Inboard fuel tank

jhickam · #1 04-19-02, 10:35 PM

I have a 1973 337G that has a leaking inboard fuel tank. We are having a hard time trying to get access to the hoses on this tank. The service manual is of little help in this area. Does anybody know if you have to remove the outboard tank to remove the inboard ? Any help would be greatlly apprieciated on this subject.

SkyKing · #2 04-20-02, 05:28 AM

If memory serves me, I think your first name is Jim...

Anyway, Jim, without the manuals in front of me tonight I seem to recall the 1973 models having only three individual tanks on each side, as compared to four on the later models, and the interconnect hoses are reachable through the underside inspection panels. Have you been able to get to all of them?

Ours is a later P-model and during annual we went through and checked the clamps... all of them were loose and you kinda have to be a contortionist to reach the upper hose clamp on the outboard side (especially if you have the Robertson/STOL conversion, where they install a different tank in the outboard position).

BTW, just as a side-note to those who have a 1977 P-model with the Roberston/STOL, your useable fuel is NOT 148 gallons... it's 143, or 71.5 per side, due to your outboard tanks being somewhat smaller than the Cessna factory installation.

In your case, I think I'd make sure those interconnect hoses are not the culprit first before entertaining the idea of removing the wing-panels, but be careful not to overtighten them or you may cause a leak. Next I'd check the inboard quick-drain valves. From there, a check of the upper side inspection ports... but to do this you do have to remove the upper wing panels to gain access and the manual goes into some detail on how you MUST prop-up and support the underside of the wing and the tail boom PRIOR to removing any of the screws up top side, otherwise you'll get some distortion and you'll play hell getting things back together again.

You seem to be pretty sure your inboard tank is leaking... but are you certain? And does the leaking occur all the time or just when the tanks are topped off?

Hope this helps a little...

SkyKing

jhickam · #3 04-20-02, 11:44 AM

Our Skymaster has 2 outboard and 1 inboard tank per side with a total of 118 gal (708lbs). We have supported the boom and wing and removed the top of wing covering the inboard tank. We still cannot gain access to the hoses coming out of the outboard side of the inboard tank. The manual provides very limited information on this subject. Buy the way we have found the leak, it is a cracked inboard tank same as last year but a different tank.

Jim

SkyKing · #4 04-20-02, 03:30 PM

Jim,

In order to reach the interconnect hoses between the outboard and inboard tanks - where they go through the boom area - you have to remove a contoured cover on the bottom side of the boom about half way between the strut to the backside of the wing. As I recall it has about 24 screws and once that's off, there's a flat rectangular plate up inside above the trim cables with another ga-jillion screws... and as I recall, it is behind that plate where you'll find the hose clamps.

SKyKing

jhickam · #5 04-20-02, 05:08 PM

Skyking,

Are you speaking of where the sumps are on the booms. I have inspected the area and it does not look like there is access even if I remove the sumps.

Thanks

Jim

kevin · #6 04-20-02, 11:21 PM

Jim,

While reading your posts, I noticed you said you have 118 gals of fuel. I think you said you have a '73, why do you think it is 118? (I imagine you have a good reason, I just want to learn.) I think the manual says 123... I have I always had 5 gals less fuel on my '73 P337 than I thought I did...bummer...

Kevin

SkyKing · #7 04-21-02, 02:11 AM

Jim,

I'm presuming you have a Cessna parts book for your plane. As mine is pressurized and a later year model, there might be some slight differences, but I'll try to direct you to the right place so you can identify the part I'm tryng to direct you to, i.e. the contoured cover I mentioned above.

If you'll look for Figure 3, or perhaps another number on yours, but look for "Complete Wing Assembly" and Item #46 and #28. #46 is the contoured cover on the lower half of the boom, located about half way between the strut attachment point and the aft edge of the wing... this contoured cover is probably about 12 to 14 inches in length... under this cover is an access door, Item #28 and there are 28-screws to be removed.

As you look at Figure 3, the area you're looking for is in the middle of the boom area shown.

Let me know if this helps... otherwise, you can e-mail me direct.

SkyKing

jhickam · #8 04-21-02, 01:32 PM

Kevin,

According to my POH it shows 708 pounds of fuel but I believe that is useable fuel. The tanks may have a total capacity of 123 gallons I will have to look. This is all from memory, I will check this week when I get back out to the airport.

Jim

SkyKing · #9 04-26-02, 02:52 AM

Jim,

Were you able to get to the hose connections through the boom access point I described above?

Let us know what kind of progress you're making and any probs we might be able to help with.

SkyKing

jhickam · #10 04-26-02, 12:07 PM

Skyking,

We are in the process of removing the fuel tank cover on the two outboard tanks. Maybe if we remove one of the outboard tanks it will allow us access to the hoses to the inboard that interconnect them. If anybody has a better way I would love to hear it. Removing the top of the wing over the tanks is hard to do because of the rustsy screws that have also been painted over.

Jim

SkyKing · #11 04-28-02, 04:40 AM

Jim,

I still maintain that you should remove the access door inside the lower part of the boom, just behind the strut to wing fairing... there are 28 screws holding the "U" shaped panel on the boom and once that's off, there's an access plate/door with another 28 screws up above the trim cable to remove... and it is through this door you can access the hose clamps.

Now, what you may run into is this: whoever did the last install of the hose clamps MAY have installed them through the wing access covers up topside, and if they did, chances arr you won't be able to loosen them from below. We discovered this little scenario on our outboard tank in the left wing, but we were able to manage a specially made "L" shaped screwdriver up through the bottom inspection panel to reach it, but it was a pain - big time.

For others reading this thread, if you have a Robertson/STOL conversion on your bird, and again depending on the model year of your Skymaster, Robertson may have removed the outboard factory tank and installed a slightly smaller tank so as to accomodate the extra linkages necessary to run the ailerons in the flap mode. And if they were in a hurry and not paying attention, you may have some interconnect hose clamps that are facing upward, instead of downward so they'd be accessible through the normal wing inspection access panel. Of course, what I'm stating here is dependent on the model year of your Skymaster, as each year had a different tanking system. Our's is a 1977 P-model and it has four tanks per wing and because we have the Robertson/STOL conversion, our usable fuel is 71.5 gallons per side instead of the nominal 74.

Hope this helps.

SkyKing

FRED-E · #12 04-28-02, 08:45 PM

SkyKing

I have been trying to follow this thread, and I have one question, what year is your skymaster and does it have a sump in the boom like my 337B 1967 does. It sounds like his 1973 does have sumps in the booms, and if that is the case maybe he has to remove the sump to get to the "door" (my book shows 28 screws) that you are referring to. Just a thought.
Fred N358

Kevin McDole · #13 04-29-02, 04:14 AM

Jim,

Remember to use structural screws when you replace all the rusty screws on the top of the wing. Don't simply use stainless screws. If you want stainless, then you specifically need structural stainless screws.

jhickam · #14 04-29-02, 03:41 PM

Fred,

It sounds like Skykings P337 must be a bit different. I have removed the bottom cowl of the boom and all it does is give you access to the fuel sumps. I have tried to look up above the sumps for a removable panel but I have not seen one. I believe this cowl on mine has 28 screws also. Thanks

Jim

SkyKing · #15 04-30-02, 03:28 AM

Jim, Fred, et. al.

Apparently the 1973 "G" model is quite a but different than our 1977 P-model. Ours has four tanks per wing, and the two quick drains are located on the innermost tank - between the boom and fuselage.

Now you begin to realize why GMAS always asks, "what year, model and serial number", because they're all just a tad-bit different.

Anyway, Jim, let us know when you get the culprit tank out and what you find.

SkyKing