Unequal fuel draw

sarangan

sarangan

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Andrew, CFI-I
Our club 182 has been experiencing unequal fuel draw from the tanks. It has been drawing more fuel from the left tank than the right. Running it on left or right separately seems to work fine, but "Both" seems to draw mostly from the left tank. This has been getting worse over time. The last pilot reported landing with nearly empty on the left side with fuel spilling over from the right. The vent tubes was identified as a potential cause. The left vent is behind the strut, as expected, but the right side seems to be slightly displaced, facing the relative wind. But I am not totally convinced this has anything to do with our problem because there is no evidence that this has been tampered with recently. I am suspecting the problem is with the fuel selector valve. The mechanic was suggesting that we fly with left and right instead of both. Parts availability, lead times and busy flying season was cited as reasons for postponing this maintenance. My recommendation was to ground the plane, but I wasn't sure if that is overly conservative. Any thoughts?
 
I’d call Bruce Hartwig and see if he has any ideas. Pretty sure he will.
Venting is usually the cause I believe?
 
Yep check the vent in the cap,, and the cross wing vent. The other possibility is the vent valve in left tank, ours had a wasp that made it in there and was holding it open, this allowed the left tank to have slightly more pressure.

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It also has a little check valve with a small spring to hold it closed, if the spring is broke or pin backed out it will cause uneven feeding.
 
My 182 always did this but never to the extreme you are describing. I now have a 206 and it does the same thing, but again never to the extreme you are describing. I full up when I am have used about 60 gallons, so about 27 gallons remaining, and my right tank typically takes about 5 to 10 gallons more than my left. Spoke to many a mechanic about this, and no good answers. On long cross countries I need a break after 3 to 4 hours and so it works well for me.
 
Model, wet wing , bladders, long or short range?

Cessna fuel systems are all similar but there are differences.

Most of the issues have been brought up repeatedly.

Long range bladder tanks can have a unique problem.

There are plastic vent lines inside the cell that are supposed to

be supported by loops the the top of the bladder.

If the tech installing a bladder is not aware of this the line will

be suspended in the fuel. This would allow fuel to leave a tank even

if it is not the one selected.
 
I think if it’s got 2 vents, per the OP, it’s a wet wing R model. The 2 vents were supposed to fix the uneven draw of previous models.

Might be a different issue than just the vent alignment, but the misaligned right wing vent should be adjusted per the tech bulletin above (both should be). I would think a misaligned vent would cause pressure and push the fuel to the other tank, which is opposite of what’s happening though, so something else may be wrong.

Also, to confirm: this is a carbureted engine, not an injected engine STC, correct?

Just because someone decided to keep it in service doesn’t mean you have to fly it. You’re #1, stay safe. You could fly with full fuel, but if it’s a selector issue, there are other failure modes added to the mix, not just an uneven draw issue.
 
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sarangan said:
Our club 182 has been experiencing unequal fuel draw from the tanks. It has been drawing more fuel from the left tank than the right.
Click to expand...

Save yourself a lot of trouble. It's due to uncoordinated flight.

One day someone will install an engine that rotates in the other direction and suddenly it will be the right wing drawing first.
 
Personally, I would defer and use the fuel selector given it appears to be working correctly. I question the ADM of a pilot that let an entire wing go empty while the other wing wasn’t drawing fuel.

Even on a gravity feed 172, uneven fuel draw occurs and is noticeable as early as 30 minutes into the flight. Manage the fuel properly and it’s a non-event.
 
martym said:
Save yourself a lot of trouble. It's due to uncoordinated flight.

One day someone will install an engine that rotates in the other direction and suddenly it will be the right wing drawing first.
Click to expand...
Doubtful. Once you're in cruise, most places fly coordinated as long as the pilot doesn't keep rudder pressure.

THe 172's and some of the other Cessnas have a problem that the CPA has well documented with the fuel crossfeeding via the vent line. You can prove this by running on RIGHT only and finding the left tank is still going down. They have a tech sheet on how to adjust the venting to alleviate this.
 
No Cessna that I have flown has perfectly equal fuel draw. But the differences should be small enough that it doesn't matter and no corrective action is needed. It seems to be a difference in % not in absolute gallons, so the difference shrinks as the fuel levels get lower, and it won't run one tank dry before the other.

