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Plugged fuel vent

videobobk

videobobk

Well Known Member
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I have about 15 hours on my 9A and came across one of those "gotcha's" today. I have been making mostly short flights and have yet to fill my tanks above 12 gal. each. Most flying has been done on the left tank, for whatever reason.

Yesterday on a 1.9 hr flight, I noticed my fuel pressure was a little low, off maybe 1 psi while on the right tank. I kicked on the electrical pump and the pressure came up to normal, but I decided to switch to the left and head back home. Today I thought I'd check everything out. It didn't take long to know I had a problem, as when I opened the right tank cap, pressure escaped! It was warm and the pressure was no doubt due to vapor pressure, but there was no venting.

I started checking lines and isolated the problem as the line inside the tank. A mirror check of the inside end revealed no pro-seal, which was my first thought. Then I pulled the coupling next to the fuse. It was plugged by a bug of some sort. It had to have been plugged between finishing the wings and mounting them. I "vacuumed" out the line and flushed it with carb cleaner from the tank end toward the inboard end.

I don't know how I missed this when I mounted the wings, but on my next build, I will cap the lines better. Something like that could really spoil your day. You might consider adding this to your pre-final assembly checklist.

Bob Kelly
 
Here on the humid gulf coast, we have what's called Mud Dobbers. They are like wasps but pick up mud from ditches, etc. and make big nests for their eggs. They are well known for clogging up anything you can imagine.

One of the first things I did during construction is buy a pitot tube cover and make covers for my tank vents. I used auto fuel hose and those "Remove Before Flight" key chains. If the plane is going to be sitting more than an hour, I put on the covers.

It would be a really bad day if one of your tank vents was blocked and wasn't sucked down enough until right on take-off. Don't even want to try that.

Karl
 
Bob:
It wasn't positive pressure, but rather negative pressure or vacuum in your tank. If the vent is well plugged and the fuel cap is a tight seal, the draw from the engine driven fuel pump is enough to collapse the tank. Really disconcerting to look out the window and see your tank crushed. Generally the fuel tank is a throw-away at this point.
 
Change in temp

terrykohler said:
Bob:
It wasn't positive pressure, but rather negative pressure or vacuum in your tank. If the vent is well plugged and the fuel cap is a tight seal, the draw from the engine driven fuel pump is enough to collapse the tank. Really disconcerting to look out the window and see your tank crushed. Generally the fuel tank is a throw-away at this point.
Click to expand...

Actually, it was positive pressure. It almost blew the cap out! I'm sure there was a negative pressure when I put it away, but over a day later when I opened the cap it was quite a bit warmer (mid-afternoon with sun shining on the tank vs. morning) and I would think the vapor pressure had more than made up for the draw down from the day before. I'm glad I noticed the drop in fuel pressure or I think I might have collapsed the tank in flight. Not a happy thought. I sure know that my tank is air tight, though!

Bob
 
Fuel Vent

On my 8, I cut some aluminum porch screen about a half inch wide just long enough to roll up to fit very tightly and stuffed them into the tank vents where they exit the fuse. Excludes all but the tiniest critters.

Allows enough air flow into the tank. Haven't had any problems in 9 months now.
 
Dirt dobbers too....

Our pitot tube laid on a shelf for about five weeks while we painted the airplane. As I took it to put back on the airplane, I tried blowing through it and it was plugged. Yup....dirt dobber nest. Vent covers are a very good idea.

A friend totalled a Callair in the seventies due to dirt dobber plugged fuel vent and the subsequent engine stoppage while he was spraying cotton.

Pierre
 
In west Texas where I live, we see as many problems with leaf-rollers as we do with mud-daubers. Most of the plugged lines I encounter are actually due to leaf-rollers. They are a flying insect that looks like a miniature yellowjacket wasp, they cut and roll pieces of tree leaf into a tight space to make a nest and it quite effectively plugs tubing. Out here you have to blow clear ANY piece of tubing that has been left laying out for any period of time, to be certain.
 
I noticed the dried mud blocked tubing problem during construction as well. Good assembly practice is to be sure all tubing is cleared of all debris before installation. Then, keep those lines blocked somehow if they are to remain unconnected for any length of time. I also bought some fine wire mesh screen to cover my vent lines and check them at each preflight.
 
vent screen source

Visit your local smoke shop for 3/4" dia brass screens. Proseal the whole thing on the angled face on the end of the vent tube. Trim off the excess after the proseal cures.
Back in the day, we called those places head shops.

Steve
 
T-connecton Left-to-Right?

I was wondering wheather it would be a good idea to connect the left and right vents together, under the instrument panel, that way if one gets blocked (at the fuselage exit), it will still vent through the other side. But.... how would you know if one side is actually blocked...... hmm?

Still, this would bring on extra safety, for only a tiny weight penalty, (3 ft of tube + 2 T's), right?

Regards, PilotTonny
 
Cessna did it

Pilottonny said:
I was wondering wheather it would be a good idea to connect the left and right vents together, under the instrument panel, that way if one gets blocked (at the fuselage exit), it will still vent through the other side. But.... how would you know if one side is actually blocked...... hmm?

Still, this would bring on extra safety, for only a tiny weight penalty, (3 ft of tube + 2 T's), right?

Regards, PilotTonny
Click to expand...

Cessna did it on the Agwagon and Agtruck series in the seventies and eighties. You could see the tank interconnect hoses in the floor since all they had were footrests above the belly skins.

Regards,
 
Learn and live

One thing I did get out of this experience is the added benefit of the fuel pressure gauge. While it wouldn't give much advance warning if the tank was full (I started with something like 12 gal in that tank,) it did show that there was a problem. If it had been full, the pressure would have dropped much earlier and more significantly, I'm sure. I think I'll pay more attention to fuel pressure in the future, not just for a few moments after I switch tanks. By the time I would have heard "Check fuel pressure" from my AF 2500 engine monitor, I could have at the least done damage to the tank. I may add a note to my checklist, just to remind myself. BTW, I do have screens on the vents.

Bob Kelly
 
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