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Fuel Vent

cropduster

Registered User
Lake Providence, LA
A friend just bought a nice '61 supercub and is having fuel vent problems. When he opens fuel cap it releases pressure. Sounds to me like a clogged vent. I've been busy flying and haven't had time to go look at it. Can some of you tell me where the vent is? Is it below the wing? Or????

is this a common problem?

Thanks,
barryb
 
The fuel caps are vented, dont mess with this just get new caps. I've seen the tank sucked down to where the cap was below the tank cover. There is a very good chance of engine failure from this. Also read the Ad on fuel caps, just search Ad on the FAA sight.
 
I wouldn't be surprised if somebody has put automotive caps on it. Most auto caps allow inflow but not outflow. In this case it'll run okay, but you could rupture a tank on a hot day. When I bought mine several years ago, somebody had put generic auto parts store caps on it. Fortunately they were old school and vented both ways. Get rid of those things and get the ram-air vented caps from Atlee Dodge or Cub Crafters.
 
The fuel caps are vented, dont mess with this just get new caps. I've seen the tank sucked down to where the cap was below the tank cover. There is a very good chance of engine failure from this. Also read the Ad on fuel caps, just search Ad on the FAA sight.

I've seen the opposite problem. The plane was sitting out in the sun; the tank expanded to the point it bent the bottom ribs.
 
Unless you really know what you are doing. You need a non- ventd cap with a little "pffft" on top to avoid the negative pressure on the top of the wing. It is not trivial to convert a vented cap to a proper aircraft cap, but that is what Piper did back in the old days.
 
Here is my hangar neighbor after taking off with a clogged fuel vent. No survivors.

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AD 78-10-03

http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgad.nsf/0/630BE8A9A4B719538625699D004F69E2?OpenDocument&Highlight=ad%2078-10-03
 
Here's the cap on my PA12 with 150 hp. I do wonder what happens to the bugs and the plastic is how I store the plane in the hangar.
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Why do you need ram vents on a 150hp airplane?
I have no idea. This is the way it was rebuilt with Air Energy tanks. When I did the Don Snyder 150 STC to my other PA12, 30 years ago, I had to put copper tubes in the original caps by boring a hole and soldering. They basically did the same thing as these store bought jobs.
 
To get out of the low pressurearea above the wing. Ran into this on my J-3 with a brand new wing tank last century - a vented cap would not allow flow to start under some conditions. The ram tube cured it. Those caps pictured above are a hundred dollars each, and are the equivalent of an auto non-vented cap with a ram tube. A 1/4" copper tube works fine.
 
Here's the cap on my PA12 with 150 hp. I do wonder what happens to the bugs and the plastic is how I store the plane in the hangar.

View attachment 24095
Marty, Try a pair of these instead of the plastic bag. They will eliminate the risk of "forgetting" to remove the plastic.
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https://www.aircraftspruce.com/catalog/inpages/ptubeprotector.php?clickkey=2017487

I would bend the tube down a little to align it with the upward air flow over the leading edge and for any stray drops of water to fall out rather than in.
 

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If you have the Atlee caps (which are vented) and you fill your tanks with auto gas to the point where the gasket in the cap gets exposed to fuel, the gasket will swell to point that it shuts off the vent in the cap. The end result is described in post #3.

Jerry
 
Pete, Marty has the plastic bags on his caps to keep the moisture out of his corn gas. Marty keep on eye on those caps while flying in the winter. CC Joe and I were flying one February day and one of his caps like yours frosted closed while flying.

Glenn
 
If you have the Atlee caps (which are vented) and you fill your tanks with auto gas to the point where the gasket in the cap gets exposed to fuel, the gasket will swell to point that it shuts off the vent in the cap. The end result is described in post #3.
I have Atlee caps. I use auto gas almost exclusively, and have had no trouble.
 
I have Atlee caps. I use auto gas almost exclusively, and have had no trouble.

My friend was in the same boat until it almost cost him his airplane. I think that the potential for this to happen is rare and takes a particular set of circumstances to create a very predictable result. Here are the circumstances.

