Fuel tank back baffle leaks

With nearly 12 years and 1370 hours on my RV-4 I found blue fuel stains at the inboard end of my left fuel tank, extending out of the back baffle area. I pulled the tank and found the fuel stains originating at the bottom of the back baffle some 6 inches from the outboard end. It is hard to tell exactly where the leak begins. Some of the staining seems to be from the edges of the access plates I cut into the back baffle for resealing back in 2000 when I was readying the plane for first flight.

What annoys me is that the 12-year-old sealant (Pro-Seal at the time) is absolutely gooey to the touch all along the back baffle and the inboard fuel tank rib where the access plate is screwed on. I can just rub it right off with a finger tip. A search of this forum reveals this is a common problem. Apparently fuel leaking outside the tank in the presence of oxygen softens the sealant. I did mix the sealant by weight using a scale, so I don't suspect an incorrect mixing ratio.

I guess I will have to remove my back baffle access plates and confirm that the sealant is still firmly cured inside the tank. I will do the balloon pressure and soapy water trick first to pinpoint the exact leaking areas. I'll report back on what I find. I really don't want to pull the whole back baffle.

My initial inspection supports the hypothesis of fuel leaks softening the sealant outside the tank. I performed the balloon and soapy water test. No bubbles anywhere except the fuel cap pivot, which I then taped off.

This shot of the left tank spar shows staining all along the lower flange:


This shot of the left tank back baffle shows where the sealant was soft. This is not the result of solvent, folks. I just wiped it off with the soapy water and a soft plastic scraper:


This closeup of the baffle shows my two access plates and the suspect rib rivets. Where you see the sealant still in place it is still firmly cured and did not wipe off with a cloth:


Investigating the back baffle showed that my two back baffle access panels are the most likely culprits. Staining and soft sealant around areas of both panels. There may also be weeping through the lower rivets of the fuel tank ribs at the back baffle. Staining and soft sealant also is evident at the inboard rib upper and lower corners at the back baffle.

What is encouraging is that the soft sealant is localized only in these suspected leak areas. Where there is no fuel staining the sealant is still firmly cured. I therefore think it unlikely I that will have to pull the back baffle. My plan is to open my access panels in the back baffle, clean out any soft sealant (hoping there is none!), and spread more sealant along the rib rivets at the back baffle and reseal the access panels. I will also pull the inboard rib access panel and reseal the corners of the inboard rib and reseal the access panel. I don't think this will be a terribly difficult job.

Tweetr said:

WApparently fuel leaking outside the tank in the presence of oxygen softens the sealant.

Click to expand...

I don't understand this theory. I admit I haven't searched the archives and read all the history on the oxygen theory but isn't there oxygen inside the tank above the fuel level? Why doesn't the pro seal soften at the top of the tank?

Just curious.

DaAV8R said:

I don't understand this theory. I admit I haven't searched the archives and read all the history on the oxygen theory but isn't there oxygen inside the tank above the fuel level? Why doesn't the pro seal soften at the top of the tank?

Just curious.

Click to expand...

I am curious about the chemistry of this also. The fact is that a little fuel on the outside of the tank turns the sealant into something approximately the consistency of peanut butter. It happened to me.

I would think there would be some oxygen inside the tank but at a lower level. I expect a lot of what is in the tank above the liquid is vaporized fuel.

DaAV8R said:

I don't understand this theory. I admit I haven't searched the archives and read all the history on the oxygen theory but isn't there oxygen inside the tank above the fuel level? Why doesn't the pro seal soften at the top of the tank?

Just curious.

Click to expand...

Here is a detailed thread on the subject in which they describe the consistency of the softened sealant as "peanut butter". That is an accurate description for the way my sealant feels in the areas where I wiped it off. Just like creamy peanut butter spread on the affected areas. Not at all like the cured sealant elsewhere. All the sealant on the back baffle is the same batch and mix too, so a bad mix seems unlikely. Here is the thread: http://www.vansairforce.com/community/showthread.php?t=83302&highlight=fuel+tank+leaks

I will report back after I get inside the tank. I hope to find all the sealant firm inside, but we shall see . . .

I guess the question is.. How far does the soft Proseal extend. If the stuff inside the tank is solid then a repair/reinforcement with additional proseal seems in order.

If the proseal is suspect then building a new tank or dismantling the exisiting tank and re-assembing would be the path forward.

Hope its good news.

Frank

I just got the inboard access plate off. I am happy to report that:

1. The access plate was stuck on there like grim death. It took lots of razor blades underneath and careful prying to get the plate off the inboard rib. Thereafter it took lots of elbow grease with scrapers and MEK to clean the very firmly cured sealant off of the flange. I still haven't gotten it all off the mating area on the inside of the plate!

2. All the sealant inside the inboard fuel bay is unquestionably firmly cured.

Before I pulled the plate I took one more run with the balloon and soapy water. By placing the tank nose down I was able to find one bubbling pop rivet in the back baffle rib second inboard (right side of this picture), 4 rivets up:
You can just see blue staining around the rivet in question. The pattern of the blue stain before I cleaned the back baffle makes it possible that the fuel pooled both directions along the bottom flange from this leak. Fingers crossed!

Next up are the two access plates in the back baffle. I'll also take the opportunity to install the anti-rotation bracket and safety the fuel pickup nut while I'm in there. For peace of mind I plan also to pull the right tank (that shows no sign of fuel stains) and open the inboard access plate for inspection and installation of bracket and safety wire on the flop tube.

I got the inboard back baffle access plate off today. It too was stuck on incredibly firmly by completely set sealant. I may have bent the cover plate too much in prying it off to get it back on! May have to fabricate a new cover.

Inside the tank all sealant, including that on the back baffle, is fully cured and firm. I can see where the sealant I added back in 2000 to the rib flange where I found the leak, though firmly cured, has a gap that apparently allowed fuel to work its way through the pop rivet hole and soften the sealant outside the tank. That should be easy to fix by working more sealant into the gap. There may also be a slow leak at the bottom of that access panel where the softened sealant worked its way almost to the bottom rivet. Also easy to re-seal.

Given my findings I don't think it necessary to remove the outboard cover. I'll reseal where I know it was leaking and pressure test. If that test fails I'll pull the outboard cover too.

My tank passed the pressure test, but for peace of mind I decided to pull the outboard access cover and reseal the back baffle in there too. I already have the tank drained and off and the can of pooky on hand, so it only took a couple of hours for the whole job.

Inspection revealed no problem areas in the sealant, but I filleted the back baffle and resealed the outboard cover. Another pressure test in a day or two should tell the story.

I had the same thing happen, rear baffle leak,,,I read on a thread here somewhere that when the leaking fuel evaporates, the chemicals remaining in the blue dye react on the sealer softening it,,it doesn't make sense that fuel tank sealer won't hold up to the chemicals in fuel dye when the dye is concentrated, but it aparently does,,,

My wings are on order, and between this issue and the problems I've read about paint blisters where proseal was used, I'm sure not feeling very good about using it. Is there nothing else that works better to seal fuel tanks without these problems?