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Thread: Unported my left tank in a slip

  1. #1

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    Unported my left tank in a slip

    I was out flying the other day and decided to descend for some sightseeing, wanted to get down quick so I put in a slip left wing low, had the fuel selector on the left tank...caught me by surprise when the engine started to sputter and lost power. My first thought was possible carb ice so I went through that drill, didn't fix the problem. It didn't occur to me until after the fact that I likely unported the fuel tank. I had approx. 6 gallons in the tank and I was established in the slip for maybe 20-30 seconds, I always assumed I'd be safe from unporting with more than about 1/4 tank. I'm still scratching my head about why the header tank couldn't keep up for that amount of time. I must have had a lot of air in the line at that point because it wouldn't make good power even after a coordinated power-off descent for what seemed like another 15-20 seconds. I selected the right tank and power was reestablished immediately, albeit closer to the trees than was comfortable. After I landed I ran the left tank at 1800rpm for a few minutes and everything was normal. My assumptions were challenged and proven wrong, be wary of your assumptions and be quicker than I was to go for the other tank. Another reason why I'm glad to have two tanks in a 18-95...
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  2. #2
    BC12D-4-85's Avatar
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    Can a slip like you describe force fuel from the full header back into the main tank? Just a question no answer here. You might try it again at altitude and see if it reoccurs.

    Gary

  3. #3
    aktango58's Avatar
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    I was thinking that the header tank will also have the fuel slide 'down' and away from the out port, and with no fuel in the line pressurizing it.

    This is the exact scenario that Jason G. talked about where the header tank is worth having, at least the fuel is super close to the tank, so when you do unport you have a chance of getting fuel back to the carb quickly.

    My guess is that the slip was mighty steep?
    I don't know where you've been me lad, but I see you won first Prize!
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  4. #4
    BC12D-4-85's Avatar
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    Header tank support for fuel in level flight versus turns...coordinated turns or slips and skids. Discussion?

    GAry

  5. #5

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    I am not an expert on wing tank vent systems, but have had an 18 quit at reduced power in level coordinated approach. Plenty of fuel in both tanks, and a switch to the opposite tank cured it almost immediately.

    Do you have those little “pffft” thingies on your caps?

  6. #6
    cubdriver2's Avatar
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    I've done a million slips on left tank only and never any problems. Even with only 3 or 4gls

    Glenn
    "Optimism is going after Moby Dick in a rowboat and taking the tartar sauce with you!"

  7. #7

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    Quote Originally Posted by cubdriver2 View Post
    I've done a million slips on left tank only and never any problems. Even with only 3 or 4gls

    Glenn
    What configuration do you have Glenn? Dual tanks with header? Headerless? Just curious since I am finishing fabric on my 11EX and will be assembling this summer. I did headerless with dual 18 gal tanks.

  8. #8
    skywagon8a's Avatar
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    A both position on the fuel selector valve eliminates this issue.
    N1PA
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  9. #9
    cubdriver2's Avatar
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    Quote Originally Posted by Dan Gervae View Post
    What configuration do you have Glenn? Dual tanks with header? Headerless? Just curious since I am finishing fabric on my 11EX and will be assembling this summer. I did headerless with dual 18 gal tanks.
    I only have left tank and front header

    Glenn
    "Optimism is going after Moby Dick in a rowboat and taking the tartar sauce with you!"

  10. #10

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    Yes Bob I have vented fuel caps, it wasn't blocked. Glenn I haven't done a million slips but I've done enough and it has never happened to me before which is probably why I didn't consider unporting as the problem in real time. I'm guessing it was just the right combination of left wing down, deck angle, uncoordination, amount of time in the slip, etc. for it to unport. The header tanks failure to keep the engine running is what I'm curious about, and the other thing about this that is striking is that I couldn't get the engine running again from the left tank for a considerable amount of time after the slip was let out. If I had a 1 tank system I would have ended up in the trees. I'm curious if anyone is running a boost pump on a cub type fuel system?

  11. #11
    Steve Pierce's Avatar
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    I run very low fuel levels a lot with header tanks and have never had an issue. I can't see the header tank unporting. I would put some more fuel in the right tank and go up and try to recreate that scenario. I take it all the lines and tanks are installed correctly?
    Steve Pierce

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  12. #12

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    I'd investigate left tank venting and check the finger screens.

