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-   -   leaking fuel tank rivet (https://vansairforce.net/community/showthread.php?t=79991)

BillSchlatterer 12-14-2011 07:57 PM

Quote:

Originally Posted by Rick6a (Post 605821)
Verified with a manometer or toy balloon, I'm guessing you successfully pressure tested your fuel tanks before installing them.

Yup, did the balloon test with helium balloons so there was no seepage. Balloons stayed up 36 hours and I called it good! Turned out not quite so good :( and glad I waited for paint!

Bill S
7a Arkansas

jcbarker 12-14-2011 09:44 PM

Thanks all for your input. I'm going to try the loctite 209 and vacuum. Hope it works because it's going into the paint shop on Jan 20.

Rick6a 12-15-2011 07:55 AM

My thoughts.....
 
Quote:

Originally Posted by Mike S (Post 605964)
I have one weeping rivet in my QB tanks in the same area...Thoughts out there???

Mike,

You have a clear idea of the dynamics involved. For the benefit of those builders who may
have some difficulty understanding the forces at work here, I will attempt to outline the 3
"perfect storm" conditions that must occur at the same time to cause a rivet to "weep."

1. The fuel within the tank enters a breach or more technically, a leak path through the
fillet dam along the rear baffle.

2. Continuing its journey, the fuel then finds an open void hidden somewhere in the
skin/baffle faying surface leading to that weeping rivet hole location.

3. The fuel (or its fumes), now pooled in that open void hidden from view in the faying
surface, then moves on to take advantage of the last bastion of proseal defense which
is the lack of sufficient sealant that should be underneath the manufactured head
of the rivet. As a consequence, the fuel pushes its way through the gap, finally escaping
and leaving its tell-tale blue dye on the surface.

All three points of failure must occur at the same time to cause a particular rivet hole
location to "weep." If any one of the three failure points had remained intact, the chain
would have been broken and the rivet would not leak. Just fix any 1 of the 3 failure points
and the problem goes away....technically, that is. I say technically because few people are
actually going to completely remove the back baffle just to get at and repair the mating
faying surfaces as described in point 2. Cutting a hole or a series of holes in the rear
baffle to gain access to the inside of the fuel tank so the fillet dam can be repaired is a far
more practical solution for addressing gross leaks.



Now in your situation Mike, you do have options. The easiest fix involves repairing the
sealant boundary underneath the manufactured head of the weeping rivet. LocTite comes
to mind. Its success rate, however, appears to be mixed at best. If you are willing to pull
the tank, replacing that solid rivet with another solid rivet makes sense. If you'd rather
not pull the tank just to fix one single weeping rivet, replacing its solid rivet with a blind
fastener is an option. There is a lot of anecdotal evidence out there suggesting that
replacing a random weeping rivet with a blind Cherry rivet works well enough.

Faced with your situation, I would try your technique of using a syringe to try to squirt a
bit of proseal into the faying surface before replacing the rivet. Seems like a perfectly
reasonable idea and it can't hurt a darn thing even though it may or may not work. But
after I did that syringe thing and prior to reinserting a new rivet into the hole, I would
make especially sure that the replacement rivet and the countersink is liberally coated
in fresh wet sealer.

If only that single point of failure (3) remains intact because of your repair efforts, that
rivet location will no longer weep because you have effectively interrupted the leak path
that continues to originate from inside the fuel tank.

Mike S 12-15-2011 09:44 AM

Quote:

Originally Posted by Rick6a (Post 606146)




Faced with your situation, I would try your technique of using a syringe to try to squirt a bit of proseal into the
faying surface before replacing the rivet. Seems like a perfectly reasonable idea and it can't hurt a darn thing
even though it may or may not work.

Thanks Rick, what you show in the illustration is exactly what I was trying to describe.

My leak is on the bottom of the wing, so the path is always under fuel, not vapor.

As I see it, the "cant hurt a darn thing even though it may or may not work" is the key to my next step.

My way of looking at things is to try the easiest and/or cheapest first. And move up the ladder if the first fix doesnt work.

As I see it, blind rivet and proseal like I described with a syringe is first step, and if it fails, pull the tank, and cut an access hatch. I do not think the pull the tank, and solid rivet is worth it if the blind rivet wont do the job.

Thanks for your input, it is greatly appreciated.

jcbarker 12-20-2011 08:52 PM

An update:
It turns out that the rivet that was weeping WAS along the baffle flange. My first approach was to drill it out and use a Cherry Max blind rivet. Well... when I drilled out the rivet my drill hit the spar carry thru structure and deflected. ****! So off came the tank so I could take it home and do it right. Drilled it out. Goobered it up with Pro Seal and replaced it with an 1/8 rivet that I shaved the head down. Then I covered the shop head with more Pro Seal. I also applied Pro Seal to the edge of the flange.
Next step is to put gas back in the tank and check for leaks.
I'll keep you posted.

AX-O 12-20-2011 09:48 PM

I just saw this post. For others with the same problem in the future, try this (worked on 2 different airplanes):

-mix proseal with MEK or Acetone to make it runny. No more than 50/50.
-use a q-tip and dip it in the container.
-dab the proseal mixture over the rivet in question.
-introduce a vacuum into the tank. Be careful with this step. Just barely enough to start sucking in the mixture.
-release the vacuum.
-repeat the process 2 more times. Be sure not to suck any air via the rivet.
-clean the outside of the rivet.
-let the proseal mixture cure for a few days. I waited a week before pressure testing them.

Worked well.

PCHunt 12-20-2011 10:46 PM

Vacuum
 
Axel, how did you introduce a small vacuum, and how did you make sure you didn't pull too much vacuum?

AX-O 12-20-2011 11:46 PM

This will sound bad and unhealthy. (with an empty tank) I place a hose over the fuel Fitting. Then placed a cap on the overflow fitting. After that i sucked the air out of the tank using my mouth. Couple of puffs then put my finger over the end of the hose and slowly watch the proseal mixture disapper. May not be the best way to do it but i was concerned to put too much vacuum in the tank.

avee8tor13 12-23-2011 04:53 PM

How I repaired my weeping rivet.
 
I let the fuel level get below the weeping rivet, then I used Loctite, the stuff used for installing liners in engine blocks (green I think) and pushed it into the weeping rivet, after cleaning it. I have refueled the tank 5-6 times and it has been over three months since using the loctite. Still no weeping.

roadrunner20 08-05-2012 04:06 PM

I developed the same problem yesterday on a rivet close to the leading edge.

Once I get the fuel level below the rivet, instead of the vacuum, I was thinking of fabricating a syringe with a gasket & push the green loctite into the rivet area.

BTW, My plane's been flying 6 years and this developed yesterday.
Strange how these things crop up out of nowhere.

I'll provide an update when available.


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