The Case of the Disappearing Fuel

[Anne]

The tanks were filled to the tabs, both sides. The fuel selector was set to Off (both tanks). For one reason or another, I haven't flown my plane in a month. I did a pre-flight this morning, and the right tank is down about 3 gallons. No smell, no wet spots, no blue stain anywhere. I even ran my hand along the inside of the wing and it was dry. It's in a locked hangar, and no-one flew it without my knowledge.

Anyone have an idea where my fuel went?

Anne.

[okmike]

Hi Anne
Just a thought, did you switch the selector to off after you fueled. If I don't switch my selector to off, R or L before I fuel my Cub & Cessna both cross feed while I'm fueling and the first tank I fill is never full when I check it after fueling, I just did this and several days later I checked before a long cross country and the first tank took about 3 gallons.

[mvivion]

Hi Anne
Just a thought, did you switch the selector to off after you fueled. If I don't switch my selector to off, R or L before I fuel my Cub & Cessna both cross feed while I'm fueling and the first tank I fill is never full when I check it after fueling, I just did this and several days later I checked before a long cross country and the first tank took about 3 gallons.

I don't believe a stock Cub fuel system can cross feed. There is no connection bewteen the tanks at all. The only common point is the fuel selector, and that can only point one direction at a time.

If you've modified it to the headerless system, with a both position on the fuel selector, and you leave the selector on "both" while fueling, you won't get the tanks totally topped off. Three gallons seems a bit much, but.....

MTV

[behindpropellers]

The tanks were filled to the tabs, both sides. The fuel selector was set to Off (both tanks). For one reason or another, I haven't flown my plane in a month. I did a pre-flight this morning, and the right tank is down about 3 gallons. No smell, no wet spots, no blue stain anywhere. I even ran my hand along the inside of the wing and it was dry. It's in a locked hangar, and no-one flew it without my knowledge.

Anyone have an idea where my fuel went?

Anne.

Did you measure your fuel with the same device when you filled up?

[Rick Papp]

Anne, Isn't your plane experimental homebuilt? Could be a none stock one of a kind configuration. Just a thought. Rick

[OLDCROWE]

Fuel it on a HOT afternoon from an above ground tank and then park it in a cool hanger?

[Anne]

Rick is right - it's a homebuilt, 2 wingtanks, 2 separate levers (one for each tank, on - off), no headers. I measure with a calibrated dowel stick. I don't remember if the levers were on or off, but after fueling it was at the tabs on both tanks. A couple weeks later the right tank was down about 2 gallons, and then it was down again. The left tank is still to the tabs. It was fueled in the hangar from a truck, and it's gotten a lot hotter. The valves have been off since after fueling and there's no evidence the fuel has expanded and overflowed. Anyway, I would think if it was expansion & overflow, it would happen to both tanks equally.

Anne.

[DW]

Any chance of an air bubble in the tank??? maybe from a quick fill up. I've never had it happen but we're looking for something. Or did someone need to mow the lawn around the hanger.

[Longwinglover]

Squirrels. They're like organized crime bosses of the animal world.

John Scott

[Chuck Avon]

Anne on my tcraft that has the same type caps as a cub i will loose fuel due to evaporation out the cap vent made a rubber cap with red streamer so as not to forget to take it off cut fuel loss by a whole lot

[Webfoot]

Low tire or floor sloped compared to fill point?

[behindpropellers]

I don't think your measuring process is accurate enough to determine a 1 gallon change over the surface area of a standard 18 gallon tank.

Tim

[Anne]

This is the first time in 10 years that this has happened, but it might only have shown up because it's been sitting more than usual. Same airplane, same hangar, same tires for the past 5 years (they lose air equally), the tanks were full to the tabs right after fueling, and one tank loses "about" 3 gallons. I may not have an exact amount of loss, but my stick has never lied to me. At the tabs, it's 17 gallons. The stick reads just above 14 gallons now. I just want to be sure it's safe to fly it and that I won't suddenly ignite a fireball.

Anne.

[S2D]

Maybe check inside the fuselage. you may have a slow enough leak, it isn't getting to the outside world.

[12 Geezer]

I don't believe a stock Cub fuel system can cross feed. There is no connection bewteen the tanks at all. The only common point is the fuel selector, and that can only point one direction at a time. MTV

The cross vent will cross feed, even with valves off. It wouldn't take much out-of-level to make a 3 gal difference - probably less than an inch??