    If temps were below freezing I might suspect ice crystals clogged the screens. If above freezing maybe a little water in the system. Isopropyl will fix either.
    Last edited by stewartb; 03-03-2020 at 08:31 AM.
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  13. #13

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    I also considered contamination and blockage. I drained the left tank, checked the gascolator screen, and the finger strainer: All was good. Ambient temp was about 60F.

  14. #14

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    FWIW I'd expect the engine to run the 20-30 seconds with the fuel turned off. It should have run much longer with a functioning header tank.

  15. #15
    skywagon8a's Avatar
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    Quote Originally Posted by MZ18 View Post
    The header tanks failure to keep the engine running is what I'm curious about, and the other thing about this that is striking is that I couldn't get the engine running again from the left tank for a considerable amount of time after the slip was let out. If I had a 1 tank system I would have ended up in the trees.
    Quote Originally Posted by Steve Pierce View Post
    .... I take it all the lines and tanks are installed correctly?
    Steve has a point. Once the lines get full of air, if there is a high point anywhere the air can form a bubble which is strong enough to oppose letting fuel gravity feed. Sometimes this would require a strong slip in the opposite direction to give the fuel in the tank a high enough head pressure to start flowing.
    N1PA
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  16. #16

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    How/where is the header vented? Up to the right tank and not the left? Vented to both?
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  17. #17
    BC12D-4-85's Avatar
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    Needs to be tested and fixed no doubt. Confirm all the plumbing and vents are correct and unrestricted. Stuff like this does bad and the aftermath is fodder for the scene's investigators.

    Gary
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  18. #18

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    Thanks for all the input with troubleshooting tips, I think the next step will be pulling panels and checking vent line from left tank to header. I agree Steve, after I've looked over everything and the weather allows I'm going to see if I can duplicate the scenario overhead the field. I'll report back with findings...

  19. #19
    BC12D-4-85's Avatar
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    How about draining the affected portion of the fuel system then inspecting and carefully blowing air through the components to clear obstructions? I've had finger screens on my Taylorcraft wing tanks restricted by insect parts. They get in the large forward cap vents apparently. Spiders and their webs won't dissolve. Removing the quick drain, blocking the vents, and blowing through the tank outlet sent them out flying again. Maybe do same for your header and fuel valve then replace any hose splices that look compromised?

    Gary

  20. #20
    courierguy's Avatar
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    Quote Originally Posted by CharlieN View Post
    How/where is the header vented? Up to the right tank and not the left? Vented to both?
    My header tank is unvented, didn't see the need when I built the S-7S 13 years ago, zero issues. Didn't know it was a thing to vent them, ignorance is bliss!

  21. #21
    SuperCub MD's Avatar
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    The left(forward) header is vented to the forward corner of the left fuel tank. The vent line is small,gets little if any flow through it, so it is very prone to corrosion and clogging in older aircraft. That is the first thing I would look at.
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  22. #22
    Gordon Misch's Avatar
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    Quote Originally Posted by courierguy View Post
    My header tank is unvented, didn't see the need when I built the S-7S 13 years ago, zero issues. Didn't know it was a thing to vent them, ignorance is bliss!
    How does it fill if there's no place for air to go?
    Gordon

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  23. #23
    BC12D-4-85's Avatar
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    Plus the header's air fitting should be upper or at least higher than the fuel fitting(s).

    Gary

  24. #24
    skywagon8a's Avatar
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    Quote Originally Posted by courierguy View Post
    My header tank is unvented, didn't see the need when I built the S-7S 13 years ago, zero issues. Didn't know it was a thing to vent them, ignorance is bliss!
    If there is no place for air to be trapped in the header, then a vent is likely not needed. It all depends on the design of your system.
    N1PA

  25. #25
    BC12D-4-85's Avatar
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    Question on front Cub header: It's been some years since but aren't the vent and fuel nipples on the same end of the tank? If so would a left slip force fuel into both and maybe slow drainage by filling the air inlet? That assumes the main tank's outlet is unported when partially filled and in a left slip.

    Gary

  26. #26
    cubdriver2's Avatar
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    I don't think it has a dedicated vent? Fuel in fuel out, vents up to tank? This is a Pa11 tank



    Glenn
    "Optimism is going after Moby Dick in a rowboat and taking the tartar sauce with you!"

  27. #27
    courierguy's Avatar
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    Quote Originally Posted by skywagon8a View Post
    If there is no place for air to be trapped in the header, then a vent is likely not needed. It all depends on the design of your system.

