Fuel loss

[redraider]

Hello all,

I have a '78J 201 (N201VG) with standard 32 gallon tanks. I have noticed on more than one occasion that after I fill the tanks full - to the tabs - and it sits for as little as a week or so and I return to pre-flight the tanks are nowhere near the level at which I left them. Each tank will be 2" - 3" down from where I left them a week or ten days prior.

I cannot figure out where the fuel is going. There are the tiny leaks around the rivets on the topside of the wing on both sides, but nowhere near large enough to lose inches of fuel. (I had a "C" that had seeps, leaky rivets, etc., etc., but never "lost" this much fuel. Smaller tanks, however, so less expansion?) There are no puddles of fuel; no drips or seeps that I can tell. Again, not near enough of a leak to loose that much fuel in that short amount of time. There is no overwhelming odor of fuel in the cockpit either. (Unfortunately, the paint color is almost identical to the color of the blue dye in 110LL, so seeps are difficult to see.)

I can only surmise that during the variable spring weather in Texas I have filled the tanks with cold fuel in a cold airplane and as the weather warms up, the tanks enlarge creating the 2" - 3" headspace in the tanks. Does this sound reasonable? Seems like a lot of fuel loss.

The loss is somewhat confirmed when calculating gal/hr. When departing with full tanks and refueling the same day I get 10.5 gph or so. However, after sitting a week or so and calculating fuel burn after a normal flight the numbers go up noticeably.

Has anyone else noticed this? The plane is hangared, so no possibility of theft.

Any thoughts?

[duke]

Possibly venting is slow from lower tanks and fuel is settling after air is vented.

[Hank]

You've got a leak, you just can't see it. For proof, you lose more in cold weather when everything contracts and less in warm weather when the sealant and metal parts expand. It's less visible when it evaporates fast in higher temperature, too.

Sorry to bear bad tidings. . . .

[N601RX]

Don't just look at the tanks. Also remove the 1st belly piece and exhaust cavity cover on the pilots side. It could be the selector valve, pump or any of the aluminum tubing under there. I had a problem with the fuel and brake line rubbing. There was an old service bulletin about it that was never complied with.

http://mooneyspace.com/topic/5456-brake-and-fuel-line-rubbing/?hl=%2Bbrake+%2Bline

[Rustler]

Red Raider--

 

This is probably a stupid question, but have you tried "filling it up," then waiting for, perhaps, ten minutes, then topping it off, then checking it a day later? This doesn't seem unusual to me. I've won several $20 bets with linemen who insist my J is full, only to find it'll take considerably more after a little settling time.

[aaronk25]

I don't think this is your problem but may have some humor to it. I kept chasing the "k" factor on my jpi which calabrates how much fuel the jpi thinks the plane is loosing. Turns out my pilots side gas cap was eaiser to open than my right side and the preload wasn't sent as high. (The bit under the cap wasn't torqued as tight).

On every "full" take off I was streaming the top couple gallons of gas out the back of the cap. The low pressure on top of the wing was literally sucking it out!!!! Didn't notice it until just yesterday. Damn. What was really screwing up my math for the JPI was when I'd run the tank low then add maybe 10 gallons, but didnt have the fuel anywhere near the top so it didn't suck out.

It was really screwing up my fuel calculations!

[Lood]

Mine was losing 2 gal per 24 hours without leaving any traces that are easily noticed. If a Mooney tank starts to leak, it does so well. Check under the carpet behind the front seats and under the belly, inside the cover.

I'm sure you should also smell fuel inside the cockpit after the airplane has been left closed up with full tanks.

[jetdriven]

Our plane has bladders, and I have no experience with wet tanks in a Mooney, but it takes 5 minutes for the fuel to distribute and even out, then you can put another gallon per side in after it evens out.

[ELT]

Hi Redraider,

I think your Mooney has 3 interconnected bladders in each wing. It is difficult to fill all bladders with 1 filling. You must ill the bladders, then refill them 2 more times to completely top them. I keep a 5 gallon can around. If bladders leak they LEAK.

[FloridaMan]

Also make sure the keyring drain in the floor is fully seated. I've come out to my plane and spotted mine dripping every so often. 

 

There was a story at a local airport where a number of people seemed to notice their planes weren't completely full. Turns out there was an employee at a paint shop on the field stealing fuel from planes. 

[redraider]

Thanks for the responses. I topped it off yesterday, so I'll check it today and explore some of the probable causes described above.

[carusoam]

My favorite M20C "easy" fix we're the seals at either end (inboard and out) of the fuel tanks where the level gauges are mounted. Mine leaked, they were made of cork. I found them, my mechanic fixed them for a couple hours labor?

If you are not familiar with where these are. Use caution to not accidentally open inspection panels into the tank. Using powered screw drivers is dangerous near fuel tanks and leaks.

This stopped most of the fuel smell in the cabin and blue stains under the rug.