[8GCBC]

I had a hydraulic leak that dripped down the rudder cable. The hydraulic fluid followed the cable and departed from the aircraft in flight with no trace of RED! It was almost impossible to find:

* The leak only happened when the pump was on i.e. taxiing, flying.
* The rudder cable was the perfect exit in flight to keep the RED stain from touching the aircraft.

I would think to myself how could I lose 1 QT. of 5056 without seeing any RED? I finally found the leak and the loose fitting during the annual

Hydro-leak.jpg
(click on the above photograph to see area)

[Anne]

I'm going out to the hangar later, and will check inside the fuselage, and down the cables. Yesterday, I added air to all 3 tires, but the mains had gone down equally. Thanks for suggestions, everyone.

Anne.

[FdxLou]

Anne
Could you have loaned it to someone and forgot?
Check the tach......

Lou

[mvivion]

The cross vent will cross feed, even with valves off. It wouldn't take much out-of-level to make a 3 gal difference - probably less than an inch??

Cross vent? In a stock PA 18 fuel system? First I've heard of one, but there are still lots of firsts for me. In any case, she's talking something other than a stock cub.

MTV

[12 Geezer]

Oh - well maybe not in a stock 18! I assumed there would be, just because there is in the stock 12. Yep, "assumed".

[OLDCROWE]

Shrinkage happens

[SteveE]

I'm with S2D on this one. I found a leak accidentally behind the fuel valve, never showed, but I could always smell fuel when I opened the door. It would leak and before it got out of the bottom it would evaporate. Over time, I am sure it could leak quite a bit, but I never did find that out.

[Cajun Joe]

Anne,
Odd there's no stain or odor.
Av or auto gas??

[sierra bravo]

Did you fill it with an auto shutoff nozzle? Not like those never give up prematurely. I'd suspect the previous fueling was less detailed than the current analysis, but I wouldn't waste any time trying to find three gallons of gas. I'd rule out the obvious potential leaks and go about my day.

[55-PA18A]

So pour in 3 more gallons and see what happens.

Jim W

[windy]

fly your Cub more, then you can justify the fuel level going down

[aktango58]

Do you ever smell gas in the cabin?

If yes, start pulling panels off and find the leak.

If no, I would relax about it, maybe check inside the wing and at the gascolator. Then go fly.

[Anne]

After running my hand around inside the wing and under the rear seat, and not finding anything, I give up. No smell, no stain, no nothing. I know the tank was fueled to the tabs because I checked it, and then used the same calibrated stick to discover the missing fuel. No-one has access to my hangar that would take 3 gallons from one side (they would use a couple hours worth!).

After I fuel up next time, I'm going to switch caps and see what happens. Maybe it's evaporation, like Chuck says.

Windy has it right - go fly, and then the fuel loss is justified!

Anne.

[mvivion]

Anne,

One more thought: Was the airplane perfectly level when you checked the fuel levels after fueling? If the plane is on a bit of a slope (and depending on dihedral, it doesn't take much slope) the uphill tank will take less fuel than the one on the downhill side. Now, park it level, and the tank that was uphill will appear "less full". That's due to the inboard location of the typical fuel filler.

I seriously doubt that much fuel would evaporate....even with a fuel cap missing.

MTV

[cubpilot2]

Anne:
I have seen a few collapsed fuel tanks on cubs. I assume caused by plugged vents with old style caps. (don't understand how on a gravity feed system but....)
If you by chance have a plugged vent and a collapse occured in flight... and then refueled and placed in a warming hanger the pressure from the expanding gases could pop the tank back into shape. This may account for the fuel quantity difference.
This may sound far fetched but worth a check. If you lift the fuel tank cover and find an odd wrinkle in the tank then it may be a clue.
Happy 4th of July

[Anne]

Thanks, everyone, for all the ideas. I'm still at a loss (so to speak), so I'll try the whole scenario again, checking what you've suggested before & after fueling. I was worried about a leak and starting a fire in-flight, but since I can't find any fuel outside the tank, I think it'll be okay. Anyway, stay tuned, I'll let you know what I find out soon.

Thanks again,

Anne.

[8GCBC]

Had a PA23-250C with a very slow fuel leak from the gascolator (right wing). About a gallon was lost in (4) weeks of storage. The 100LL drip left a blue round stain on the hangar floor. No way not to SEE the BLUE STAIN. 100LL avgas evaporates and leaves a heavy BLUE STAIN with only (1) gallon!

PA23-leak.jpg

Your results could vary.