I have gas caps that leak when I go flying. This is evident by the fuel streaks down the top of wings from the filler necks after I land. I suspect this is from the position of the filler neck & fuel caps is steep turns. High wing tank leaks in a turn. I also noticed that the right fuel tank level was always higher than the left after going flying.

The tanks would start out level if left in the hangar over night. I went to dip the tanks after flying and heard a "whoosh" upon opening the right fuel tank...."a vacuum condition". I tested these tanks for fuel flow during construction and no problems. I can drain the tank by selecting the right tank, it will drain but takes much longer than the left tank to do so. For now I replaced the cap with a spare I know works off my CH601HD header tank. My CH601HD uses a CESSNA vented capP/N C156003-0101 or Aircraft Spruce #06-00985.

This is a vented cap with a red rubber anti-leak check valve on the inside of the cap. I removed the rubber anti-leak part so I can pump fuel from the leading edge tanks into the header tank without over pressuring it. To solve the messy gas leak stains issue on my CH750 I will install this same vented cap with the red rubber check valve removed. This will prevent the fuel tank in the wing of the CH750 from building pressure as this fuel pressure could possibly build to the point of overpowering the 3 psi Rotax 912 carb floats limit or even bulging the wing skins while sitting in the hot sun. Just like how a red plastic gas can goes sitting in the sun with gas it it. These CESSNA fuel caps dont leak with the rubber anti leak check valve removed, it solved the constant gas smell in my CH601HD cockpit issue.

Fly safe.

BobFirst Home C-IKIM 006.jpg

The picture shows the right wing fuel level lower than the left, the Dynon D180 is calibrated accurately.

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Comment by Don Herbel on February 4, 2013 at 6:47pm

Guys: I have noticed uneven fuel drain between tanks (CH701) during my phase one 40 hours.  Most flights were under 1 hour and rarely did I start the day with full tanks.  Last week-end I flew my first 150-mile cross country in perfect CAVU conditions both ways.  I started first leg with full tanks and landed 1 hour and 50 minutes later with my right tank reading empty and my left tank still reading full.  Visual inspection of the fuel levels confirmed the gauges were correct.  After two hours on the ramp (empty wing higher than the full wing on a sloping ramp) about 1/4 of the full tank had traveled to the empty tank.  I put 7 gallons in the empty tank which put both near full again.  I switched fuel caps to see if a venting problem might exist.  Blessed with a tail wind both ways (return flight was the next day) I flew back home in the same amount of time withl the same result: right tank  reading empty and left tank reading full.  I checked the empty tank with my new Fuel Stik and it did not register any fuel at all.  This morning both tanks measured equal.  Has anyone experienced this radical of an imbalance?

Comment by Bill Bear on October 21, 2011 at 3:54pm

I found two interesting analyses on this subject by the Cessna Pilots Association.  There were two sections at this link:



.... one about fuel caps and another about uneven fuel feed.

Also, this pdf file went into greater detail.  Very informative.

I also found a CPAs Tech Note 003 on Uneven Fuel Feed that gave me a better understanding of the dynamics and concerns involved with this issue.  If I can find a way to upload a pdf file to this post, I will. 



Comment by John Ellis on August 5, 2010 at 8:21pm
Just an observations. If your turns are perfectly coordinated the tanks will not leak from the cap. The G vector is perpendicular to the wing, even in steep turns. However, if you slip, slide or pull negative Gs all bets are off.

So as the song says.... Slip Sliding Away :-)

Comment by Chris Aysen on August 2, 2010 at 7:17am
Observation on fuel level this past weekend: My right tank was lower than the left. It was reading a little over 3/4 as compare to full on the left (VDO gauges). Visually inspected and confirmed. Took off and flew for 1.1 hours about 35 minutes into the flight gauges equalized. Upon return visually inspected and confirmed same amount in each tank. Just an observation.
Comment by Mike Hammond on July 31, 2010 at 5:54pm
I should have added that if you have a three or four position fuel selector you can feed from the left or right tank only and keep the fuel load as balanced as you wish. If operated in the both tanks position you would experiance the same uneven tank flow than if you had an on/off only valve. Maybe it is possible to fabricate new gaskets for the "tractor style" gas caps. My Piper has some part number gas caps that gaskets are no available for. I went to the local rubber supply house and found some butyl rubber material the correct thickness (actually a few thousandts thicker) and cut out some new gaskets. They work great.
Comment by Bob McDonald on July 31, 2010 at 3:19pm
I have an Andair 4 position fuel selector. The other issue here is the ugly fuel stains down the top of my wings that comes from the leaking "tractor style" gas caps. This stains the top of your wings and leaves a gasoline smell around your aircraft. The CESSNA fuel caps don't leak like this in a steep turn.
Comment by Mike Hammond on July 31, 2010 at 9:13am
The Cessna 150 that I used to own, as well as every other C-150 in the world has this same fuel flow issue. like Mike said it will eventually even out and should not really be an issue. The "whooshing" sound when a cap is removed would indicate that the cap is not venting correctly, and it should be fixed or replaced. One way to solve the fuel flow issue is to install a three or four position fuel selector.
Comment by Chris Aysen on July 29, 2010 at 8:01am
I'm going to 3rd motion the uneven tank levels. I noticed myself after flying uneven tanks. But would even out after sitting 24 hours or so. That proves you have different flow rates from the tanks. I have a giant gasculator (about 2 quarts worth) so I'm comfortable with my fuel availability. But this does seem to be rather confusing. I'm suspicious of distant to artificial (fuel pump) fuel management system rather then pressure differences; can't prove it though. My fuel feeds the engine from the left side (as viewed from pilot position) of the fuselage; which is also where my gasculator is positioned. I have a transparent fuel filter at my left side where I can visibly watch fuel flow. I scan it just like the instruments; looking for air bubbles..... Bob, I'm going to look into that Cessna cap.
Comment by Ken Ryan on July 28, 2010 at 10:18pm
I'm confused. Both Bob and Mike report a "whoosh" upon removing a cap. What is the scenario whereby the (vented) cap is not functioning correctly? Is there any conclusion other than the cap is not doing what it needs to do (vent)? Or, to put it another way, how could you get the "whoosh" if the cap is functioning?
Comment by Mike Schlichtman on July 28, 2010 at 9:00pm
Bob, I have been going round and round with the fuel flow from the tanks. I had a long talk with Caleb about the fact that the tanks don't empty evenly. I even replacedone cap because I suspected it was not allowing air into the tank due to the fact that I once heard a "whoosh" of vacuum air when removing the cap. Roger sent me a new cap and that solved the vacuum issue. But it is a known fact (according to Caleb) that the tanks will not drain evenly. Everyone of the factory planes do not use fuel evenly from the tanks. It is due to pressure...high or low....created by the wing that causes the tanks to drain faster than the other. I have flown enough now to realize that it is nothing to worry about as the tanks do drain, they will eventually even out before you would drain one and not the other. I have flown down to about five gallons and at one point I would have 6 to 8 gallons more in one tank...but the flow will eventually increase from the fullest tank. Just my observations...hope this helps.

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