P337 Right Inboard Fuel Tank Leak + Pictures

[SkyKing]

In case any of you happen to top-off your tanks one of these days and the line boy says, "HEY... you've got fuel running down the side of your fuselage!"... know that with the all-metal tanks on our Skymasters, the MOST USUAL thing that happens is, the cork gaskets on top of the inboard tank usually dry out or sjrink - as the case may be - and when they do - well, it kinda grabs you by surprise. And the worst part is thinking the worst thoughts, like.... "Oh, my God, I hope the tank didn't develop a CRACK!!!" Not to worry too much about that, since the design engineers at Cessna had the supplier/vendor do some fancy metal work on those tanks with ridging to absorb the load and flexing. They are really built quite well, eliminating most chances for a crack developing.

Last summer, or was it the summer before... I forget now... this very thing happened to our '77 P337. I was a couple hundred miles from home, picking up some cheap juice -- ah, back when it was $1.90 out here on the left-coast! Anywho, as soon as the lineboy yelled, he at least had the presence of mind to grab one of those 30-gallon plastic garbage cans and rags, and we did manage to catch some of it. It's really disconcerting to see all that fuel streaming down the side of your beautiful paint and plexiglas windows, not to mention the anxiety of a spark or something lighting off all that fuel on the ground... and POOF! Goodbye aeroplane!! Luckily, nothing like that happened. Whew, that could have been a REAL nightmare.

First thing we did was take a pair of dikes and cut the safety wire on the inboard quick-drain. By chance there were a few of those red plastic 5-gallon gasoline cans in the line-shack, so with a five-foot ladder in place under the wing and the help of the lineboy, we pulled the quick-drain and drained off 5-gallons at a time. You gotta be quick and have a few rags so things don't splash all over, and momentarily placing the qucik drain back up in the hole while another 5-gallon can is placed into position.

By the time we drained off five of those 5-gallon jugs, the gushing fuel down the side of the cabin went to a trickle... and then nothing. Because of the diehedral of the wing the fuel tended to level out in the four tanks... and thus the leak stopped, since it was no longer filling the inboard tank to the top at the rear. So, the quickdrain was reinstalled, resafteyed, and after moving the airplane to a safe location -- away from the spilled fuel -- we prepared for a night liftoff and head for home. I recall my words to the lineboy, "If you see a Roman Candle in the sky ___________" (you fill in the blank!)

On the '77 P337 there are THREE inboard tank inspection ports on the top side: One for the Penny Cap fuel sensor wiring at the leading edge nearest the cabin; one that's smack dab in the middle of the tank; and one at the aft-inboard corner nearest the cabin. Farther below we'll show you a sequence of the pictures of the before and after. The culprit in this case was the aft-inboard gasket... but the middle one was also letting go.

Let's see... since the plane was built in the beginning months of 1977, doing the simple math says these cork gaskets lasted 25-years before letting go. Not bad! You could tell they'd never been touched as the paint on the screw heads on the tank cover plate had never been touched. Moral of the story: If your bird's tanks haven't been touched, more than likely after 25-years.... yours too might be getting ready to surprise you! *When you LEAST expect it! So, before the price of the gaskets goes totally astronomical, you might want to get three for each side, plus the Viton gaskets that go under each of the six bolts that hold the solid aluminim plates over the gaskets. Seems we paid about $20.02 each for the gaskets and .88 cents apiece for the Vitons.

The biggest task in this whole ordeal was removing the ga-zillion screws from the inboard tank cover -- just to get at the tank gaskets, and there are a few tricks involved in this project which we'll share at a later date -- especially involving reinstalling the lead-fairing cover on the boom. Since the screws had never been out, some of them were ornery, requiring the assistance of Snap On Tools easy-outs which do the job nicely with application of a simple rachet with a half-inch drive. More on this later. For now, we'll leave you with the pictures of the before and after.

SkyKing

[SkyKing]

This is another shot of the right inboard tank with the top cover removed... you can see where the fuel had been leaking from the middle and aft inspection ports.

At the edge of the tank, nearest the cabin side, you can see the slots in the top of the wing structure where the 'HAT' sections of the top tank cover fit ... and where the longer screws are used. Yes, the tank cover is part of the wing... structurally speaking.

SkyKing

[SkyKing]

Sorry for the slightly 'fuzzed' quality of the pix, but this was my first time using the digital gadgetry.

Here we see the boom support... kinda crude looking, but it did the trick... with a nice big cushy soft rag on top of the 4X4. The jacks were also placed under both wings. The Service Manual gives you a good explanation why this is necessary -- before trying to take out or reinstall the ga-zillion screws on the tank cover.

And, yes... you DO have to remove the lead-fairing boom cover BEFORE you can remove the tank cover... at least on the 1977 model.

SkyKing

[SkyKing]

Here's a close-up of the tank after cleaning the fuel stains off. And by the way, just using a little avgas on a rag does the trick well. Here you can also see the wiring for the Penny Cap sensor.

SkyKing

[SkyKing]

Here's the underside of the inboard tank cover... notice the "HAT" sections with the anti-chaf tape... these "HAT" sections provide carry-thru strength of the wing structure across the top of the tank.

We'll post a few more pictures later on showing the inside of the tank and the Penny Cap fuel sensor.

SkyKing

[SkyKing]

BTW fellow Skymaster owners/operators, affecionados and enthusiasts... were you at all aware that a lot of the techie stuff, like what you've just been seeing here in this thread concerning the fuel tank leaking problem... is regularly being discussed and kicked around over on the other e-mail board? You might want to swing on by and join in sometime! Just send us an e-mail to "Cessna337Skymasters@Yahoogroups.com" and I'm sure the invitation carpet will be extended.

SkyKing