Uneven Fuel Usage Between Tanks in a C172

inav8r

inav8r

Line Up and Wait
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Indiana, US
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Mike B.
On my trip out and back from WV I noticed that I was sucking fuel from mostly the right tank.

When they topped me off for the return flight home I noticed that I had burned 9.1 gal from the right tank and 4.3 gal from the left.

On the way home, I noticed the same thing and wound up selecting just the left tank for about 45 minutes to help even out the burn (I was using the fuel gages to judge this, which in hind sight probably wasn't the smartest thing) and according to the line guy who topped me off after getting home he said they were pretty even.

I'm a big guy - and I was flying solo. I loaded my luggage on the "right side" but my luggage weighed way less than I do. The ball seems to be centered when I'm flying solo - but I do require a bit of an adjustment to keep from banking left - but the AI shows me as wings level...

I've not discussed this with other members of our club or the FBO, I'm just curious and trying to confirm it's me and my inexperience and that it's not anything more serious.
 
I've noticed this also. Not exactly sure what causes it. But I beleive it was using the right more then the left for me also. I ended up burning from the left for awhile.
 
I used to see this all the time when I fueled airplanes for the flight school.
It was always with the 172SPs. I'd put 8 gallons on one side and 2 on the other.
I talked to one of the maintanence guys about it and he had a theory as to why, but I can't remember exactly what he said. Something about uneven pressure between the fuel tanks having to do with the fuel bleed and/or the little holes on the gas caps. Sorry I can't be more specific. I'm pretty machanically reclined.
 
Sounds like maybe one of the tanks isn't venting correctly, and hence getting a better flow from the other side?


inav8r said:
On my trip out and back from WV I noticed that I was sucking fuel from mostly the right tank.

When they topped me off for the return flight home I noticed that I had burned 9.1 gal from the right tank and 4.3 gal from the left.

On the way home, I noticed the same thing and wound up selecting just the left tank for about 45 minutes to help even out the burn (I was using the fuel gages to judge this, which in hind sight probably wasn't the smartest thing) and according to the line guy who topped me off after getting home he said they were pretty even.

I'm a big guy - and I was flying solo. I loaded my luggage on the "right side" but my luggage weighed way less than I do. The ball seems to be centered when I'm flying solo - but I do require a bit of an adjustment to keep from banking left - but the AI shows me as wings level...

I've not discussed this with other members of our club or the FBO, I'm just curious and trying to confirm it's me and my inexperience and that it's not anything more serious.
Click to expand...
 
mgkdrgn said:
Sounds like maybe one of the tanks isn't venting correctly, and hence getting a better flow from the other side?
Click to expand...

The trouble I have with that idea is that AFaIK there is a cross vent between the two tanks that should equalize the pressure. Assuming the cross vent is unobstructed, the only thing besides unlevel wings that could account for the difference would be a serious restriction in the fuel outlet on one side, or a leaking cap that's supposed to be sealed. When one cap leaks that creates airflow through the cross vent and the result is different pressures in each tank.
 
I own a C172SP. During cruise, fuel from the left tank is used faster than fuel from the right tank because the fuel system vent is located in the left tank, right behind the left wing strut. Nevertheless, inflight fuel transfer from one tank to the other occurs. I did this test. I dried one tank inflight until the engine coughed, then I switched the fuel selector to BOTH, did a few maneuvers, turns, climbs etc. When I landed, 30 minutes later, I almost had the same amount of fuel in each tank.
 
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lancefisher said:
The trouble I have with that idea is that AFaIK there is a cross vent between the two tanks that should equalize the pressure. Assuming the cross vent is unobstructed, the only thing besides unlevel wings that could account for the difference would be a serious restriction in the fuel outlet on one side, or a leaking cap that's supposed to be sealed. When one cap leaks that creates airflow through the cross vent and the result is different pressures in each tank.
Click to expand...

Lance is right. This is an often-debated subject on this and other boards. The facts as we know them: 1) Uneven fuel burn is a fact in these planes. 2) Theoretically, a plane in perfect condition with a test pilot flying will not exhibit uneven fuel burn.

In between these two there are several possible explanations, including the possibility that several are exising at the same time:

1) The plane is slightly out of rig, and therefore flying in a slight crab all the time.

2) Passenger/cargo loading causes a heavy wing, held up by aileron, induced drag caused by that causes a slight slip.

3) Obstructed air vent in between the two tanks

4) Turn indicator not level, so the ball doesn't read right.

5) Pilot flying with the ball not centered.

There may be others.

-Skip
 
I don't mind it. I don't mind managing the fuel burn by switching tanks. It just struck me odd because the vent on our 172N is on the left... So I would have expected it to draw more from the left than the right and not the other way around.
 
I just did a quick search in the CPA forum and found that this guy was experiencing a similar problem, uneven fuel burn, more from the right tank than from the left tank. The problem in this case was that the vent in the left tank had slipped into the wing. After correcting this discrepancy the problem was solved.
 
TeenDoc said:
It may be entirely normal, at least for the C172S. It is described in the POH.
Click to expand...


Every 'hawk in our local fleet seems to do this. It's just a thing, me thinks. If you fly uncoordinated, i.e. without the ball centered, you can make it happen in a more pronounced manner. Don't ask me how I know this, I always fly coordinated.;)

Jim G
 
In my Cherokee - I notice that I have to burn the left tank first, because otherwise I'll be constantly correcting to the right because I'm fat.

Are ya a big boy like me? Could be the cause.
 
NickDBrennan said:
In my Cherokee - I notice that I have to burn the left tank first, because otherwise I'll be constantly correcting to the right because I'm fat.

Are ya a big boy like me? Could be the cause.
Click to expand...
Nick, sounds like you need a substantial girlfriend.:rofl:
 
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