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ALT Fuel Tank Vent Valve

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  • ALT Fuel Tank Vent Valve

    The thought of having fuel in the tanks but no way to get to it bothers me. In other threads we talked about tank vents getting clogged, iced, or put on incorrectly, making fuel in that tank unavailable. A pretty significant single point of failure to be able to inop a whole tank of fuel from a single 1/8" hole, seems to me. I was thinking I'd design and fab a custom check valve and maybe terminate the vent line in an empty wing bay with a weep hole, but that solution is sub-optimal for all the obvious reason.

    My latest idea is to have a vent line run from the outbd of the tank near the filler neck to a ball valve on the wing root. If I suspect a vent has been clogged or put on backward I can open the ALT vent valve and vent from the cabin. No exposure to the outside, no cross vent, can be easily checked/drained, a completely redundant tank vent. 1/4" lines and fittings would be extra weight sure, but not that bad for the functionality. Plus I'd now have a cabin-side fitting to plumb a ferry tank if I ever needed.

    Question is do I do this to the aux tanks too? I don't think I want to connect the aux and main tanks via the same vent, don't want to have to worry about siphoning and whatnot. Again more stuff, but might be worth it.

    Just watched a video about a guy ferrying a new plane across the Atlantic. He couldn't get fuel out of his ferry tank because of what sounds like a clogged vent, ended up rigging a tube to blow in the tank and pressurize it to get him across. Got me thinking about this.
    Dave B.
    Plane Grips Co.
    www.planegrips.com

  • #2
    I am going to cross vent the tanks together, with a valve to shut it off if I have full tanks and am parked non-level. It may be a 12v valve that is normally closed, power off. Ultimately, a BH can take off and land just about anywhere, and N America has so many airports, landing and cleaning out a vent shouldn't be that big of a deal. I think that is enough redundancy for me.

    One of the problems with complexity becomes troubleshooting and verifying that each part works. Even with an extra vent per tank, one can be plugged and you won't know it, until the other vent is also blocked and you can't use the fuel in that tank. You could fly around with one blocked vent for a year. If you have redundancy, aviation convention generally requires testing that redundancy each flight, first flight of the day, alternating redundant systems each flight, or have some warning system telling you the primary has failed.

    One thing good about the fuel cap vents is you can easily inspect/blow them out each time you fly or fill up with gas. One easily and often inspected vent might be better than a more complicated system that is more difficult or inconvenient to verity its' operation.
    Last edited by svyolo; 12-04-2020, 07:57 PM.

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    • #3
      Good points. To clarify these vents would be normally closed, so I'm running the stock system most of the time, and this is per tank. I was thinking the valve would be in the fwd root in the same area as folks locate air vents. You can easily blow them out too with a stub of rubber hose and lung power, though I doubt this would be an every flight check. Like an ALT static source I think an ALT vent source may be prudent. Granted there are plenty of "I've run stock BH vents 1000+ hrs and never had a clog" which is good info, but for me a ALT vent source seems prudent.
      Dave B.
      Plane Grips Co.
      www.planegrips.com

      Comment


      • svyolo
        svyolo commented
        Editing a comment
        I was thinking something similar. I wonder if Cessna requires periodic inspections/verifications of their 180-185 fuel systems.

    • #4
      Just a thought as I’m also planning to cross vent the tanks together.
      The plans show the filler cap to the outside with a small dihedral. If a cross vent is added in the vicinity of the filler neck, will fuel transfer to the other tank if parked on a slope ? I think it would require a siphon to start, but it seems unlikely as the fuel would flow away from the uphill filler neck. Therefore would a valve be needed, or does it just add complexity?

      I would think it may transfer through the main fuel selector when left in the BOTH position, or flow out the downhill vent.
      Nev Bailey
      Christchurch, NZ

      BearhawkBlog.com - Safety & Maintenance Notes
      YouTube - Build and flying channel
      Builders Log - We build planes

      Comment


      • svyolo
        svyolo commented
        Editing a comment
        I think if the tanks are cross vented from the high point (top, outboard) they shouldn't xfer fuel from one side to the other. I am going to vent mine from the top inboard, so if the tanks are very full I might need a valve to shut on the ground. If I was scratch building I think I would install an internal tube in the tank from the high point outboard, to the high point inboard and put a fitting there to cross vent them. That would allow high point venting, and make installation and servicing a lot easier. I tried to figure out how to put a vent outboard, but there is very little room to work there. To me it seems like it would make removing and installing the tank a lot more work.

        Hopefully my inboard cross vents are "good enough".

      • svyolo
        svyolo commented
        Editing a comment
        When I installed the tanks I put them as far outboard in the bays as I could to leave room to work on the fittings on the inboard side. I believe I also read that in the old kit builder manual. It really doesn't leave much room to work on fittings if they are on the outboard side, at least on mine.

      • Nev
        Nev commented
        Editing a comment
        Would the auxiliary tank feed ports on the main tanks be high enough?
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