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How to compromise a good fuel tank

D

David-aviator

Well Known Member
I submit this boo-boo message and perhaps it won't happen to you.

This is mud dabber season, they are here or will be here soon and will clog a fuel vent line or pitot system. That's why we use screens or tube covers.

I've been using simple yellow plastic quarter inch tube caps for some time. Drill a small hole in the leading edge and they work fine. This year I forgot to make that small hole none of the caps.

Three days ago the OAT rose to the mid 80's for the first time this year. I was next to the 8 and notice fuel on the floor under the left drain valve, thinking it was leaking. Wrong. Fuel, a nice stream was flowing from the aft end of the tank. And then I saw the yellow cap on the vent line - darn!!

Opened the fuel cap and there was a rush of air and the leak stopped immediately. But I knew the tank had been compromised. It took 48 hours for the seepage to start, a small blue ring of fuel stain appeared today just outside the first rib along the aft baffle rivet line. Darn, darn, darn!!

Already drained the fuel, began tank removal and ordered a couple tubes of sealant from Vans. It will be repaired but what a dumb thing to do - close the vent line with no weep hole.

PS By way of explanation, if you use the plan vent screens this could not happen. I use a 90° tube intake to add a little head of pressure in the tank to facilitate fuel feed, hence the need to cap the tube openings.
 
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Vent

After I put the plane in the hangar I release both caps and leave them in that position until my next preflight . The design of the fuel vent can allow it to build more than .5 psi , leaving the caps loose guarantees no pressue build up .
 
I just took several inches of safety wire, doubled it over, and stuffed it up into the vents. What little protruded got bent 90º to prevent any further movement and there they have been for ~420hrs of flight.
 
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same for me

g zero said:
After I put the plane in the hangar I release both caps and leave them in that position until my next preflight . The design of the fuel vent can allow it to build more than .5 psi , leaving the caps loose guarantees no pressue build up .
Click to expand...

I do the same.
 
What's the load limit?

I hope we don't have to worry about 0.5 psi build-up being bad for the tanks.

Gasoline has a specific gravity of 0.75; water has a column pressure of 43 psi/100 ft. Pull 6G's, and a tank with 12" fuel depth (dihedral counts, here) sees 1.93 psi on the bottom. Makes one wonder how 0.5 psi can be even close to catastrophic.

Pretty sure I did that math correctly.

-Stormy
 
Vent

.5 psi and fuel vapor might be the cause of rivet blisters , cheap insurance by leaving the caps released . ( might be BS ....but what do I have to loose )
 
ChiefPilot said:
I just took several inches of safety wire, doubled it over, and stuffed it up into the vents. What little protruded got bent 90? to prevent any further movement and there they have been for ~420hrs of flight.
Click to expand...

Hmmm... this sounds like a good solution. Does it introduce any additional potential failure modes? .025 wire?
- Roger
 
safety wire

I did the same on my -6, 5 years and 400 hours ago. Can't remember who posted the idea, but it was one of our more experienced and knowledgeable members here. No issues whatsoever here in the bug capital of the world. Easy enough to remove when/if needed.
 
It's what I did also

Both my tank vents and my pitot have a squiggly length of safety wire stuck into them, with a little hook hanging out for visual verification and removal if necessary.

I have also done this to the inlet of every air tool in my hangar (and regretted it when I didn't). Very effective way to keep the mud daubers from destroying expensive toys.

Wish I could find an equally effective remedy for English starlings. Shotgun shells are an expensive and time-consuming answer to that infestation.

-Stormy
 
I really like the safety wire idea, thanks!

My attempt at using RTV to put screen on my air vents failed miserably.

Any chance you could post some pictures of your vents with the safety wire installed?

-Dan
 
Seems like the sealant application in the are of the leak was probably maginal to begin with. I have seen many tanks with plugged vents that didn't spring leaks at seams or fasteners.
 
Vent protection

I've always used long pipe cleaners. I turn a nice round loop into one end and then double the other end over and stick it into the vent openings. The round loop is very easy to see in preflight inspections and the fuzziness of the pipe cleaner seals the bugs out. It will allow the vents to work if you forget to take the pipe cleaners out too. I usually buy RED pipe cleaners.....very easy to see.
 
What is the simplest way to cover the vents to prevent insects getting in? Mine are cut at a 45 facing forward as per the plans. The builder did not see a need for screens, but I do.
 
I did mine with little discs of aluminum window screen, prosealed over the end of the vent tubes. Pain in the backside to do, but I've never had to think about them again.

Charlie
 
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