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I went for a 1.5 hour flight in a 1977 Cessna 172N, with several takeoffs and landings. I was alone. When I left, I had 18 gallons in each tank. When I arrived back, I had 15 gallons in the left tank, and 10 in the right tank.

Why was there such a disparity? Is this at all dangerous? Why did the right tank have less?

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  • $\begingroup$ semi clog in the fuel line, pump not working optimally $\endgroup$
    – ratchet freak
    May 30, 2014 at 21:05
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    $\begingroup$ Well, on the bright side: 7.3 gallons per hour is a pretty nice fuel burn :-) $\endgroup$
    – voretaq7
    May 30, 2014 at 21:47
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    $\begingroup$ I've flown a number of Cessna 172s (N and S models) the last few years. Every single one of them burned noticeably more from one tank than the other. Consistently too, it was always the same tank on a particular airplane. I too am very curious as to the reason, since the fuel system is so simple and it looks like it should stay even. $\endgroup$
    – Brian Knoblauch
    May 31, 2014 at 13:35
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    $\begingroup$ @rachet freak No fuel pumps in a C-172N. Gravity feed only. $\endgroup$
    – Skip Miller
    Jun 2, 2014 at 15:33
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    $\begingroup$ I think Skip pretty much just answered the question. Gravity feed isn't consistent, so one side may flow better than the other. Or maybe you made more left turns. I wouldn't worry that much about that kind of fuel burn. You can report it to the mechanics and have them take a look. But unless there's a BIG difference (like one wing doesn't seem to be burning fuel at all), then the most they're likely to do is clean or replace some filters. $\endgroup$
    – Shawn
    Jun 3, 2014 at 19:24

4 Answers 4

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Consulting your handy POH (or one for a 172N that I Googled up) you'll find the fuel system diagram looks like the one below (click to enlarge - most high-wing gravity-fed fuel systems are similar).
Cessna 172N Fuel System

So there are a number of ways you could wind up with uneven fuel burn with this plumbing.
The most common ones are:

  1. (Uncoordinated) Turns
    If you're flying lots of left-hand traffic patterns (or you're making lots of left turns for some other reason) that means your left wing is frequently lower, and gravity favors pulling fuel from the right tank. (Note that it doesn't have to be a turn - prolonged slips can also cause uneven fuel flow.)
    If I had to take a guess, I'd say this is probably your culprit. Were you making a lot of left turns?

  2. Pressure
    The fuel vent tube on the left tank faces into the wind - this pressurizes the left tank first (and the right tank through the breather tube connecting the two) - it's a little extra encouragement for fuel to leave the tanks and flow into the engine. In some aircraft this leads to the left tank draining faster (because it's at a slightly higher air pressure than the right tank) - this can become particularly pronounced if the breather tube between the two tanks is blocked.
    Cessna fuel vent

  3. Various blockages
    A partial blockage in either of the tank fuel lines, the fuel selector valve, the vent, or the breather line between the two tanks could theoretically cause uneven fuel burn (the vent/breather blockages being a special case of "pressure" from (2) above).

  4. Pilot Error
    It's possible to mis-position the fuel selector in such a way that you're getting adequate fuel flow to run the engine, but one tank is more restricted than the other.
    This is unlikely if the aircraft is properly maintained (the fuel selector should have nice, identifiable detents that it clicks into, letting you know it's in position).

For more detail on causes for uneven fuel burn (and tips for troubleshooting it with your mechanic) check out Cessna Pilot's Association Technote 003.

Is uneven fuel burn dangerous? Maybe.
It's certainly something you should be aware of (part of why you want your fuel gauges to be at least somewhat accurate is so you can tell if your fuel burn is uneven in flight and correct it by switching to the fuller tank briefly).

