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#244586 - 04/28/15 10:43 PM Non equal gas consumption from tanks
R Chisler Offline
Safety Pilot

Registered: 01/19/15
Posts: 88
Loc: Arizona
I have a 1961b 172 that is real low time plane that I just purchased a couple mos. ago. After every top off I can go fly and when I come back there is always more fuel used out of the left tank vs. the right. I had one A&P look at it and said the left vent tube was in the air stream and it was over pressurizing the left tank and may be pushing fuel to the right side. We adjusted it so it is directly behind the wing strut out of direct forced air, still no change. I ordered 2 new vented tank caps and still no change. I did notice that the tank sump valve on the left wing is damp and turned black compared to the brass look on the right side. The left side drips every now and then but there is no marks on the wings or on the ground where it appears to be leaking any more than a drop every once in a while. Yesterday I used 6 gallons in the right tank and 11 in the left and the selector was on both tanks the whole time. I was wondering if anyone else may have experienced this and may have an idea of what I need to do to fix this. Thank you in advance for any ideas.

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#244589 - 04/28/15 10:59 PM Re: Non equal gas consumption from tanks [Re: R Chisler]
NightHawk XP Offline
Second in Command

Registered: 09/22/14
Posts: 197
Loc: WY
My 1977 XP does this. Same tanks, uses the left faster than the right. I don't know how to fix it, I just run off the right tank only about every half hour or so. I have my GPS set to remind me to switch tanks every half hour and then I check the gauges and decide whether to change from both to right or not. I always land with both tanks as that is part of my pre landing check list. It would be nice to just leave them on both and forget it. I've never just left it on both to see what would happen when the right ran out...
_________________________
If you have integrity, nothing else matters, if you don't have integrity......
Nothing else matters!

Nuff said,
Hawk !!

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#244599 - 04/28/15 11:23 PM Re: Non equal gas consumption from tanks [Re: NightHawk XP]
Desert Hawk Offline
Silver Pilot

Registered: 06/30/13
Posts: 919
Loc: Phoenix, Arizona
You might want to check your right vented cap. If its not venting, what you are describing can happen.
_________________________
karl
1975 C172M-180

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#244600 - 04/28/15 11:32 PM Re: Non equal gas consumption from tanks [Re: Desert Hawk]
NightHawk XP Offline
Second in Command

Registered: 09/22/14
Posts: 197
Loc: WY
Thanks, I'll check this out on mine!
_________________________
If you have integrity, nothing else matters, if you don't have integrity......
Nothing else matters!

Nuff said,
Hawk !!

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#244601 - 04/29/15 12:42 AM Re: Non equal gas consumption from tanks [Re: NightHawk XP]
Flubber4.0 Offline
Gold Pilot

Registered: 08/22/14
Posts: 1103
Loc: blue marble
Unequal tank usage is a neverending story in C172s ... Mine does as well and it depends on W&B, Wind, the Moon and the impact angle of Voodoo. Mine is often preferring left tank and when wifey is aboard the right one, guess who has more weight? After changing caps to venting ones and repairing the upper tank connection tube it improved slightly. How can this construction (http://www.directupload.net/file/d/3972/knfh4mf3_jpg.htm) ever provide equal use of tanks?

Did you check the overflow pipe overhead?
Is the 6-11 consumption from full tanks?
If yes - usually this equals out when going to bottom - starting to be par at about half tanks, as the more filled tank generates more gravitational push and the level comes below overflow pipe. I did worry at the beginning, but am relaxed now. If you have checked tanks and pipes, just keep an eye on the needles and everything will be fine.


Edited by Flubber4.0 (04/29/15 01:00 AM)
_________________________
Rules without reason engender resentment and foster suspicions of all rules. They are an anathema and should be fought at every turn.

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#244627 - 04/29/15 04:08 AM Re: Non equal gas consumption from tanks [Re: Flubber4.0]
Joeman434 Online   content
Diamond Pilot

Registered: 02/19/14
Posts: 6752
Loc: Ocala, Florida
I dug this up out of the archives. Obviously it doesn't apply to later model aircraft or ones that that have Cessna Kit No. SK172-31B or SK172-32 installed.