    What skywagon said....when I run my floorboard mounted fuel transfer pump when refueling out of a 5 gallon jug or bush bag, when the pump starts sucking air I can see the air bubble gurgle up the header tank's sight gauge, up into the wing tanks where it vents out their vents. 2650 hrs TT and no in flight anomalies of any kind.

  28. #28
    BC12D-4-85's Avatar
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    Subject plane is a PA18-95 with dual tanks and headers. The PA-11 with single left tank originally had none? (http://www.univair.com/content/partc...ndex.html#p=30) but I guess one could be added later. My PA-11 had dual tanks and two headers.

    Either way the fittings I believe should be one above the other not horizontal so any air when empty can be replaced by fuel and the header fully fueled to capacity. PA-18 fuel system (https://www.univair.com/content/part...dex.html#p=120) The UNIVAIR front header might be drawn wrong for the outlet location (see 61-63). See Atlee's PA-18 front header tank picture below.

    Gary
    Attached Images Attached Images  

  29. #29
    jjack's Avatar
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    I did a left 180 , non coordinated descending dive towards my brothers dock and 1/2 way thru the turn the engine quit. Switched tanks , engine came to life and averted a crash landing in his yard.
    Rebuilt early cub, 24 gallons in each wing. No other issues in 300 hours.
    Never forget that incidence.
    JohnnyJackson

  30. #30
    SuperCub MD's Avatar
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    Quote Originally Posted by cubdriver2 View Post
    I don't think it has a dedicated vent? Fuel in fuel out, vents up to tank? This is a Pa11 tank



    Glenn
    That is the stock 18 front header. It was added to the 11 by a SB. Fuel line on the bottom goes down towards the selector,vent line on the top goes up to the main tank. Atlee front header has pipe fittings as shown in the other picture.

  31. #31
    cubdriver2's Avatar
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    Glenn
    "Optimism is going after Moby Dick in a rowboat and taking the tartar sauce with you!"
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  32. #32
    BC12D-4-85's Avatar
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    All good info and hard to recall. I guess the questions may be: Is the stock PA-18-95 setup described here prone to fuel starvation in a prolonged 20-30 sec left slip with about 1/3 in the left tank? Is his front header positioned correctly or lines (vent/fuel) restricted?

    Even in subsequent coordinated descending flight it apparently ran poor until switched to the right tank. If the front header was not full to begin with, or emptied fast, or when isolated from the main tank prone to low flow to the carb somehow, I can understand a lag in response. Make sure the lines don't have an air lock bump. Fix it.

    Gary

  33. #33
    Gordon Misch's Avatar
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    Tangential - I sure do like my headerless system, not having to worry about adding fuel to the header tank before fuel gets to the carb in the event I mess up. I get a re-start in less than 5 seconds from either re-porting an unport, or an incorrect fuel selector setting.
    Gordon

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  34. #34
    BC12D-4-85's Avatar
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    One reason for header tanks is in case the fuel tank in use lacks a front outlet. Low on fuel and nose down or uncoordinated flight can unport the rear and cause quiet times. Placards don't fill carbs.

    Gary
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  35. #35
    Gordon Misch's Avatar
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    Placards don't fill carbs.
    Love that line!
    Gordon

    N4328M KTDO
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  36. #36
    courierguy's Avatar
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    Another way of putting it is with a properly configured header tank like mine apparently is, one advantage is 2 less tank fittings, less fuel line/fittings busy work. Plus I gained 3 gallons capacity, and it's all useable.
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  37. #37
    aktango58's Avatar
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    60F... Humidity?

    Possible that it built up carb ice, less direct inlet air suffocated the carb, and once you opened the throttle it came back to life?

    Often times we have one thing go wrong, but our moving of other controls make us think it is something else.
    I don't know where you've been me lad, but I see you won first Prize!
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  38. #38

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    Screw the weight, I’ll keep my header tanks. AD up front and stock aft, never had an issue and I’ve slipped full left and full right in all fuel loadings.
    Remember, These are the Good old Days!

  39. #39

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    In Post 31 there is a diagram of the fuel system. Up until now, I believed that all header tanks had an inlet were fuel flowed from the main tanks into the header, PLUS an outlet from which fuel flowed out to the engine.

    I believe this diagram has enlightened me. It looks like that diagram shows only one fuel line connecting the header to the fuel system, plus a vent line. The fuel line acts as both an inlet and and outlet to supply fuel to the engine. So, under normal operations, the header fills up with fuel and and sits there on standby. If fuel stops flowing from a main wing tank, then the header will start to drain.

    Am I reading the diagram right?

  40. #40

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    Rear headers have three ports.

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