Does failure to top off fuel tanks eventually cause fuel leaks?

[Dale]

I looked through the Mooney forum for this answer but didn't see anything specific.  Does anyone know if you fail to top off your fuel tanks (when you hangar or park your plane), does that eventually cause leaks?  I spoke with someone at my airport working on their Mooney and they were doing a reseal of their tanks.  They mentioned that if you don't keep you plane topped off that the top seal becomes brittle and the seals are more likely to fail and the tanks are more likely to leak. 

 

I didn't think keeping the tanks topped off would keep the seals from failing.  I assumed that the failure of the seal was due more to age, the seal just becomes old and more brittle, and when the plane is exposed to constant heat (or constant extreme cold) sitting on the ramp, the heat (or cold) eventually dries out the seal and it fails. ( I have also heard hard landings may be a culprit)  I keep my Mooney hangared and so far, I have not had any fuel leaks (knock on wood), however, I do not keep it topped off and I don't like to fly with the extra weight when I don't need to.  Most of my flights are under 300 miles.  I generally keep about 50 gallons in the plane.  Also, I have long range tanks.  I never top the long range tanks (If I ever topped the long rang tanks it would eat up a lot of my useful load).  So on long trips I just put 5 or 10 gallons in each long range tank after the mains are topped off.  If topping the tanks off helps make the seals last longer then I guess my long range tanks will go first because I never top them off?

 

So after all that longwinded explanation, does anyone know whether the failure to top the tanks off really leads to early seal failure and, consequently, a fuel leak or is this just an old wives tale?  Thanks.

[OR75]

think of it in term of temperature and temperature cycle. plastic type of material do not like high temperatures or temperature cycles. a full tank act as a reservoir of temperature.

if a planeis sitting in the sun, probably best to keep the tank full.

[N1026F]

Call Edison at Wetwingologists, he'll give you the straight poop on it...

[Piloto]

If you keep your plane in a hangar I would not worry about it. The sealant like any rubber is susceptible to harden and become brittle when exposed to excesive heat. The leaks are caused by minor cracks on the hardened sealant just like those you see on vinyl seats exposed to the sun. To reduce the sealant deterioration it is recommended to leave the tanks at least half full when the plane is left out in the sun, specially in the summer. The fuel helps to maintain a lower temperature. The upper part of the tank deteriorates earlier due to the direct sunlight exposure and minimum fuel contact. Occasional exposure at FBOs ramp will not create a siginificant degree of degradation. My 30 years old M20J (factory sealant) never had a leak, but it has been hangared all its life. Hard landings do not cause leaks. If they would you would have more serious structural problem than a leak.

José

[FBCK]

I also have long range tanks and had a real seal done last year by Paul in Willmar. I also never really used the long range tanks, you can go for 6 hours with the regular tanks, the longest I have ever sat in the plane was 5 hours and I needed to get up and stretch after that. I asked Paul about removing the long range tanks as they rarely had fuel in them and I didn't,t want to have to do another reseal in 5 years or something of that nature, Paul said as long as the plane is hangared it shouldn't,t happen, for the reason quoted above.

[N201MKTurbo]

My 67 F never leaked. It spent half of its life outside in Colorado in sub zero weather, and the other half of its life outside in Arizona where it is hotter then hell ( I love it ). I never fill my tanks after a flight I fill them when they are empty. I owned that plane for nineteen years.

My current J leaked like a sieve when I got it 10 years ago. It is 10 years younger then my F. The previous owner was a hard lander, I can set a Mooney down like a feather.

So I did everything wrong with my F except landing hard, and it never leaked a drop. The previous owner of my J did everything right except for his landing and it leaked everywhere.

One other difference between the two planes was the F always lived in low humidity, and the J always lived in the southeast. There is no corrosion on the J.

One other thing that may be a factor is I flew both my planes at least once a week and most times two or three times a week.

I don't think there has ever been a study done about what causes Mooney tanks to leak. After all it would take thirty years to do.

Asking the repair stations is a waste of time also, they don't know how the plane has been operated for the past thirty years, they just know its condition when it shows up for repair.

I know there are plenty opinions about the subject, I would take them all with a grain of salt, including mine.

[Piloto]

My 67 F never leaked. It spent half of its life outside in Colorado in sub zero weather, and the other half of its life outside in Arizona where it is hotter then hell ( I love it ). I never fill my tanks after a flight I fill them when they are empty. I owned that plane for nineteen years.

 

Well that's one way to keep it from leaking.  

[RJBrown]

I also have long range tanks and had a real seal done last year by Paul in Willmar. I also never really used the long range tanks, you can go for 6 hours with the regular tanks, the longest I have ever sat in the plane was 5 hours and I needed to get up and stretch after that. I asked Paul about removing the long range tanks as they rarely had fuel in them and I didn't,t want to have to do another reseal in 5 years or something of that nature, Paul said as long as the plane is hangared it shouldn't,t happen, for the reason quoted above.

My understanding of the long range tanks is the line between them is open and allows fuel to flow both ways.  Fuel will flow from the regular tank into the long range tank if you never fill the long range tank. With both tanks full fuel will flow out if you remove the regular cap. I think you may be carrying more fuel than you think. This post assumes your tanks are plumbed like the Monroy ones I had in N231NH.

[FBCK]

Yes I have the monroys and they do as you described, if I fill the mains to full, after 5 minutes or so there will be an inch or two the the outboards, but that leaves about 6 inches dry in the outboard tanks, which I never fill and you are correct I always have more fuel than I think, but it bette to have too much then not enough. It's difficult to get an accurate reading on the fuel when it going from inboard to outboard. I do have the Monroy manual, but I don't care to take my ruler out and measure the outboards.

[carusoam]

Ways to make an old Mooney leak... (10 years of M20C experience)

(1) let the o rings in the fuel cap get old, water will leak in.

(2) let the non SS fuel neck rust, water leaks in there at a very high rate.

(3) let the rust bits (from the same source) collect in the sump valve at the bottom of the tank. Water and fuel with both leak out.

(4) let the plane get old, the seals around the fuel level sensors at both ends of each tank will leak fuel. Inner end will soak the carpet in the back seat.

40 years without a reseal....decades of living outside in NJ.

None too challenging or expensive to fix.

I only filled the tanks PRIOR to flying.

That was my single point experience. Ymmv.

Best regards,

-a-

[butchgilbert]

I was told that the fuel fumes actually keep the fuel tank sealant pliable, not the fuel itself, that's why I only fill the tanks before I go flying.

[Alan Fox]

Hard landings and age are the reasons that make the most sense.....The polysulfide gets hard and brittle with age , hard landings will cause rivets to loosen up over time......