RE: C-172 Power Loss at altitude
Airworthiness Directive
DEPARTMENT OF TRANSPORTATION
Federal Aviation Administration
14 CFR Part 39
Amendment 39-1415; AD 72-07-02
Airworthiness Directives; CESSNA Models 172, 172A, 172B, 172C, 172D, 172E, 172F, 172G,
172H, 172I, 172K, Airplanes
AGENCY: Federal Aviation Administration, DOT
DATES: Effective March 25, 1972.
72-07-02 CESSNA: Amdt. 39-1415. Applies to the following airplanes:

MODELS SERIAL NUMBERS AFFECTED
172 28000 thru 29999
172 36000 thru 36999
172 46001 thru 46754
172A 46755 thru 47746
172B 17247747 thru 17248734
172C 17248735 thru 17249544
172D 17249545 thru 17250572
172E 17250573 thru 17251822
172F 17251823 thru 17253392
172G 17253393 thru 17254892
172H 17254893 thru 17256512
172I 17256513 thru 17257161
172K 17257162 thru 17258855
Compliance: Required as indicated, unless already accomplished.
To reduce the possibility of engine power interruption at altitudes above 5000 feet caused by vapor formation in the fuel lines, accomplish the following:
(A) Effective now, the airplane must be operated on a single fuel tank immediately upon reaching cruise altitudes above 5000 feet.
(B) On or before April 1, 1972, install at the fuel selector valve applicable Cessna placards P/N's 0509021-1, 0509021-2 or 0509021-3 as provided with Cessna Service Letter SE72-7, dated March 17, 1972, or any FAA-approved equivalent placard which reads as follows: SWITCH TO SINGLE TANK OPERATION IMMEDIATELY UPON REACHING CRUISE ALTITUDES ABOVE 5000 FEET.
(C) Compliance with the provisions of Paragraphs A and B is no longer required when the fuel system has been modified by the installation of applicable Cessna Kit No. SK172-31B or SK172-32 referenced by Cessna Service Letter SE72-7, dated March 17, 1972, or by the accomplishment of any equivalent method approved by the Chief, Engineering and Manufacturing Branch, FAA, Central Region.
This amendment becomes effective March 25, 1972.
_________________________
Do it right the first time.

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#244631 - 04/29/15 06:13 AM Re: Non equal gas consumption from tanks [Re: Joeman434]
Henry M. Offline
Gold Pilot

Registered: 08/23/11
Posts: 1985
Loc: Leander, TX
My 172E burns fuel unevenly too, but that has never been a real problem. Eventually the fuel level evens out.

At the risk of stating the obvious, be sure you either turn the fuel off or that the airplane is level when fueling and conducting experiments in this area. A lot of fuel can move from one tank to the other when parked even on a slightly sloping surface. Depending on where your fueling area is, and how long it takes to get the fuel in, that in itself can cause strange readings.
_________________________
Henry
1965 C182H

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#244637 - 04/29/15 08:26 AM Re: Non equal gas consumption from tanks [Re: Desert Hawk]
R Chisler Offline
Safety Pilot

Registered: 01/19/15
Posts: 88
Loc: Arizona
I checked the caps and replaced both tanks with new vented caps on both sides with no change.

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#244638 - 04/29/15 08:32 AM Re: Non equal gas consumption from tanks [Re: Flubber4.0]
R Chisler Offline
Safety Pilot

Registered: 01/19/15
Posts: 88
Loc: Arizona
Yes this was from full tanks. I fill up after every flight and turn selector to right when filling and start filling right tank first. The FBO is on level ground so the plane is also sitting level. I have never ran it past 1/4 tanks but I do think they do start to level out as the tanks get lower like someone also stated. It's good to know that this is a common trait of this plane so I don't spend a bunch of time and money chasing something that seems to be normal.

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#244639 - 04/29/15 09:04 AM Re: Non equal gas consumption from tanks [Re: R Chisler]
magman Online   content
Gold Pilot

Registered: 10/10/12
Posts: 1378
Loc: buffalo NY
" Vented Caps" really are NOT.