As others have mentioned, an obstructed vent and uncoordinated flight (or parking!) can make it worse, but that will tend to equalize in coordinated flight. As for coordination in straight & level flight, if the airplane has a trim tab instead of a pilot adjustable trim wheel, then it's coordinated in level flight only at one particular airspeed. If you fly faster or slower than that, coordination will be slightly off.
 
The 150 I fly draws uneven. I can even it up by flying uncoordinated for awhile. If I don’t, it evens out by itself by the time it’s empty. Not quite sure how. Eventually the higher head pressure wins out I guess.
 
Cessna vented caps are normally closed. They only pop open if a vacuum forms in the tank due to a stopped up main vent.
 
That vent check valve's flapper is to let air in rapidly and out slowly. The flapper opens easily enough if it has to, but there's a small hole in it that serves the venting function adequately even if the flapper is closed.

The more likely explanation is misaligned underwing vents causing a pressure differential between the tanks, or a leaking right fuel tank cap. Bad cap gasket, loose cap fit, or the silicone umbrella inlet check valve on it might be shot. The low pressure on top of the wing can suck air out of the tank and leave that tank with a lower pressure, holding back the fuel a little. That cap's rubber gasket can crack, as can the thin nylon washer under it. If this airplane still has the flush caps, those commonly leak even though the bladder wrinkle AD demands an annual inspection of the cap's O-rings, and there are two of them, one that seals the cap to the tank inlet and the other around the locking stem.

If a cap leak is big enough it can suck the fuel out as the bladder collapses as the vent cannot keep up, and that bladder will squeeze all the fuel out of the tank and hold the fuel sender float all the way up, showing a full tank. Neat, huh?
 
Witmo said:
Cessna vented caps are normally closed. They only pop open if a vacuum forms in the tank due to a stopped up main vent.
Click to expand...
That's one design. It is very important on Cessnas to use the proper caps for the aircraft. Some are valved as you describe, some are not, some are not vented at all. You do not want to use the wrong cap.
 
455 Bravo Uniform said:
I think if it’s got 2 vents, per the OP, it’s a wet wing R model. The 2 vents were supposed to fix the uneven draw of previous models.

Might be a different issue than just the vent alignment, but the misaligned right wing vent should be adjusted per the tech bulletin above (both should be). I would think a misaligned vent would cause pressure and push the fuel to the other tank, which is opposite of what’s happening though, so something else may be wrong.

Also, to confirm: this is a carbureted engine, not an injected engine STC, correct?

Just because someone decided to keep it in service doesn’t mean you have to fly it. You’re #1, stay safe. You could fly with full fuel, but if it’s a selector issue, there are other failure modes added to the mix, not just an uneven draw issue.
Click to expand...

This is exactly correct. It is a R182. The vent tubes are misaligned, but it is producing the opposite effect of one would expect (ie the the tank with the vent tube facing the relative wind is not drawing fuel, but the other one is).
 
sarangan said:
This is exactly correct. It is a R182. The vent tubes are misaligned, but it is producing the opposite effect of one would expect (ie the the tank with the vent tube facing the relative wind is not drawing fuel, but the other one is).
Click to expand...
That's because it's not a venting problem, never has been. The position of the vent tube specified in the service manual is to minimize ice accumulation.
 
flyingron said:
That's one design. It is very important on Cessnas to use the proper caps for the aircraft. Some are valved as you describe, some are not, some are not vented at all. You do not want to use the wrong cap.
Click to expand...
The typical raised caps on almost every Cessna single have the vent. They were mandated 44 years ago by AD 79-10-14R1. The R182 was not one of those. From the beginning it had the vented flush fuel caps, and most of those airplanes were converted in accordance with AD 84-10-01R1 to the raised caps due to water ingress and sealing problems with the flush caps.

The '77 and '78 models of the R182 only had the more common vent under the left wing and the crossover tube to the right tank. In '79 they got vents under each wing.
 
And the 120/140 has different caps (and it varied with the year of production there). Again, it's important to make sure you have the right caps on the tanks based on your model (and ADs).
 
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