8KCAB fuel leak

W

wpbarnar

New member
While preflighting my '99 8KCAB this am, I discovered av gas dripping out of the weep holes in the fabric on the trailing edge. It appears as a small amount of fuel had accumulated inside the right wing and started dripping out the weep holes after I moved the airplane. I pulled the gap cover between the wing and fuselage inspecting the fuel lines & connections and found all OK.

The airplane is flown regularly. I topped the tanks off yesterday and the plane sat a couple hours before flying it for 45 minutes. It was in the hanger overnight before discovering the leak this am. I monitored it carefully today and estimate only a few ounces leaked out in total. Very small leak or perhaps limited to when the tanks are full. The plane was flown inverted during the short flight.

Is there anything else to inspect before becoming intrusive, removing fabric and pulling the fuel tank?

Any other ideas or recommendations?

Bill
 
Bill,
We had a fuel leak in the right tank on our 74 7ECA. We found that one of the hold down straps had rubbed a hole in the top of the tank near the front. We only got a leak when we had full tanks as the hole was on top in front.
We had to cut a couple of holes in the fabric before we found the leak.

Ron
 
Bill,

In my experience, there are a couple things that can lead to this.

This past summer I had the tanks in my 1977 Decathlon topped one day and I believe that I may have replaced the cap incorrectly leaving it a bit cocked. The result was that some fuel got siphoned from one tank during my flight and some of that ended up inside the wing and exited via those weep holes. I've not seen any blue stains since, which leads me to this particular conclusion.

The other source of this, which I fear may be more common in the later model Decathlons, is that the fuel tank is busted. I've observed this personally on another Decathlon of similar vintage to yours, and I know of other examples of this problem.

What happens is that when we go do our aerobatic thing, there's typically a good plug of gas that sloshes back and forth in the tanks. That gas basically flows, or crashes, through the baffles. Those baffles are secured by a series of small tabs which protrude through the skin of the tank and are welded to the tank skin. Over time the action of the gas crashing around flexes the baffles and breaks those welds.

There are two solutions to this problem, neither of which you are going to like. Both require removal of the tank, which in turn requires a lot of fabric work.

You can have the broken welds re-welded and the tank re-installed.
But these welds will likely break again down the road.

You can buy a new tank from ACA. My understanding is that ACA is building tanks with a thicker sheet of aluminum, which supposedly cures this problem.

If the leak is a recurring problem then it's likely a broken tank and you will need to fix it. If you do not fix it then over time then the gas will compromise the glue that holds the tapes, etc. together and fabric will start to come unglued.


Sorry to convey an expensive-sounding message, but this is what I've observed in these airplanes. Our local aerobatic school operates a Decathlon (878AC) and over the past few years I know that they've repaired or replaced a gas tank three times.

Regards,

Farrell, N1126E
 
Actually, its throwing a third bead on the outside weld that seems to do it...and we significantly reinforced the center baffle in recent years.

Acro and fuel tanks are NOT friends! Never have been.
 
Hi Jerry,

In your estimation, is it a coincidence or a fact that the gas tanks in the planes with wood spars fare better? Or is this basically a wash with the better baffles and welds?

Regards,

Farrell, N1126E
 
The data is like a shotgun. IMO has nothing to do with the spars.
Has more to do with the whole assembly being hand made and expected to support loads athat are difficult (at best) to model. Well, until we get that NASA funding... :lol:
 
Hi Jerry,

Just a wild thought: I wonder if you folks could get help from a school which features aeronautics/aerospace programs. You'd like to get stress analysis data on the airframes and perhaps model some improvments, and the students could get an interesting real-world learning experience helping you with that exercise.

Regards,

Farrell, N1126E
 
It is a good idea, but you have to time your project with school semesters. They are great for reducing costs on tests you dont want to do, like vibration.
 
Hi Jerry Jr. It always a pleasure to talk to you as I did a few weeks ago regarding that control stick question. My question now is: You mentioned a while back that the factory had, in recent years, added an extra bead of weld on the baffle tabs and increased the wall thickness of the center baffle....would my 2001 8kcab have been made after these improvements were instituted?
Jerry, do you ever get the feeling that you're the answer man :)

Thanks

Tom
 
Jerry, folks,

First of all, Merry Christmas to everyone. I hope folks are having a good holiday.


I was speaking recently to my mechanic; he finished up the annual on my plane a short while ago. We talked a bit about wood spars (mine appear to be just fine fortunately) and we spoke about fuel tanks and leaks.

My mechanic has on his wish list that a factory-approved mod be created that would allow for the installation of a removable metal panel on the wings above the fuel tanks. Kinda like the panels above the strut attach points but obviously lots bigger. Then if the tanks need to be serviced it would be lots easier and less costly to do that.

I've little idea what work is involved in creating such a mod, particularly, gaining the feds' approval these days. But I didn't think it would hurt to ask. :)

Best regards,

Farrell, Decathlon 1126E
 
1) a 2001 would have SOME of the changes. We added more slosh holes and then added a bigger flange for a bigger bead and then added 2 stiffeners. You may or may not have the extra holes. Even if you dont have the extra items, it doesn't mean you are prone to fail.

2) You honestly dont want a skin on top. Its several pounds of weight and the rivets like to get beat upon by the prop wash. When we did the 8GCBC we developed a skin for the top at the request of the bush pilots. Well, its back to fabric now. Oh, and it did not seal very well.

Once we give birth (thats what it feels like) to the Champ, a clam shell tank for the 8KCAB is still on the list. The clam shell is great for the 7's. We just need to increase the size and make a good 'slosh wall' for the 8. Oh, and bite the bullet for the tooling. ouch.
 
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