They will vent when a Negative Pressure exists.

There is a red rubber "umbrella" on the bottom

that acts as a Check Valve.

Paint or wax can disable the valve.


You can do a quick check of the Vent System by:

1. Removing Right Fuel Cap

2. Slip a piece of 3/8 " ID hose or tubing over

the Vent Tube beneath the Wing.

3. Blow in the Tube (Mouth ONLY-no compressors)

A helper @ the Rt Cap should be able to observe

fumes coming out of the Filler Port.

4. No fumes = blockage likely in the crossover in fuselage.

You may want to back up & see if fumes will come out of Left

Tank Filler.

5. No fumes from Left Filler is likely

"Mud Daubers" or other nests in the Vent Tube.

If you feel like the line is blocked it likely is.

There is a restriction caused by a Flapper Valve

in the Left Tank Vent Inlet.

It will allow Air in by flapping open

but will then close to prevent ( not really) fuel

from dripping out.

There is a small hole in the Flapper.

6. I've had good luck cleaning the Vent Tube with a speedometer cable.

YMMV

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#244646 - 04/29/15 10:06 AM Re: Non equal gas consumption from tanks [Re: magman]
R Chisler Offline
Safety Pilot

Registered: 01/19/15
Posts: 88
Loc: Arizona
Thanks for the idea... This was done prior to ordering new caps. The new caps are doing the same as the old caps, lines are clear, venting out of tank when blown in.

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#244669 - 04/29/15 10:26 AM Re: Non equal gas consumption from tanks [Re: R Chisler]
MrOutdoorsie Offline
Second in Command

Registered: 10/09/12
Posts: 125
Loc: Colorado Springs, CO
Just out of curiosity, how would changing the fuel selector valve to Off or Right affect cross feeding from one tank to the other? Doesn’t the cross feed line go directly from tank to tank?

Also, I’m assuming that the vent tub is subject to positive pressure, even if located behind the strut. Wouldn’t that explain why the left tank would feed faster, due to the slightly higher pressure in that tank? Vented caps shouldn’t change this, given that they relieve negative pressure, not the positive pressure provided by the vent (unless of course my earlier assumption is incorrect).

And lastly, are the cross feed lines located in the upper section of the tanks, as depicted in the graphic provided by Flubber4.0? If so, wouldn’t a full tank then tend to impede airflow between tanks somewhat until it’s drained below that level? And could that not account for why full tanks tend to show more unequal consumption, but that it tends to even out as the tanks drain?

Keep in mind, this is all based on theory and my limited understanding of how these systems are engineered. Translation…I may be completely full of crap.


Edited by MrOutdoorsie (04/29/15 10:27 AM)

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#244670 - 04/29/15 11:42 AM Re: Non equal gas consumption from tanks [Re: MrOutdoorsie]
R Chisler Offline
Safety Pilot

Registered: 01/19/15
Posts: 88
Loc: Arizona
I have read if you fill the tanks with the selector on both you can get some cross feeding. I don't know if that is true or not so I have filled with selector on both and on right and I haven't noticed a difference. You may be 100% correct since early models only had 1 tank vent tube, mybe that is why Cessna went to 2 later on? I have looked at 25-30 vent tubes on 150's & 172's at the airport and the position of tubes are all over the place...

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#244696 - 04/29/15 04:29 PM Re: Non equal gas consumption from tanks [Re: R Chisler]
vettdvr Offline
Club Sponsor
Diamond Pilot

Registered: 06/24/10
Posts: 7724
Loc: Slidell La
Mine also burns more from left tank than right. Pressure in the vent line gives a very slight pressure on the left tank over the right due to the restriction in line size. Gravity takes over from there and is equal on both sides. Tanks vent out from the cap. if they took air in they could get water in also. Venting in comes from the "under wing" vent. If the under wing vent is off center from the strut more air can enter. The closer you get to blocking ram air from entering the vent the more equal the tanks should burn. Valve in L/R Both or off makes little difference in filling unless when parked the plane doesn't sit level. Then it will of course flow to the lower wing. How the plane sits on the ramp overnight will cause fuel to be more in one tank than the other.

