RV-10 fuel tank vent tube leak fixed!

[Lew Gallagher]

Upon first fuel fill on Grant Smith's RV-10, there was a tiny leak from the vent tube on the pilot's side. The other end of the vent tube by the filler cap was above the fuel level. When I researched it I found others that had this problem with quick build wings. It is due to the flare nut on the inside of the tank not being tightened enough so that the gas leaks into the vent tube when that union is under gas level. I found a picture of the tool that others have used to tighten that nut reaching in through the hole after removing the float sending unit. I made my own tool and tightened the nut which stopped it some but not all. Further research showed that someone cut an access hole in the spar side of the tank, found and fixed a bad flare. I was resigned to eventually doing something similar when an EAA member, Tim Crouch, remembered how his RV-9 vent tube was designed - which led to my solution.

I drilled two holes in the fuselage wall in the wing root at the vent tube location, had a 26" Teflon hose with fittings made, connected one end to the tank fitting, ran the hose through the first hole in the fuselage, ran the hose up to the bottom of the door sill between the fuse wall and the interior kick plate, and back down through the second hole with a 90 elbow which connects to the original vent tube down through the grommet through the lower wing root faring.

Done. This keeps any leakage at the faulty flare inside the tank from going above the fuel level since the loop at the top of the interior hose is above the fuel level in the tank. As fuel is used, air is easily drawn past the faulty flare until the fuel level is below that flair which then drips back into the tank.

Done! I'll attach pictures.
Later, Lew

 

[PaulvS]

Upon first fuel fill on Grant Smith's RV-10, there was a tiny leak from the vent tube on the pilot's side. The other end of the vent tube by the filler cap was above the fuel level. When I researched it I found others that had this problem with quick build wings. It is due to the flare nut on the inside of the tank not being tightened enough so that the gas leaks into the vent tube when that union is under gas level. I found a picture of the tool that others have used to tighten that nut reaching in through the hole after removing the float sending unit. I made my own tool and tightened the nut which stopped it some but not all. Further research showed that someone cut an access hole in the spar side of the tank, found and fixed a bad flare. I was resigned to eventually doing something similar when an EAA member, Tim Crouch, remembered how his RV-9 vent tube was designed - which led to my solution.

I drilled two holes in the fuselage wall in the wing root at the vent tube location, had a 26" Teflon hose with fittings made, connected one end to the tank fitting, ran the hose through the first hole in the fuselage, ran the hose up to the bottom of the door sill between the fuse wall and the interior kick plate, and back down through the second hole with a 90 elbow which connects to the original vent tube down through the grommet through the lower wing root faring.

Done. This keeps any leakage at the faulty flare inside the tank from going above the fuel level since the loop at the top of the interior hose is above the fuel level in the tank. As fuel is used, air is easily drawn past the faulty flare until the fuel level is below that flair which then drips back into the tank.

Done! I'll attach pictures.
Later, Lew

What about the reverse direction, i.e. when the contents of the fuel tank expands, is the intention for the fuel that has leaked into the vent line to be blown out onto the tarmac?

 

[Bill Boyd]

I tried the same principle, and Paul's concern is well-founded.

In my case the leak is on one side only, and only present when the tank bulkhead vent fitting is submerged in fuel. A peek with the boroscope through the tank sump drain hole shows that a large amount of pro seal has been slathered over the B-nut, making it likely impossible to get a wrench on it through the fuel level sender hole as has been popularized here. It occurred to me that the steady drip of fuel when the tank is more than half full could be mitigated by extending the vent tube upwards through the top of the wing root fairing instead of downward per the plans. I figured if it didn't work I could undo it with just one extra top hole as collateral damage.

It looks ugly, but it does indeed work. The problem is that the P-trap this creates fills with fuel that gets blown out onto the wing upper surface when the tank warms. Now I have mogas stains all over my wing walk area and the stink that goes with it. I will probably revert back to the original routing of the vent tube and hope the stains scrub off.

One way to mitigate this shortcoming might be to include a vertical section of tubing fat enough to defeat the gasoline's surface tension and let the air bypass the fuel as it burps upwards out the end. If that doesn't work, it looks like I will be joining the ranks of 10 drivers who have had to remove and cut open their tanks to fix this.

Zoom in to see the stains. Bending the tube upwards a bit from the original profile probably increases drag but does not stop the fuel from spurting out when the tank contents expand. I might try the expansion chamber in the wing root that I mentioned above. I really don't want to pull that tank.

 

[flightlogic]

I find this occurrence odd and unfortunate. Having chased the same vent problem on my 9A, and reading others comments on how tricky it is, I WOULD HAVE THOUGHT ALL THE future builds WOULD BE TIGHTENED, by the time the 10 came into existence.
I feel bad for builders who have to repair this. It should be so easy to have on a builders checklist... "those who go before us" saves headaches...
Makes me think of the loose bolts on the Boeing Max. Not rocket science to put a torque wrench on a multi-million dollar build. But I digress.
Glad you got your vent issue resolved.

 

[Lew Gallagher]

What about the reverse direction, i.e. when the contents of the fuel tank expands, is the intention for the fuel that has leaked into the vent line to be blown out onto the tarmac?

