Inboard fuel tank

[jhickam]

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]

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]

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]

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]

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]

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]

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]

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]

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]

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]

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]

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]

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]

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]

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

[jhickam]

Skyking

Well we got the inboard tank removed and and it was cracked as we thought it would be. We had to remove the top fairing of the boom to access the hoses and tubing between the inboard tank and the middle tank. We hope to have it flying in about a week with any luck. One problem with removing the top of the wing over the fuel tanks was that at a previous time somebody drilled a few of the screws and broke off screw extractors in the screws. The best thing I found to do was to drill out the broken extractor with a small masonary bit with a sharp cutting edge ground on it. You must use a green stone to grind the carbide on the masonary bits or a diamond wheel.

Thanks to everyone for the help.


Jim
N1871M

[GMAs]

hate to say it... but, you have to cut the hoses ... to get the tanks out... else you will bend the fittings that are welded on the things... it works the best... using a hacksaw blade by itself... and making sure you don't get the fitting on the tank... split the hose...

They get hard with age and you can't get them out any other way... now when you put them in...use new hoses and lots of engine oil... on the inside and around the fittings... I use a mixture of 50/50... engine oil and stp...and makes the hoses slide on...

On the inside tank... their are some little buttons on the underside of the wing... that we used to call chassis plugs.. then you reach in with a screwdriver and undoo the clamp....

Now when you go to put them in... or take them out... start with the outside tank and work towards the wing root... and when you put them back.. start on the inside one and work out to the tip... even though you only have to remove one tank... it is a lot less time to break them all off and re-do them... with new hoses...

AS to tightening the clamps.....Don't go using a lot of pressure on the clamps... the hose clamps will make a flat spot on the inlet/outlet fittings... try and put the clamp back exactly where the old one came out... and when you tighten it up... only 1 to 1.5 turns after tight... only....

You can always go back and check it if its leaking and tighten it up.. but, if you do too much... you will crack the fitting and then you get to take the tanks back out again...

As to the gaskets up on top for the inspection cover... and fuel sender... New...New... new... and use the right sealant... and torque on the bolts and then safety wire.. as you can't do it after the wing cover is back on...

While you have the inboard tank out... make sure you check the flapper inside that seals the pocket... you may want to replace it if it looks old...hard... or shows signs of cracking...

and use plybond on the cork tank straps... with masking tape...only... other stuff will cause the alu to start to corrode... clean the bottom of the wing where the tank sets is a good time while they are out... and when you go to put the tank straps back in... you don't again torque them up to where the tank starts to flex... just tight enough to keep the tank in place when empty... empty.... the wing top and bottom keeps it from going up and down... and if you torque the straps up too much.. your going to see the bottom of the wing start to show signs of stressing at the rib... (bending)

do not use actone or mek... on any of the wing pannels... as they have the hat section epoxied to the sheet... if you cause it to break free... you have more problems... so only stodard solvent or a product called aluaprep... and then real weak solutions... paint brush and lots of flushing after you get done cleaning the wing panels... just use water... it will delute out... and not hurt a thing... but, it will clean the alu....

Sorry that I didn't get back to you sooner... but, this is the only computer that seems to work... to get in to the board... the second and third one still can't make it... and even though I was reading the mail... could not get back to respond to you... but, were working on it... G.M>

By the way... here is a little trick we learned from boeing.... they dip the screws in zinc cromate.. the good stuff... and then put them in wet... it keeps them from rusting... and taking the cad plating off... when you run them in... then next time you have to take them out... they will come a lot easier... smile... G.M>