Cessna 172 Fuel Imbalances

[SM227]

G'day,
I have am just wondering if this is normal?

I recently undertook a flight in a Cessna 172R and discovered on landing after the engine had been running for about 2 hours that the left tank was half empty and the right tank was still completely full. Concerned it may be blocked, I did a fuel drain and found nothing, so I tried running it on the right tank before i took off to see if the engine would stop, after about 3min the engine was still running so I put the selector back to both and headed off to a airport with fuel that I could get to on the left tank.

On landing I checked the tanks again before filling them and found this time that no fuel had been used from the left tank and the right tank was now just over half full.
I re-fulled to be safe and headed home, and on arriving both tanks where pretty much the same with the usual few liters imbalance.
After asking around i was told this was normal, and then a few days later someone else had the same thing happen in the same aircraft.

I just wanted to see if this is a common occurance and if so, what happens if my flight had been for say, 5 hours, using more than one tank of fuel?

Thanks

[Trojan1981]

It is pretty common with high-winged Cessnas. I have been told it can be caused by differences in the tank vent (one more effective) but is nothing to worry about.
If you want to check there are no other problems, fill it, switch between tanks every 30 mins during flight and keep an accurate log. You will know then, how much should be in each tank when you land and will be immeadiatley aware if there is a problem.

[VH-XXX]

Also depends on the flying attitude, in particular rudder and wings level and that goes for most aircraft with wing tanks. In some aircraft it will drain from one tank to the other and use one first before the other 90% of the time with little reasoning. In the Airvan, flying with one wing high will drain fuel from one tank to the other, however in the reverse to the direction in which you will think that it should happen. (eg right wing high drains from left to right) A trap for the young (or experienced) player and a good discussion point.

[james michael]

SM

Quite normal situation in all my C172 ops.

I checked it via the CPA USA years ago and confirmed it.

Try Cessna Owner Organization :: Maintenance :: Fuel Tank Feed Imbalance

[172driver]

While your example is a bit on the extreme side for a 172 (it's more pronounced normally on the 182), this is quite normal. The reason is the way the Cessna tanks are vented. In simple terms, if you fly on 'crossfeed', fuel is 'easier' to suck out of one tank (the better vented one) than the other. I sometimes fly a 182 where this leads to some serious imbalance. Solution: fly on one tank only and switch tanks regularly (I do on the watch hemispheric rule).

PS: don't worry, even if you stay in crossfeed, it will not suck one tank dry and leave the other full. After an equilibrium is reached (IIRC, mostly after a bit over half the 'first' tank is gone), it will balance itself out.

[27/09]

Can't say I would call it normal for this to happen with a C 172.

On the earlier models e.g. the "N" series 172, one cap is vented and the other cap non vented. The non vented cap is fitted to the tank with the vent. Assuming the "R" series are the same, if the caps are reversed or the wrong caps are fitted the problem you speak of can happen.

I would check to see the correct caps are fitted to the correct tanks.

[BrazDriver]

Flying in balance usually helps too!

[Westaussielame]

Get a LAME to check correct positioning of fuel vent .Inlet should almost directly behind strut.You usually can see by looking at the paint on the vent where it was sitiing when installed at factory.
Westaussie

[MCKES]

Never seen more than 10 liters difference in the two sides on 172R. The caps aren't vented on either side - unless it is not visible to see that the cap is vented. As people say you might not be in balance all the time. Otherwise I would be seeing what is going on, because in 30 hours on the 172R never seen more than 10l diff. Maybe just luck. Happy flying

[kingRB]

While your example is a bit on the extreme side for a 172 (it's more pronounced normally on the 182), this is quite normal. The reason is the way the Cessna tanks are vented. In simple terms, if you fly on 'crossfeed', fuel is 'easier' to suck out of one tank (the better vented one) than the other. I sometimes fly a 182 where this leads to some serious imbalance. Solution: fly on one tank only and switch tanks regularly (I do on the watch hemispheric rule).

only thing to remember when doing that, is (as per the POH) only select single tank feed in level flight / in the cruise. Both tank feeds must be selected for all other stages of flight or if there will be prolonged slips / skids.

[172driver]

Correct

[185skywagon]

MCKES,

The caps aren't vented on either side - unless it is not visible to see that the cap is vented.

On later C172's and others, I think you will find a small soft rubber "button" on the tank side of the cap. This lets air into the tank, and hopefully stops fuel going out. Sometimes, this "button can have something under it, or not be seated properly and your cap will let a bit of fuel out in flight.

Earlier 172's had one cap non-vented on the tank with the pipe vent, and the vented cap on the non-vented tank.

Pipe vent alignment behind the strut is quite important for even burn.

The 185 has pipe vents on both sides and uses fuel fairly evenly, provided you keep the machine in balance with the rudder.

[MCKES]

185 - Sorry you are correct. section 7-26 of the POH says:

"FUEL VENTING
Fuel system venting is essential to system operation. Blockage of the system will result in decreasing fuel flow and eventual engine stoppage. Venting is accomplished by an interconnecting line from the right fuel tank to the left fuel tank. The left fuel tank is vented overboard through a vent line, equipped with a check valve, which protrudes from the bottom surface of the left wing near the wing strut. Both fuel filler caps are also vented."

I have never been told or noticed this in my flying I know the rubber button you are talking about in the cap. Just thought it was a seal.
Thanks very much for clearing that up, I really appreciate it.

[Pilotette]

I noticed this too and on doing a bit of research found this Tech Note which answered my questions..Safe flying http://www.moonflight.org/Documents/Fflow.pdf

[boardpig]

Warning light

This does happen in many 172 ops but as already been said, happens more in the 182.
I remember doing the V1 scenic and having the left "low fuel" warning light come on about 3/4 of the way around. Selector was on both of course but the left had been used while the right was still about half full. In "theory" this should be fine (not sure how much needs to be in there for the light to come on) but I did have a job trying to convince my pax it was nothing to worry about, especially when he pointed directly at it and asked "what does that mean?"

;-) BP

[nick2007]

Would it be possible for the tendency to cross feed to cause fuel to spill overboard the tank with the vent, if the tanks were almost full, and the apron had enough of a slope to completely fill the downward tank?

I'm wondering if this is the same reason I was always taught to "isolate" the tanks from cross feeding when the aircraft is parked overnight, by turning the fuel selector to either left or right tank only....

Yep, that's the reason why we switch the fuel off at night in ours.

[kingRB]

I was thinking more about that - and the POH says the left vent line (at least POH for the 172RG) has a check valve - which I assume means that its not possible for fuel to actually vent overboard?

I noticed this too and on doing a bit of research found this Tech Note which answered my questions..Safe flying http://www.moonflight.org/Documents/Fflow.pdf

thanks for that - excellent read

[loopylune]

Fuel imbalance

Yeah,
Happens all the time on my 172 P I swap from both to the right tank until the gauges show just about level and then swap back.
Remarkably the AI is a good indicator, when the tanks are level so is the AI!
cheers

[death667b]

Just adding my 2 cents - I fly a 172SP and it happens alot.

Only recently completed circuit solo and it is normal for the left tank to be about 10-20 liters less than the right tank.

[frigatebird]

Try this link. Its for 182, but is similar for many things on 172.

Cessna Pilots Association