Uneven fuel burn can be an annoyance (a 5 gallon difference is about 30lbs - potentially enough to notice on the controls), but it can also be a safety hazard: If allowed to go uncorrected you could theoretically get yourself into a situation where the right fuel tank is empty, and in a steep enough left turn you may not have enough pressure from gravity to get fuel from the left tank into the engine.

In an extreme case your right tank may be empty and your left tank may be so low as to unport the fuel pickup tube in a turn, potentially leading to attitude-induced fuel starvation ("The noisy part of the plane gets quiet until you level the wings").

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  • $\begingroup$ Thanks for the detailed and interesting reply. I looked at my GPS tracks, and made 5 right-hand patterns and 3 left-hand patterns. I didn’t do any other significant turns. So that makes your most likely explanation less likely... time to file a squawk. $\endgroup$
    – Peter
    May 30, 2014 at 23:03
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    $\begingroup$ @Peter An uneven ramp is sometimes a culprit too (same principle: fuel flows across through the breather line or the valve) - I assume you checked your fuel right after the flight though, and that you'd probably notice if the plane wasn't level. It sounds like you definitely had fuel flow from both tanks (those 3 gallons went somewhere) but if you can't figure out why it was so uneven it's definitely worth having a mechanic look at. Even if they say "no trouble found" at least you'll know there's not a gremlin waiting to bite you on a future flight :) $\endgroup$
    – voretaq7
    May 31, 2014 at 3:24
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    $\begingroup$ I fail to see how (coordinated) turns affect your fuel balance. $\endgroup$
    – DeltaLima ♦
    May 31, 2014 at 13:39
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    $\begingroup$ In a coordinated turn, the resulting gravitational and centrifugal force is always orthogonal to the wings and therefore cannot cause any imbalance in the fuel system. Only uncoordinated manoeuvres can cause any imbalance and one normally aims to fly coordinated. $\endgroup$
    – Jan Hudec
    May 31, 2014 at 16:02
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    $\begingroup$ If this was a fuel-injected 172, the answer is easy - the excess fuel return line only goes to one tank. No such luck! $\endgroup$
    – egid
    Jun 1, 2014 at 2:03
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I asked my instructor and this was his answer!

The Nefarious Skyhawk Gas Gremlin !

Fact: When a high wing plane (such as a Cessna 172 or a Cessna 150) is flown on a training flight with the fuel selector in the "Both" position (C150 fuel valves are always on the "both" position - even when "off") with a primary flight student at the controls, the plane will almost always land with more fuel in the right tank than in the left (but the very same plane may occasionally land with more fuel in the left tank with a slightly more experienced student, or - eventually - with equally balanced fuel tanks).

"Why is this?" you ask. "How can this be?" you demand to know! I ask all my students this, but I try not to give them the answer. The answer is completely obvious once they have flown for a while. Once they understand the most fundamental basics of flight, they will be able to tell me the secret of "The Nefarious Skyhawk Gas Gremlin". (But once in a while a CFI is compelled to point out the obvious).

WHAT IT IS NOT:

It is not because of a design flaw in the Cessna.

It is not because of an obstruction in the fuel line or the vent (while that is possible, this rare obstruction problem is not germane to the nearly universal "Gas Gremlin" phenomenon).

It is not because of turns and banks during flight.

An aircraft can do steep turns all day long; loops, wing-overs, and barrel rolls, ad infinitum, with the fuel selector in the "both" position, and never have a fuel imbalance. Yet a simple training flight will usually result in more fuel in the right tank than the left.

WHAT IS THE CAUSE?

Hint, the answer is related to the "native call of the CFI in his/her natural environment".

The cause is revealed, not by the trusty aircraft mechanic, nor by the psychic aviation consultant, but by the most simple indicator found in all airplanes: The slip/skid ball. The force that causes the ball to be pulled to the right side of the curved glass tube during takeoff, climb, and power-on stall practice, is the very same force that is pulling the fuel from the left tank into the right wing tank.

