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Thread: PA-12 Fuel System and Landing Gear - Tilting aircraft with unwanted fuel transfer between tanks

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    PA-12 Fuel System and Landing Gear - Tilting aircraft with unwanted fuel transfer between tanks

    Hello all!

    I have a PA-12 and I have two issues with it and I was hoping to get some advice.


    1. For some the aircraft tilts to the left. We have changed bungees in the landing gear but still can not figure out why it tilts. It is about 20 cm difference from one wingtip to the other. Has anyone experienced this?
    2. Because of the tilt, the fuel keep on running between the tanks in the vent line. This of course makes the tilt worse and if you have a lot of fuel in it when you park it will fill the bottom tank and push the fuel through the cap eventually, resulting in a waste of fuel.

      Do you think it is a good idea to put some sort of valve on this line so that it can not transfer fuel when it is turned off?
      Could that line be removed or closed all the time? I do not want fuel to transfer between the tanks as standard ops.


    Hope to hear some advice!

  2. #2
    skywagon8a's Avatar
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    Look at the forks which attach the wing struts to the fuselage. One side could be screwed way in and the other side way out. Unscrew both forks on the short side an equal amount and screw in both forks on the long side by the same amount. All of the rigging dimensions will remain the same. Check with a level that the fuselage is actually square with the ground.
    N1PA

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    PerryB's Avatar
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    The fuel is pushing across through the supply lines, not the vent. Shut either tank off and it'll stop. As to the tilt, assuming the wings are rigged properly my first suspicion is the bungees need to be replaced. You may have one stretched badly or broken on the left. If it's a stock 12 with the bungees inside the fuselage, it's not a fun job.
    Last edited by PerryB; 04-15-2018 at 08:33 AM.
    After Monday and Tuesday, even the calendar says WTF !

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    180Marty's Avatar
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    Also, assuming it is original landing gear and the rubber bumpers under the seat are up against the stop, maybe measure and see if the gear legs are the same length.

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    I’d find a hoist or crane and lift it wings level, then slowly let it down and watch what the gear does.

    Is this a new behavior? Do you pin one brake and spin the tail around with power to park?
    Last edited by stewartb; 04-15-2018 at 09:19 AM.
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    55-PA18A's Avatar
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    You need to resolve the problem of why your aircraft isn't sitting level before modifying your fuel system to compensate for it.

    Have you checked for differences in your tires? Landing gear? Wing struts? Is the fuselage sitting level, with wings not level?

    Please let us know what you find. Too many ask for help with issues, and we never find out if any of the suggestions helped.

    Jim

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    Quote Originally Posted by skywagon8a View Post
    Look at the forks which attach the wing struts to the fuselage. One side could be screwed way in and the other side way out. Unscrew both forks on the short side an equal amount and screw in both forks on the long side by the same amount. All of the rigging dimensions will remain the same. Check with a level that the fuselage is actually square with the ground.
    Thanks, I'll check that!
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    Quote Originally Posted by PerryB View Post
    The fuel is pushing across through the supply lines, not the vent. Shut either tank off and it'll stop. As to the tilt, assuming the wings are rigged properly my first suspicion is the bungees need to be replaced. You may have one stretched badly or broken on the left. If it's a stock 12 with the bungees inside the fuselage, it's not a fun job.
    The handbook says that you should have one tank OFF and one ON which I have tried and it is still the same so therefore I suspect the fuel line.
    The bungees has been replaced, actually two times(!), by the previous owner in the last years so I do not think it is that either.

    Quote Originally Posted by 180Marty View Post
    Also, assuming it is original landing gear and the rubber bumpers under the seat are up against the stop, maybe measure and see if the gear legs are the same length.
    I will do that, thanks!

    Quote Originally Posted by stewartb View Post
    I’d find a hoist or crane and lift it wings level, then slowly let it down and watch what the gear does.

