Fuel tank venting for dummies...:)

[way_up_north]

hey guys...hope you're staying warm...

as I'm working away things that come to mind

ive always wondered about the diffrent types of fuel tank venting....

on an RV there is a aluminum tube from inside the tank beside the cap....vs the Bearhawk method of a vented cap....

the pros and cons of each system

Does the rv system prevent fuel spillage in a full tank in a turn....vs Bearhawk leaking a little out the cap in turns with a full tank out of the low wing...

[Chris In Milwaukee]

Well, I can tell you with my experience with a Maule, I've lost plenty of fuel from the vents after fill up, especially if it's got a bit of a lean going. For that I appreciate the vented caps.

[tailwind]

Yes, my caps spill fuel. I have a small hole and I have some concern about ice closing them off or a bug plugging the opening. They are simple, light and they work.


I did give some thought to venting the left tank from the left side across the cabin out to the right wing and the right tank over to the left wing. If I was flying IFR I would certainly look at options.

In in my case simple won out.

[Bcone1381]

Thats my favorite design.

[Battson]

The advantages with the tank cap vents compared to an under-wing vent tube are:
- simplicity of construction
- low cost
- elegance (fewer parts poking out of the wing)
- ease of repair / replacement
- easy to check for a blockage (blow test)

The disadvantages of tank cap vents are:
- one more pre-flight step is required every time you refuel (to avoid the following)
- ability install them with the vent hole facing backwards and suck some fuel out of the tanks, but not all the fuel
- if fuel is draining from the vent, then it goes onto the upper wing skin and flap fabric instead of onto the ground. This can permanently stain a single-pot paint, including Polyfiber's Polytone for fabric.

All but the most convoluted fuel systems will drain fuel out of the tank vents, whatever their design. Provided the tanks are full and that plane's on a sideways lean, the vents will drain. They will also drain fuel if you fill the tanks to the brim on a cold morning or from an underground tank, and then leave the plane parked in the heat of the day with full tanks (expansion of fuel).

In general, tank vents should not spill fuel in a normal turn. The positive air pressure is too high, particularly with a small vent hole. If you are flying like a bat from hell, or way out of balance, then I am sure you can get fuel to drain out.

[jaredyates]

I agree with the other replies. When the tanks are full and I'm parking on a slope, I select one tank at the fuel selector and that stops the overflow. Unless I really need to, I usually don't fill the tanks completely full. To expand on what Battson said, in flight, bank angle is not related to draining. The airplane can be in a 60 degree bank (coordinated) and not drain, or it can be at zero degrees of bank (uncoordinated) and drain. The condition that will cause fuel to flow from one tank to another is indicated by the inclinometer (ball) instead of the bank angle. This is a common misconception for non-pilots and surprisingly common among pilots also.

[Battson]

G-force is a big considerations in flight.
I think the fuel will go to the side of the tank and out the vent. Cross-feeding from one tank to another in flight would be an uncommon cause of overflow in a two-tank system.

[Startin22]

In a coordinated turn the g-force is perpendicular to the wing so the fuel will stay in the tank the same as it would in straight and level flight. You could fly inverted at one g or more and fuel wouldn't leak out the caps.

[rodsmith]

I used tank vents but ran the vents from the inboard forward corner to outboard of the tanks. This should prevent fuel loss on the ground on moderate slopes. That is an idea I came across in one of the Tony Bingelis books.

[stinger]

It's cold and wet, yes, even in North Florida. Fuel tank vents, I have no loss of fuel except when complete full and hard braking. My vents are under the wing, I built my tanks with the vent line in the tank and positioned to the highest location . As the vent line exit the tank I made three loops keeping the line high in the wing, from there the vent line goes outboard and down through the bottom of the wing just behind and outboard of the wing strut about six inches. This works for me as I did not like the above wing vent. Stinger

[way_up_north]

It's cold and wet, yes, even in North Florida. Fuel tank vents, I have no loss of fuel except when complete full and hard braking. My vents are under the wing, I built my tanks with the vent line in the tank and positioned to the highest location . As the vent line exit the tank I made three loops keeping the line high in the wing, from there the vent line goes outboard and down through the bottom of the wing just behind and outboard of the wing strut about six inches. This works for me as I did not like the above wing vent. Stinger

On a new thread on here....what would you do diffrently...a flying pilot listed fuel tank venting as the #1 concern in cold weather....stupid question....could you build one tank cap vented and one tank internal vented ....was thinking you could get the best of both worlds..with the
least amount of complexity...or better to go all one way or the other?...I live in Canada so I'll be following this topic carefully....

[Battson]

I used tank vents but ran the vents from the inboard forward corner to outboard of the tanks. This should prevent fuel loss on the ground on moderate slopes. That is an idea I came across in one of the Tony Bingelis books.

Presumably the tube remains near the top wing skin until it is well outboard of the tank, or has a hump in it? Assuming the vent tube sticks out of the bottom of the wing. Otherwise fuel will simply drain out of the uphill wing when the tanks are full to the top. Just trying to understand how the concept works.