This is also the origin of the "native call of the CFI in his/her natural environment" ... "RIGHT RUDDER! RIGHT RUDDER! RIGHT RUDDER! ... STEP ON THE BALL !!!"

Note: this unbalanced fuel condition will also be caused when even the most experienced pilot is practicing side-slips and forward slips. But this is OK, and elicits only a silent smile from the CFI.

JT Rairigh, CFII, MEI, Helicopter, A&P/IA

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    $\begingroup$ An amusing way of reiterating point #1 in the heavily upvoted and accepted answer by voretaq7♦. $\endgroup$
    – FreeMan
    May 5, 2016 at 16:32
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I have an f model 172 and as with most all skyhawks I have the fuel imbalance problem as well. I found that in the Cessna Service manual, the fuel vent has very specific measurements it is to be set at with only 1/32" + or - tolerances. My 1965 172 shows it to be 4 5/8" down and 1 3/16" behind the strut. I can tell you from experience it makes a notable difference if it is not positioned per the specs. When set correctly the difference is minimized to 2 gallons and always less in the right tank! While there is allot to be said about coordinated flight involved as part of the problem, I have always been very diligent to have the ball centered and found it didn't make any difference in the imbalance. But the vent position sure does.

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This is a small discrepancy in a tank that is quite difficult to measure accurately: it's wide, short, 2 meters in the air, the filler hole is at one end and it's on a contraption that is rather sensitive to the angle of the pavement.

If you have (nearly) ZERO burn from one tank, you have a reason to see your mechanic and probe the fuel lines for a blockage.

The weight difference will not affect your flight. If one tank is empty and the other nearly full you may bank slightly to that side, but Cessnas can easily handle 800 pounds hanging on the wing strut.

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    $\begingroup$ I care because it’s a potential safety issue or potentially indicates something wrong. I want to understand the plane more. $\endgroup$
    – Peter
    Jun 2, 2014 at 4:44
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    $\begingroup$ I applaud your curiosity but you need to understand that fuel imbalance is endemic to 172s. One reason is that the fuel vent hangs below the wing in the slipstream and if it isn't positioned perfectly it can partially pressurize one tank, diverting fuel to the other. However, this is not really a safety of flight problem. If the gauges are showing a severe imbalance in fuel remaining, fly on the fuller tank only for a while. Yes discuss this with the mechanic but don't be surprised if he shrugs and goes back about his business. $\endgroup$
    – Skip Miller
    Jun 2, 2014 at 15:40
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    $\begingroup$ Paul's answer may have come off a little callous, but it really doesn't deserve the downvotes. And he's right. I will also agree with Skip again. This level of burn difference in a 172 during a training flight really isn't that much of a concern. If it makes the pilot comfortable, report it to Mx (just as anything wonky with the plane should be reported). But don't be surprised if the response is a shrug. 15 gals / 2 gals would raise eyebrows. 15/10 isn't really that much of a problem in this aircraft. Especially during a training flight. $\endgroup$
    – Shawn
    Jun 3, 2014 at 19:49
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    $\begingroup$ Airplanes are complex machines that never work "perfectly". An important part of training is learning what to pay attention to and what can be dismissed. Notice, ask someone who knows (like a mechanic) and file their reply for future reference. But don't pester maintenance with every tiny detail or they will ignore you when you say the engine runs rough (and the next guy loses power on takeoff). In this case, we clearly have fuel flow from both tanks, a burn rate within normal limits, and a quick look at the fuel system diagram shows how it can easily move back and forth. Mystery solved. $\endgroup$
    – paul
    Jun 4, 2014 at 10:33
  • $\begingroup$ I get that a fuel imbalance in a Cessna may be 'normal' and dismissed as not a problem. I am sorry, however, but "Why would you care?" ... it is the fuel system, which affects the whole, as @voretaq7 said so eloquently, "the noisy part of the plane gets quiet..." $\endgroup$
    – CGCampbell
    Jun 4, 2014 at 17:55

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