Now that's my story and I'm sticking to it.
_________________________
Vettdvr

Single/Multi/instrument/type/commercial But then I am still learning.

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#244701 - 04/29/15 05:05 PM Re: Non equal gas consumption from tanks [Re: vettdvr]
Willie Offline
Platinum Pilot

Registered: 12/28/10
Posts: 4728
Loc: Ft McMurray Alberta Canada
My plane used to sit in a stall that had the left wing low . When the fuel truck filled the tanks the vent didn't leak if the fuel valve was in the off position . When I started my walk around I opened the valve and the vent would start to leak . Got me a few times when I forgot to avoid walking under it
With my normal 2 to 3 hour flights I haven't had a problem with left and right tanks being at slightly different levels


Edited by Willie (04/29/15 05:10 PM)
_________________________
Flying is the 2nd greatest thrill known to man
Landing is 1st

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#244702 - 04/29/15 05:32 PM Re: Non equal gas consumption from tanks [Re: Willie]
Joeman434 Online   content
Diamond Pilot

Registered: 02/19/14
Posts: 6752
Loc: Ocala, Florida
This is a Cessna VENTED fuel cap...it's obvious it lets air in the $1.75 little Viton flapper. If you have one on each tank, it will let air in even if the vent tube is plugged.

P/S...it doesn't let air out...

Cessna fuel cap by JZuraw8073, on Flickr


Edited by Joeman434 (04/29/15 05:42 PM)
_________________________
Do it right the first time.

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#244704 - 04/29/15 07:34 PM Re: Non equal gas consumption from tanks [Re: MrOutdoorsie]
flyhayes Offline
Pilot in Command

Registered: 12/13/12
Posts: 395
Loc: New Smyrna, FL
Originally Posted By: MrOutdoorsie
Just out of curiosity, how would changing the fuel selector valve to Off or Right affect cross feeding from one tank to the other? Doesn’t the cross feed line go directly from tank to tank?


Looking at the service manual there technically isn't a "cross feed line". What you're most likely referring to is the "vent crossover line". In the diagram this line is attached to a relatively high point in the tank. Unless absolutely brimming, there should be no fuel in this line. The fuel leveling occurs in the lines running through the fuel selector. By switching the selector off of the Both position, gravity can no longer equalize between the tanks.
_________________________
-Andre
CFI, CFII, MEI
www.flyhayes.com

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#244708 - 04/29/15 07:59 PM Re: Non equal gas consumption from tanks [Re: R Chisler]
Don Tedrow Offline
Platinum Pilot

Registered: 07/16/10
Posts: 3659
Loc: Bastrop, TX
Tech Note #003 from the Cessna Pilot's Association deals with this issue. It's supposed to be available only to members, but I've found it on the web before, you just have to do some Googling. It's titled "Uneven fuel feeding in Single Engine Cessnas".
_________________________
1972 F33A Bonanza


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#244743 - 04/30/15 10:07 AM Re: Non equal gas consumption from tanks [Re: Joeman434]
R Chisler Offline
Safety Pilot

Registered: 01/19/15
Posts: 88
Loc: Arizona
Yep I purchased 2 new ones and still made no difference.

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#244745 - 04/30/15 10:27 AM Re: Non equal gas consumption from tanks [Re: R Chisler]
R Chisler Offline
Safety Pilot

Registered: 01/19/15
Posts: 88
Loc: Arizona
Go to this link

http://www.avcom.co.za/phpBB3/viewtopic.php?f=104&t=64352

and click on the Uneven fuel feeding in Cessnas there is a great article written on this problem.

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#244824 - 04/30/15 09:05 PM Re: Non equal gas consumption from tanks [Re: R Chisler]
SunDog Offline
Pilot in Command

Registered: 09/01/13
Posts: 307
Loc: Maryland
I think the vented caps don't do anything, unless the underwing vent is blocked. I have burned down to VFR minimum fuel remaining, with long range tanks (butt numbing) and it all evened out in the end. Fuel burn, I mean; we dipped the tanks post-flight, to check the POH prediction; Cessna's numbers were spot on.