    Is this a new behavior? Do you pin one brake and spin the tail around with power to park?
    Also a good idea. It has been like this for a while, I have only owned it for a year. I do not think it has been turned sharply like that on the same side.

    Quote Originally Posted by 55-PA18A View Post
    You need to resolve the problem of why your aircraft isn't sitting level before modifying your fuel system to compensate for it.

    Have you checked for differences in your tires? Landing gear? Wing struts? Is the fuselage sitting level, with wings not level?

    Please let us know what you find. Too many ask for help with issues, and we never find out if any of the suggestions helped.

    Jim
    I agree that I should fix the tilt which also will more or less fix the fuel issue. Soon I am up for the annual and then I will be able to investigate it further. I will get back to you as I go along!

  9. #9

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    Have you checked fuselage symmetry? I know of at least one airplane where the fuselage was tweaked and rigging was problematic. Check the diagonal distance across the fuselage at the wing attach fittings to the opposite cluster at the floor.


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    mvivion's Avatar
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    I once flew a Cessna 185 that didn’t handle real well on the runway. After first flight, I got back and looked at it, and noted a slight list, as you describe. Turned out that one tire was a Goodyear 8.50 x 6.00, the other was a “brand X” 8.50 x 6.00. Took out a tape and there was an inch and a half difference in diameter. I’ve also seen similar discrepancies in tires both from that same outfit.

    MTV
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    Quote Originally Posted by mvivion View Post
    I once flew a Cessna 185 that didn’t handle real well on the runway. After first flight, I got back and looked at it, and noted a slight list, as you describe. Turned out that one tire was a Goodyear 8.50 x 6.00, the other was a “brand X” 8.50 x 6.00. Took out a tape and there was an inch and a half difference in diameter. I’ve also seen similar discrepancies in tires both from that same outfit.

    MTV
    I'll check that, thanks!

  12. #12
    www.SkupTech.com mike mcs repair's Avatar
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    level top deck & or floor side to side, are they same? set turn & band ball in center now, also in this position in level flight(firewall vertical) see if you have same dihedral on each wing...

    we usually put a valve in any cross vent with a both

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    I had the same problem with the 12 I built and the only way I could stop it was to turn the fuel selector to "off."

    If you think about it, fluid in two inter-connected tanks will always seek to level themselves out, regardless of whether one tank itself is placed a little higher or lower than the other--the fluid level will alway equalize over time.

    If turning off the fuel selector doesn't work for your airplane, finding and fixing the reason fuel transfers from one tank to the next is your primary concern, otherwise you will always find yourself with tanks with equal fuel.

    The "why one wing sits lower" question isn't important until you fix the fuel transfer issue.

  14. #14
    PerryB's Avatar
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    The original Imperial valves were of the plug type and somewhat known for not stopping fuel flow in the off position. That was why I replaced mine with the Dakota valve.
    After Monday and Tuesday, even the calendar says WTF !

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    Take another look at the bungles under the seat. Are they the same-examine 'code thread color markings', do both have the same codes, are there the same number of bungles on both sides, do they lay on the saddles the same way.
    Staying alive in an airplane has a lot more to do with mastering ourselves than mastering the aircraft.

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    12Geezer2's Avatar
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    Does it lean to the left ?? Have seen a couple of 12s with "tweeked" fuselage. Stock fuselage is known for weakness on left side. Hard landing will push gear closer to wing fitting. Not sure who has the paperwork for adding a vertical tube from left front gear fitting area to left front wing attach area. Think this is something to do with float mods---?????? Not a 12 expert but there are some here ---geezer Dan--- OH YAH---my 12 will leak fuel out the low wing when parked sideways on a hill---have two fuel valves--both off but fuel will transfer through the vent line between the tanks then out low wing fuel cap vent.
    Last edited by 12Geezer2; 04-15-2018 at 06:40 PM.

  17. #17

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    I am on a flying trip to North Cape now but I will check it all when I get back home!

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