Citabria fuel vent issue

[ebetancourt]

My Citabria ('99 with 600 hours) has an interesting problem.

I was headed out Tuesday on a long X-Ctry (>500NM) and was planning just one fuel stop. About 45 minutes into the flight I noticed the right tank wasn't feeding at the same rate as the left. Tried flying left wing low and finally landed early. 2.8 in right tank, 14+ in left. Not right. I suspected vent problem, called and discussed with my mechanic and he thought the same thing. He said he had seen cases of fuel in the vent lines preventing normal flow. Went out to the airplane and looked at the vent line (it is behind the windshield between the left and right tanks) and noticed it had sagged over the years and had a dip in the middle. Took off, and while flying pushed the line up and held it for a minute. I also slipped aggressively toward the fuel vent in the left wing. On every leg after that I pushed the vent line up and fuel flow (without a slip) was normal. (~10 hours total.) Yesterday had a very short (45min) flight and forgot to lift the line. Noticed an imbalance again, but waited to land. Taxiing in, pushed the vent line up, and sure enough fuel drained from the vent. Airplane is going in for an annual shortly so will look for permanent solutions, but thought I would post for other Citabria owners who might encounter the same thing.

 

[BillTIZ]

Interesting problem. It's good that you can reach the vent line in flight to troubleshoot.

But now you have a known fuel distribution issue that requires maintenance. Would it be wise to fly with a known problem? Why wait until annual, unless you plan on being grounded until the annual is completed.

 

[Dan Thomas]

I was headed out Tuesday on a long X-Ctry (>500NM) and was planning just one fuel stop. About 45 minutes into the flight I noticed the right tank wasn't feeding at the same rate as the left. Tried flying left wing low and finally landed early. 2.8 in right tank, 14+ in left. Not right.

The vent line might be at fault, all right, but I would have a real good look at the right tank's fuel filler cap. For one thing, the Citabria must have unvented caps but sometimes, if one is lost, someone might put a vented cap on it instead. Or the cap is right but its rubber gasket is leaking or the cap isn't seating properly on the filler neck.

See, if the RH cap is leaking even a bit, the pressure in that RH tank is lower than in the left. The low pressure on top of the wing draws air out of that tank and the crossover vent tube can't quite keep up. The fuel will flow more from the tank with higher pressure (the LH tank) to the engine, and if the pressure difference is large enough some of that fuel will flow through the system and upward into the RH tank and might even overfill it so some goes overboard.

I've had these things happen.

Dan

 

[McBuzz]

The vent line might be at fault, all right, but I would have a real good look at the right tank's fuel filler cap. For one thing, the Citabria must have unvented caps but sometimes, if one is lost, someone might put a vented cap on it instead. Or the cap is right but its rubber gasket is leaking or the cap isn't seating properly on the filler neck.

See, if the RH cap is leaking even a bit, the pressure in that RH tank is lower than in the left. The low pressure on top of the wing draws air out of that tank and the crossover vent tube can't quite keep up. The fuel will flow more from the tank with higher pressure (the LH tank) to the engine, and if the pressure difference is large enough some of that fuel will flow through the system and upward into the RH tank and might even overfill it so some goes overboard.
Dan

Dan is correct about this. I have a 2001 Citabria with a similar situation. I just got off the phone with American Champion and their first suggestion is to check the filler cap gaskets. If they are cracked or no longer seal completely this could be the source of the problem. If they are not cracked, but dried out, you can try flipping them over after rubbing in a little engine oil to soften them up.

To detect the leak you can attach a rubber hose to the vent tube under the left wing and gently blow with the caps in place to try and pressurize the tanks. If there is a leak, you may be able to hear where the air escapes.

Lastly, try gently sucking on the tube and you should be able to tell if the internal baffles are operating.

If you have any other questions, I'd suggest you contact American Champion directly. They can be pretty helpful.

 

[ebetancourt]

Actually, I checked both caps and reversed them at that first stop. The fuel coming out of the vent after landing and pushing up on the vent line is pretty conclusive to me. My guess is if the right cap had not sealed well, the vent problem would not have existed and the right tank would have provided fuel flow.

Ernie