No matter what pipe system you install, if taxiing with full tanks over bumpy ground causing the wings to rock to and fro, you are going to lose some fuel out the vent eventually. About all I can think of is using a small vent tube and a labyrinth to minimise fuel loss. Of course the volumes of fuel concerned are very small, so it doesn't really matter.

[rodsmith]

Yes the vent lines run along the top skin until outboard of the tanks.

[stinger]

Actually there are three so called humps ( three loops ) with the line near the top skin and continues high until going down through the bottom skin . Moving forward the relative wind force keeps the fuel in the tank. The small dihedral also helps . Stinger

[Ed.Meyer]

I am thinking that it will not matter much if the vent tube is routed with high spots before exiting out the bottom of the wing. At the point that fuel runs through it, a siphon action will begin allowing fuel to flow until the tank fuel level is below the vent port. We used the stock vented fuel caps. Real simple.

[Battson]

I am thinking that it will not matter much if the vent tube is routed with high spots before exiting out the bottom of the wing. At the point that fuel runs through it, a siphon action will begin allowing fuel to flow until the tank fuel level is below the vent port. We used the stock vented fuel caps. Real simple.

Yes I wondered about that too. Because of the high point, it would only happen when the plane is moving around. When moving, the fuel sloshes around a lot(!), so it will only let a short slug of fuel through at a time. It wouldn't be able to siphon continuously.

The design RodSmith describes sounds like a good one which will work well, if vents caps aren't your thing and you don't mind the added weight and complexity. I went vented caps too.

[wpackard]

I have under-wing vents in my BH and if I fill the tanks, a fair amount of fuel immediately leaks out of the vents even on level ground. Taxiing makes it worse.

I was thinking of installing a check valve inline with the vent tubing (something like this: https://www.aircraftspruce.com/catal...checkvalve.php).

Anyone have experience with that approach or have an opinion on why it's a good or a bad idea?

Thanks.

[yateselden]

The expanding fuel has to go somewhere, not sure of your line routing. Possibly push your caps out

[wpackard]

Yeah, I thought about the fuel expansion problem, but there is a version of the Andair check valve that has a bleed feature that is intended to account for it.

Here's a quote from the site: "Optionally a bleed can be fitted to the valve, here the internal shuttle has been drilled with two small holes. This allows for bleed back to occur. This particular valve is useful in the vent line of a fuel system where allowance for fuel expansion is necessary."

[yateselden]

I didn't think of this until my wings were sealed up, but thought venting the unvented main through vented extended wing tank caps with a separate line would have worked well. Also looked into the check valves, didn't do that because of price.

[Bdflies]

[QUOTE=wpackard;n44094]I have under-wing vents in my BH and if I fill the tanks, a fair amount of fuel immediately leaks out of the vents even on level ground. Taxiing makes it worse.

I was thinking of installing a check valve inline with the vent tubing (something like this: https://www.aircraftspruce.com/catal...checkvalve.php).

Anyone have experience with that approach or have an opinion on why it's a good or a bad idea?

Thanks.[/QUOTE

My first thought was to recommend against such, for fear of pressurizing the tanks from warming. Something about the concept seemed familiar though. I still have the old parts catalog for my old Husky, so I looked at the fuel system diagram. Sure enough, the (certified) Husky employs a "check valve" in each vent line. The vents enter the tanks high on the outboard ends and are routed through the "check valves" to exit at the trailing edge of each wingtip. The valve is described simply as a "check valve", but I suspect that it also functions as a pressure relief as well. I know that warming gas will push out the vent lines, but I don't recall nuisance dumps from taxiing. Soooo, I think I'd look for check/relief valves, or install a low pressure relief circuit around the single function check valve, which complicates things even more, doesn't it?

FWIW, I have the kit supplied vented caps. No complaints here.

Bill

[wpackard]

The C 172 fuel system diagram also shows a "check valve" where the vent line exits the tank. I've never seen fuel leaking from the vents of a 172 during taxi, but I have seen leaking while it was parked in the sun so it must be using a valve with a "bleed" feature.

One variant of the Andair check valves I linked has this feature and it is mentioned specifically for use in fuel tank venting. I don't think the installation would be particularly complicated other than the fact that I'd have to cut and flare the tubing while it's already installed in the wing. There definitely could be something I'm missing though.

I'm curious now what type of check valve certified aircraft are using.

[Baloo]

Sorry to beat a possibly dead horse but.....

My BH has the under wing tube vents. I have to be very careful when fuelling and stop about 3” from the top of the tank otherwise it pisses fuel everywhere.

I recently bought and and installed the vented caps and have been running both methods. I’d like to cap/seal off the under wing tubes but was warned by someone that I might be prone to collapsing a tank if I did that.

after reading the above thread I see that is most likely not the case. Can anyone confirm that the vented caps on their own are sufficient for venting.

[jaredyates]

I've only ever had the vented caps, and never had any problems with them not venting enough. They will still allow fuel to leak overboard if I fill the tanks full enough or park on an uneven enough slope. Usually selecting only one tank at the fuel selector will stop the overboard dumping, but not always, if the slope is steep enough or the tank full enough.