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#244921 - 05/01/15 04:32 PM Re: Non equal gas consumption from tanks [Re: SunDog]
Rod Man Offline
Safety Pilot

Registered: 12/20/13
Posts: 38
Loc: anywhere it's VFR
1977 N model here. I also have uneven consumption from the tanks. I think the most drastic I've encountered on a flight was 2.5 gallons used in the left and nearly 10(!) used in the right. It's good to hear from some here that it evens out as you get close to empty (I have yet to use more than 16-17 gallons on a trip). I scoured the POH and found a fleeting reference to this phenomenon. The book says something to the effect of "excessive number of turns, skidding turns or anything other than straigt and level flight" can cause uneven fuel burn. (I admit, I am sometimes *cough*OFTEN*cough* lazy with the rudder pedals during my turns. eek) By the way, my POH also instructs me to turn the fuel selector to the L or R position while fueling to prevent cross-filling.


Edited by Rod Man (05/01/15 04:40 PM)

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#244928 - 05/01/15 06:04 PM Re: Non equal gas consumption from tanks [Re: Rod Man]
samiam95124 Offline
Safety Pilot

Registered: 04/26/15
Posts: 24
Loc: California
Mine does this (unequal gas flow). I found the "solution" on internet was to "check the cap venting". After tasting lots of 100LL blowing through the right cap I discovered exactly nothing. It passes air.

I have a standard procedure, I see the fuel gauges showing significantly different levels, then I set a 15 minute timer and flip to that right tank. Usually one or two times of this evens it out. Get an obnoxiously loud timer so you won't forget.

There are plently of airplanes that only have left/right selectors, and you have to flip/flop all of the time. Its just the way it is.

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#244932 - 05/01/15 06:39 PM Re: Non equal gas consumption from tanks [Re: SunDog]
Desert Hawk Offline
Silver Pilot

Registered: 06/30/13
Posts: 919
Loc: Phoenix, Arizona
Originally Posted By: SunDog
I think the vented caps don't do anything


Well, once the fuel level is low enough so that the internal tank-to-tank vent is no longer covered with gas, then the venting action of the cap on the right tank is no longer necessary. But until the internal tank-to-tank vent remains Not Covered with the fuel in the tank, the Right cap vent is needed. For the periods of time when the internal tank-to-tank vent is covered with gas, there is no way for air to enter the Right tank and so the Right tank can not empty.

If it weren't for the gas sloshing around and partially unblocking the internal tank-to-tank vent periodically, the right tank would never empty (with a non-venting right tank cap).

However, once the fuel level gets down about a third of a tank or so, then the tank-to-tank vent remains open and gravity slowly starts to balance out the fuel in the tanks.

I had this fuel flow imbalance problem on my plane until I replaced the right cap (which was not venting). Before replacing the cap, short to medium flights would reveal a pretty drastic difference in tank usage. But on the longer flights, some of the miss-balance in fuel usage seemed to even back out once the inter-tanks could vent air.

However, as others have indicated, there are many reasons why the fuel draw can be imbalanced. A bad right tank vented cap, is just one cause.
_________________________
karl
1975 C172M-180

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#244939 - 05/01/15 07:30 PM Re: Non equal gas consumption from tanks [Re: Desert Hawk]
Steve N8809B Offline
Second in Command

Registered: 10/01/14
Posts: 136
Loc: Ohio
I get that I would like my tanks to feed evenly. They don't.

Playing out the scenario, I leave my selector on “both” and run the left tank dry. The right still has, say, ten gallons. I would suggest that even then, past the fuel selector I will see fuel, not air. I will simply continue to feed fuel, but from the right tank only.

Right? And if “right”, who cares? If they care, why?

Has anyone, anywhere suffered an actual adverse effect, an engine failure or whatever, from the unequal suck problem?

Thanks,
Steve

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