.

I played with your drawing (nicely done by the way!) and came up with a couple other ways of doing things. "C" is how they originally vented the 6gal BCM tanks; it was vented over board and not to the other tank(s) but would work just the same. "D" is probably the simplest way of doing things. Don't have to worry with the internal 45' bend, and just accept a little fuel in the vent line until the level drops. The key is to have a positive run on the vent line between tanks, so you don't trap fuel.



And then there is plan "E" where you scrap the whole inter-tank vent system and rely on the cap vents. But I would only do that if these were Aux tanks.

nkh

Nate (and BobK)

It's the 13 gal variety.
I also thought about the possibility of having to remove a tank.
If that became necessary, you could (at that point) cut the line and put in the coupling, also if you had to take the tank out, (scary thought)
then that entire fuel line would be accessable because of the size of covering you'd have to cut (to remove the tank).

My drawing shows the line going under the tank, but that's not actually the case, it was easier to draw (and show) that way.
It actually comes out of the tank fitting and goes around the front of the tank. (sorry for any confusion)

Neither your "C" or "D" would not work because covering is directly on top of the tank.

As for fuel in the vent line, just "toppin' the tanks" and taxing is going to fill up that vent line, not to mention bouncing around
the air and banking and slipping. Also, even though the vent line exits level with the tank fitting, it dips below the level
of the bottom of the tank as it goes though the fuselage on it's way to the other tank, I suspect there is fuel in the vent line continuous.

...guess I'll just put the coupling in, Jim
"(nicely done by the way!)" ....thanks for the compliment

Jim,
This line provides a positive pressure to the tanks not just venting. This tank uses non-vented caps.
Paul