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Fuel vent

Started by lbpa18, December 16, 2010, 12:41:57 PM

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This is really just a single vent fuel system with two main tanks tied to each other as if they are one. Each auxiliary tank is its own tank/system that simply transfers fuel into the main tank. The auxiliary tank vents only vent their respective auxiliary tank when fuel is being transferred by the transfer pumps to the main tanks. They do not provide any venting when the pumps are off and obstructing the fuel/airflow to the main tanks. The main tanks share the single vent. If the main vent becomes plugged in flight one could try running the transfer pumps to provide alternate fuel/venting. If the aux. tanks have fuel in them this will replace the fuel being drawn by the engine with fuel instead of air but serve the same purpose and prevent a vacuum and fuel starvation. One must be very careful doing this as with the main vent plugged pressure in the tanks could build resulting in an overpressure situation and a risk of a line coming off, the bladder itself rupturing and possible fuel in the cabin or high fuel pressure to the carburetor. The transfer pumps may cavitate and minimize this but I'm not positive on this and have not tried it. If the aux. tanks are empty running the pumps would provide venting through auxiliary tank vents, through the pumps and into the main tanks in this case the pumps will be cavitating and no risk of pressure build up should occur. These pumps are supposed to be self priming and able to be run dry although I would not try this for an extended period of time only long enough to get on the ground. I would also consider running the boost pump to help smooth out the fuel flow if the vent was restricted.


Very good analysis.  I would be now a lot more suspecious of using tranfer pumps as a way to provide vent to the main tanks in case of a blocked goose neck. Four PSI is a lot of PSI on a surface as big as the tank.   I will now choose to land ASAP with electric pressure pump ON if necessary until then.



Doug Johnson

I just found this post Nathan Mackeys site and thought I'd repost it here kind of an old thread; the part that caught my interest was what happens when a  fuel vent is turned backwards. If I remember correctly there is nothing to keep this from happening but how tight you make the fitting its just a T in a copper line.

When I was putting 1233 back together my vent got broke off it was a homebuilt replacement. I was able to find a factory built vent and replace it. It seemed like it was a lot better. Later on when sweeping snow off I bent that one, but was able to bend it back without breaking it.

I always intended to make a copy of the factory vent out of stainless steel tubing but never got around to it. Another thing I briefly thought on was a better way to support it where it went through the the skin but didn't come up with anything simple what I wanted was a rubber grommet like you use for spots that wires pass through but couldn't come up with anything.

I was re positioning my U-10 from Hurlburt to Spartanburg for a Swift Strike exercise. Was told to get a low level nav mission on the way. Well, you know, weather was great so there I was skimming the tree tops when the engine quit!! Popped up and saw ONE cotton patch within gliding distance. All fuel tanks turned on, check everything and land. Found that the fuel vent on top of the wing was loose and turned around and instead of pressurizing the tank it siphoned all the fuel out! Even sucked the bladders up so that the gauges read Half tanks as they should have at that point in my flight. There was a big red fuel stain on the fuselage and tail that saved MY tail!

Ken Berger

Yes, the reversed fuel vent problem you describe is just one loose AN nut away for all of us.  I agree that a simple fix could easily be designed. 

The only positive thought I have about this is that after rebuilding this area of my 295, I noticed that it doesn't take very much tightness on that fuel vent to keep it from turning.  Naturally, I tightened it down regardless.