We were both heading to the Brooks Range from Palmer so my friend and I both topped our tanks completely full for the flight. He has stock tanks (requirement #1) and filled them full with auto gas (requirement #2). The tank that collapsed had an Atlee cap (requirement #3). He usually burns his tanks down in a balanced way, switching between tanks periodically. This trip he decided to run the tank without the Atlee cap totally dry (setting up the ensuing engine out) before switching to the tank with the Atlee cap (requirement #4????). This left the Atlee cap immersed in auto fuel (not fumes) for an extended time causing it to swell enough to close the vent in the cap. His engine out took a bit of time (30-45 minutes or so). The symptoms mimicked carb ice at first, since he could counter the slow loss of power with the application of carb heat. This was a loosing proposition since the tank was being slowly crushed by increasing head pressure. He made it across the flats to Fairbanks and the engine died on final. This was all good.

The end result was quite amazing. The tanks was wadded up under the tank cover. As I was standing up on his airstreak with this perplexed look on my face, another pilot walked by and said "Let me guess - you are running auto gas with an Atlee cap". We were able to save the trip by reflating the tank without it cracking anywhere. My friend mentioned this to Atlee himself and he was very Atlee when said his cap is not for use with auto gas.

The question marks on requirement #4 are because I'm not sure how long the gasket has to soak in auto gas to cause this situation. Collapsing the tank required a fairly unique series of events and the engine out happened because he had no other tank with fuel.

In the world of logic if a stock cub tank crushes to point of killing your engine is there any other culprit in the line up beside the gas cap and it's vent?? The incident is also not unique and I was glad that other pilot passed by when he did because he helped us with reflating the tank and getting on our way.

I did hesitate in my posting of this because of the long winded nature of this event but think it's worth mentioning. You might try soaking your cap in auto gas for an extended period of time to see what happens.

Jerry
 
I'm glad you posted since it proves that there are some gasoline molecules that cause problems, not just my favorite fuel that usually gets the blame.
 
Thank you for sharing the details. Do you happen to know whether that particular load of gas was tested for ethanol?

I have the Atlee tanks and caps, and have filled them completely with mogas a few times. I wonder if the gasket formulation might have changed over time? I would imagine mogas formulations can vary. In any case your suggestion to test the gaskets sure sounds like a good idea. Thanks again -
 
Jerry I have heard similar incidents. I choose not to run autogas partly due to that & other rubber stuff that goes to $hit from running that stuff. Really saving money then, huh. To each their own. If I did run it I would be anal about checking cap gasket.

Sent from my SM-G920V using Tapatalk
 
I've burned over 17,000 gal of auto gas, about 1000 gls of that was ethanol auto gas. Never had any gasket swell on Efree auto gas. Had problems with both cap and gauge gaskets when burning ethanol. Cork and leather work better with ethanol based gas. If I remember right the day that Joe's cap froze it was about 10F and an ice haze was in the air. Joe, remember anything?

Glenn
 
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If you run the caps with the ram air tube, it makes no difference if your gaskets swell. We had a 160 Cub that did not do well at all with the vented caps, but no trouble with the ram tube caps.

Some suppliers sell those toilet tank fuel neck gaskets, and they interfere with some caps. If you use these, probably less interference, although they are not PMAed like the toilet tank. They look a lot more like the original Piper parts.

http://www.ebay.com/itm/230748152034?_trksid=p2055119.m1438.l2649&ssPageName=STRK:MEBIDX:IT
 
Do you happen to know whether that particular load of gas was tested for ethanol?
I think I've seen posts on this forum indicating that Alaska doesn't have ethanol in gasoline.
 
I don't know if the fuel contained ethanol. This happened in the mid nineties and I remember there was a time where we pilots talked about where to get ethanol free fuel. However, for some time now Alaska has been exempt from it's use.

I doubt if the gasket formulation has changed since the certification process deters any change to an approval but thats just a guess.

The things that I learned was how wonderfully soft and flimsy cub stock tanks are. They crush easy and can be reflated with the careful use of an air hose or shop vac. The other thing I took away from this was what a cool helpful place FAI was. Here we had a cub that had an engine quit on landing (at an international airport) and all the controllers said was your buddies engine quit and you may need to help him off the runway. Today you would be surrounded with a bunch of self important agency types dressed like neon traffic cones handing you papers to fill out and no way in hell would we have been able to just be on our way like we did.

Jerry
 
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