Good point, Paul. Our drip is only tiny at totally full (barely 1/8” in a paint mixing cup over a week - just enough to be annoying on the hangar floor) and none after 8 gallons are used. I’m thinking (hoping) that this loop is high enough to tolerate that amount during expansion/contraction or as was stated “burp” it. We’ll be able to give you real time results soon since first flight and refilling the tank will happen soon.

 

[Bill Boyd]

I find this occurrence odd and unfortunate. Having chased the same vent problem on my 9A, and reading others comments on how tricky it is, I WOULD HAVE THOUGHT ALL THE future builds WOULD BE TIGHTENED, by the time the 10 came into existence.
I feel bad for builders who have to repair this. It should be so easy to have on a builders checklist... "those who go before us" saves headaches...
Makes me think of the loose bolts on the Boeing Max. Not rocket science to put a torque wrench on a multi-million dollar build. But I digress.
Glad you got your vent issue resolved.

I do wish the builder I purchased the partially completed kit from had attended to this before he sealed up the tanks. It was his second -10 build. I had no way to know, and like many found out the hard way.

 

[Lew Gallagher]

Good point, Paul. Our drip is only tiny at totally full (barely 1/8” in a paint mixing cup over a week - just enough to be annoying on the hangar floor) and none after 8 gallons are used. I’m thinking (hoping) that this loop is high enough to tolerate that amount during expansion/contraction or as was stated “burp” it. We’ll be able to give you real time results soon since first flight and refilling the tank will happen soon.

Here's a follow up. Paul was correct, when the tank in the hangar warmed up enough, the increased air pressure "burped" the fuel that was in the loop out the vent to the hangar floor - not much, but still not right. I'll include a picture of my next experiment. I just today received tiny Teflon tubing that is 1mm id and 2mm od. I'm planning on inserting it into the 1/4" vent tube from the end at the filler cap past the loop I put in the fuselage. Theoretically this should allow the air pressure to gradually bleed past the fuel in the bottom of the loop without blocking the vent tube and without blowing the fuel in the loop out. I welcome feedback on thoughts of cautions, etc. and I'll continue to post results.
Later, - Lew

 

[Lew Gallagher]

Here's a follow up. Paul was correct, when the tank in the hangar warmed up enough, the increased air pressure "burped" the fuel that was in the loop out the vent to the hangar floor - not much, but still not right. I'll include a picture of my next experiment. I just today received tiny Teflon tubing that is 1mm id and 2mm od. I'm planning on inserting it into the 1/4" vent tube from the end at the filler cap past the loop I put in the fuselage. Theoretically this should allow the air pressure to gradually bleed past the fuel in the bottom of the loop without blocking the vent tube and without blowing the fuel in the loop out. I welcome feedback on thoughts of cautions, etc. and I'll continue to post results.
Later, - LewView attachment 55372

Here are pictures of the "tube within a tube" experiment. The plane is currently waiting for the pilot's back to cooperate for first flight, and we haven't had enough temperature change to see if it passes the sitting-in-the hangar test. Picture of the tube in the fill opening, with the tube safety wired and tucked under the lip, and the exit at bottom of the wing root fairing. We'll probably put an angle facing forward on the aluminum exit tube with a critter screen on it after we find out how it works in real time.
Later, - Lew

 

[fixnflyguy]

I am the guy who did the surgery on the RV-10 tank and found the bad flare that leaked slightly even when tight. I would have re-flared it , but there was no way to get the vent line pulled in any further. I reassembled the flare / B-nut with Proseal, then encapsulated everything . Its a hard pill to swallow cutting open a new quickbuild tank, but at least this -10 hadn't been painted yet and the leak surfaced during phase 1.

 

[Bill Boyd]

Lew, you've got me wondering about one more way to fix my vent leak - cannulate the existing vent tube with a Teflon tube such as yours (except the largest i.d. tube that will pass through the aluminum tubing) and seal the inner tubing to the outer tubing with a fuel-proof sealant where it exits the wing underneath. The fix should work in theory - as long as the tank can breathe through the narrower lumen created. I believe flow resistance is inversely proportional to the tubing radius to the 4th power, so it's a real concern.

Anybody have any idea what the real-world numbers would look like for vent air being pulled through this tube to replace fuel burn, and what kind of negative pressure the resistance to flow would impose on the tank structure?

 

[Lew Gallagher]

OK, real world results: THE TUBE WITHIN A TUBE WORKS PERFECTLY !!! I've waited until I could test it with all varieties of temperature change, full tanks, etc. Not a drop. Totally looks and functions as it should. I would think the other RV's that have the loop designed into the build could use this tiny tube insert if you are tired of the gas being burped out in the hangar.
I did bevel the end of the vent tubes because in one of the initial flights, the pilot noticed some cavitation in the good tank. The added pressure from the bevel fixed that.
Later, - Lew

 

[NTex]

You could also try this

A.N. Male to A.N. Male One-Way Check Valve - Red - Aluminum

A.N. Male to A.N. Male One-Way Check Valve - Red - Aluminum

www.anplumbing.com