Uneven Fuel Burn 2001 GCBC

T

tkearns

New member
Have checked fuel caps and swapped sides. Straight and level, fuel burn from right sometimes goes as low as 1/4 before borrowing from left. Thoughts?
 
Basically, the tanks are lightly pressurized. So if the left is not burning down, it is not retaining/recieving enough pressure. The difference in pressure is equal to the static head difference between the tanks. If there is a break in the seal, it won't behave the same.

Anyway, if you swapped caps already, you can eliminate the cap gasket as a varible. Your filler neck might not be letting it seal. Also, the check valve might not be opening properly.

You might try to pull the right tank vent line off the check valve and plug it. Then blow in the vent tube. might need a buddy to hear if you are getting much air into the left tank.

Has it always burned uneven, or something recent?
 
Thanks, Jerry for the quick reply. Just got the plane a week ago and it seems to have this problem intermittently. Flew it 18 hours home with the ferry pilot and it seemed only to act up on the last leg. Since then I've been training in it and have noticed it once. The filler stems seem fine. Regardless, I think I'll start with a couple of fresh cap seals and try your check valve idea. Great advice....will advise.
 
Kathy and I had a 2001 GCBC and the same situation. I know this is very unscientific but we found the problem existed when we were flying cross country in a strong crosswind. We finally decided that flying a crab angle into the wind may had been a contributing factor. We also tried new gaskets for the tank filler with no real change.

Tom
 
Interesting...when I noticed it there were in fact, strong winds...but I thought I had that little ball where it was supposed to be...I'll pay attention to this. Thanks.
 
It is rare but there have been filler necks damaged by heavy handed linesman. The fuel nozzle can act as a pry bar. Look and make sure the top of the filler neck is not distorted. It needs to be flat for the cap to seal. Don't look at it from above, sight across it in from the same plane with a short straight edge.
 
It's been behaving mostly. Filler necks are perfect. About 1 in 3 fillups seem to offer a significantly uneven fuel burn. I don't fly this plane far, so I'm going to replace the seals and live with whatever result I get. Thanks for the input guys.
 
I agree with your conclusion.
If its not happening every time...its likely the caps.

Its a little un-nerving, but the uneven part is not dangerous unless you are flying by your watch only and forget that you have gauges. The uneven part is simply a difference in static head. Even when one tank goes dry, the other one will continue feeding at its slightly reduced head pressure. This reduced head pressure is still several times more than the engine needs under full power. (You can kiss the high wing for this side effect) :p
 
Jerry:
As you say the fuel pressure is the sum of the pressure due to the weight of the fuel and the pressure of the headspace above the fuel. If the amount of fuel in both tanks is equal but the air pressure is higher in one, that tank will feed faster. If the headspace pressure imbalance is small then at some point the total pressure in tanks will equalize once the fuel level imbalance matches the air pressure imbalance. If the air pressure imbalance is great enough it is possible to transfer fuel from the higher pressure tank to the lower pressure tank. I have seen this when I had a split in one of the fuel cap gaskets. In such cases when there is a significant air pressure difference between the tanks and the high pressure tank runs dry, it would seem that the possibility exits for air to be forced into the fuel line going to the engine despite the presence of fuel in one of the tanks.
Mark
 
Yes, but you would need a HUGE differential in air pressure for that.
In such a case, the 'high' tank would still be full when the 'low' tank ran out and would continue to remain full. Your asking a lot for a half inch diameter vent opening. You would need a lot of negative pressure that also does not syphon gas out of the top.

Basically, rather than the uneven pressure to be equal to a few inches of static head, it would have to be about 3 feet of avgas static head pressure.
 
" but the uneven part is not dangerous unless you are flying by your watch only and forget that you have gauges. "

Hey, Jerry...what's that mean?....my density altitude is especially high this morning, and I'm willing to admit, "I don't get it"....anything with 'dangerous' in it, I'm OK with trading any perception of my competency, for knowledge!

If you're flying by your watch, you've got X number of hours and don't care which tank it flows from first, cause ya can't switch tanks on our birds anyway...right, or what am I missing?

Let the humiliation begin... :?
 
shazam said:
"
If you're flying by your watch, you've got X number of hours and don't care which tank it flows from first, cause ya can't switch tanks on our birds anyway...right, or what am I missing?



Unless, you have a vent line that is to close to the strut. Then a leaky cap or two, and your watch method wont work because the fuel can siphon out of the vent tube.

Always check vent tube location behind strut when having a fuel imbalance with long range tank system.
Click to expand...
 
Yeah, my only point was that if you are flying by your watch, that you dont have a really lose cap syphoning fuel out.

i.e. you put in 35 gallons, you time your flight for 35 gallons, your cap sucks 15 of those away from you over the 3 hours...

As long as you are not losing gas, the watch should still be the best way. I think I was typing that up too quickly to make that clear.
 
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