If you have this challenge, you want to get on it quickly. Having a rug soaked in 100LL is bad...

Bet regards,

-a-

[redraider]

I checked under the rugs behind the front seats. No evidence of wetness, staining and no smell of fuel in the cockpit - some good news there I suppose. Leaves two options; a leak in the under belly that is "holding" the fuel to evaporate and not leaving evidence on the belly or hangar floor, or not venting enough when filling. I hope it is the latter. I will have more time to explore this weekend. Thanks for the suggestions. Stay tuned. I will report my findings.

[jetdriven]

You can have a fuel leak on those inboard senders and it wont necessarily wet the carpet. The floor does not extend to the wing root rib, there is a bog gap in the floor just behind the sidewalls. The fuel runs down that and evaporates from the inside of the belly skins.

 

Further reading, our fuel selector was leaking, it was last rebuilt in 1985.  We noticed unsteady fuel pressure above 9K feet, and it appeared to be drawing air. Disassembly showed the blue stains, at the floor level and leaking to beneath the floor. 50$ worth of O-rings we are doing great.

[jdrake]

You might not have a leak.  Ever considered the geometry of your tanks and how the differences in "level-ness" between your tie down/hangar and the pad at the fuel pump might impact how full your tanks appear?  When I top off my plane at the pump, I fill it to just over the flange - no more room to fill.  I'll then taxi the 50 yards to my tie down and check the tanks again where I routinely see maybe 1/2 inch of free space.  Not as significant as the 2-3 inches you report but meaningful nonetheless. 

 

Perhaps try topping the tanks in the tie down or hangar with fuel cans.  Then inspect after not having moved the plane for a week to see if there is an appreciable difference.  Just a thought.

[OR75]

2 to 3 inches is quite a bit to be due to some settling in my opinion. since it happens the same way in both tanks, have you tried to shut the fuel valve ( inside the cabin) ?

i found it strange you cannot detect the smell of gas by opening a few panels around the tanks.

[redraider]

After many months of dealing with my mysterious fuel loss, I believe I have found the main culprit(s).

On the ramp a week or so ago, during some pretty cold weather, I was rocking the wings (for reasons I won't go into here) and saw fuel sloshing out of an underwing inspection plate just outboard of the tank itself. Today, after removing the plate the fuel loss was discovered. The fuel gauge sending unit was clearly leaking and had been for some time - especially when the tanks were full and especially when cold. The 5 or 6 screws to the sending unit were all loose and very darkly stained by fuel.

There was no dripping to the ground because the fuel was leaking into the wing cavity and evaporating before it could find a way out (especially in hot weather). When it did find a way out, the long distance the fuel had to travel down the wing to the wheel well and eventually drip off the gear allowed for lots of time for the fuel to evaporate and not leave any tell-tale blue drips on the hangar floor. This was further exasperated by the color of my bird - Aviation Fuel Stain Blue. Until I saw the fuel sloshing out from beneath the wing, the leak was virtually undetectable.

For good measure, we checked the other wing and found the same problem: loose screws, fuel leaking into the outboard wing, and very little staining. We tightened the screws and added a good dose of pro seal around the sender fitting. After a few days to set and cure, I'll fill the tanks and report back.

I believe my fuel burn numbers will come down substantially now. A repair that pays for itself.

Thanks for all the input.

[rbridges]

glad you found the problem.

[kmyfm20s]

Isn't this something that should be seen at the annual with all the inspection plates off? Just asking so I know to tell them to look next annual. Glad you found the leak.

[redraider]

That is a very good question and one for which I have no answer, other than I have done 10 plus owner assisted annuals and never have I seen every inspection panel removed. I was never 100% sure I had a leak until few weeks ago, so it probably wasn't as urgent an inspection item as it should have been months earlier and in the heat of the summer (July annual). I believe the cold of winter allowed the leak to better manifest itself and therefore easy to overlook or miss in the hot south Texas heat. Lessons learned. It will be checked at annual from now on.

[carusoam]

If you are doing the owner assisted annual, it is easy to take them off. Just don't open the tank ones...

Best regards,

-a-

[redraider]

Pro Seal has cured, tanks filled... Still leaks. We slowed the leaks from the sender screws, but there is still a substantial leak from somewhere else. I suspect from the tank itself. Back to the drawing board. Ugh.

[mike_elliott]

Pro Seal has cured, tanks filled... Still leaks. We slowed the leaks from the sender screws, but there is still a substantial leak from somewhere else. I suspect from the tank itself. Back to the drawing board. Ugh.

 

Since your in Texas, Might I suggest you take it to Don Maxwell, he will find it and fix the leak in short order. If it requires a total strip and seal, he will send you to the best, Paul Beck at Willmar Mn. I cant remember what Don charges for a patch job, but it is well worth it. Less than a .5 AMU I believe. (A plug for both, they are both sponsoring the Mooney Summit, as are a number of great companies with door prizes)