Fuel siphoning from Main to Nacelle tank C340A

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  • #84751

    Hi all

    I am the new owner of a 1977 C340A – I’m presently ironing out the ‘new ownership’ gremlins, one of which was the mysterious loss of around 20gal (75 litres) from the left tip tank. The plane has been sitting over Christmas and New Year since the last flight due to a leaking threaded McCauley hub. When I last checked, there were 140 litres in each tip tank. I was changing data cards in the hangar today when I noticed a significant discrepancy on the fuel gauge. The AUX tanks are empty, both on the gauge and to inspection, and the dipstick confirmed that I only had 60 litres remaining in the left main (140 in the right). No signs of external fuel puddles on the hangar floor or green streaking anywhere on the wings – lo and behold though, Santa had filled my left nacelle tank for me – with the missing 80 litres!

    I have run the nacelle tank pump once, in the hangar a couple of weeks back, just to confirm that it works, with minimal volume in the tank. The siphoning would seem to have occurred since then. Anyone have any idea where the fault is likely to lie? Presumably there is a check valve in the line from the nacelle to the main that may be stuck, or something in the pump I have blocked with crud?

    Any thoughts gratefully received

    #102675
    rwelsh
    Participant

      Martin, your post doesn’t seem to follow. What does running the nacelle tank pump have to do with the tip tanks? Then again, maybe you are not familiar with the 340 fuel system. But I think your main problem is the o’rings in the fuel selector valve are allowing the right tank to siphon over to the left tank. This happens during very cold weather, infrequent use and/or just worn out o’rings. I would suggest you rotate the fuel selector valve quite a few times in a warm hangar to try and reset the o’rings. This may cure your problem and it may not, but it is worth a try. Removing the selector valve is not an easy job besides you need to drain all the tanks to remove the selector valve.

      After rereading your post, I think you mentioned the nacelle tank pump only to put a timeline on the siphon event. I doubt you have had a cold snap like the Midwest and east has the past 10 days, that could have shrunk one o’ring enough to allow the siphon over a few day period particularly if the plane was not on a level surface. Even the tanks out at the gas pump and see what happens in a few days after you exercise the fuel selector valve. If you have any questions how the 340 fuel system works, be sure to ask here as the system can cause grief if used wrong.

      #102677

      Thanks, but I think I need to clarify my post. I don’t think I have explained it very well. I have a fuel system with the 31.5 gal Aux tanks and a single (left) nacelle tank. The leak seems to be from the left tip to the nacelle tank. I only mentioned the right main tank as there was supposed to be 140litres in both tanks (so the right level was as it was before, but the left had dropped to 60 litres). From my reading of the fuel system, the fuel selector probably doesn’t have much to do with the line from the left nacelle tank to the left main, and something in this line is allowing fuel to flow in the wrong direction – from the main tank downhill into the nacelle tank (which was totally empty when I put the plane away). I mentioned the nacelle tank pump as I wonder whether running it to test it might have pumped some gunk into something and causing a stuck-open check valve or something similar. The nacelle tank had next to nothing in it when I ran the pump (for 5 seconds or so) just to make sure I could hear it running.

      I guess I could troubleshoot by running the pump again to empty the nacelle fuel back into the main and seeing if the problem recurs. Maybe running fuel through that line might unblock or unstick whatever is causing the siphoning. I’m all ears though….

      #102693
      rwelsh
      Participant

        Martin, I’ll have to say I have never heard of fuel siphoning back to a nacelle tank. I don’t have a fuel system diagram at home so I can’t say if there is a check valve in the line from the nacelle tank to the tip tank. I believe the line is only 1/4″OD and the nacelle pump is a small pulse pump. It will be interesting to see what you find out.

        #102697

        I ran the pump today and emptied the locker tank fuel back into the left main. Still no idea how or why the siphoning happened, but we’ll see if it happens again…

        #102698

        From POH shows a check valve in the system.

        Andrew


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        #102699

        IPC relevant pages, must be late can’t locate the check valve.

        Andrew


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        #102700

        Thanks for that Andrew – it seems to be a pretty unusual occurrence. Our mutual engineer Mike hadn’t heard of it happening before either. All a bit weird. I’ll just watch and wait and see if it recurs. I don’t know whether all that super-hot weather last weekend could have contributed (it must have been about 50 deg C in the hangar, it was over 40 shade temp at the airfield) – maybe a blocked main tank vent with lots of hot vapour could have precipitated siphoning down the pipe to the locker tank if a check valve was stuck open? Just a WAG on my part…

        #103011
        kkushner1
        Participant

          Martin,

          This happened to my 1973 340. I had the check valve replaced and problem solved.

          Ken

          #103014

          Thanks Ken – good to know. I was starting to feel a bit lonely, no-one else seems to have seen it before!

          #103048
          rwelsh
          Participant

            Ken, where is the check valve for the nacelle tank that you changed? I am wondering if it is shown on the tip tank drawing #136?

            #103051

            By my reckoning there would need to be 2 points of failure -firstly a ‘leak point’ into the fuel line (the opening of the line in the main tank sits high, above the level of the fuel so a direct siphon would be unusual). From the parts catalog illustration, there is a connection just as the line enters the main tank, which may be a potential point of entry of the main tank fuel into the nacelle tank fuel line. Secondly, the check valve (not sure where it is, the POH diagram says there is one, but I can’t see it on the IPC schematic) would also have to be defective.

            #103056

            Simple answer is the fuel selector is a manifold that the tanks go into i.e. main aux and crossfeed, when the seals or Teflon plates are not sealing they seek the same level, ui.e form higher main to lower aux. These are rebuildable and I believe the 340 manual has a section on this but hey must be tested before reinstalling, I have put new parts in and they leak still slightly but leak, trick is to lap the parts for a perfect fit (there is a phenolic bushing that this applies to.

            Good luck they are a bear to get out and in!

            #103064

            Except we are talking about a main tank to nacelle tank transfer, a system that is independent of the fuel selector..

            #103097
            kkushner1
            Participant
              quote RWELSH:

              Ken, where is the check valve for the nacelle tank that you changed? I am wondering if it is shown on the tip tank drawing #136?

              Wow, I can’t find the check valve in the attached schematic, unless it’s incorporated in the transfer pump? (which I doubt).


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