Esoteric design question about wet wings

[nrshida]

I figure there must be some people here with maintenance knowledge working on aircraft with wet wings or fuel tanks otherwise incorporated into the structure of aircraft.

What are the typical issues with this design solution? Are leaks an issue? How are the tanks sealed, with flexible bladders? How is feed and return handled? Do they require periodical refurbishment because of corrosives in the fuel? Is it in your opinion a good solution or are the ongoing issues a pain in the a**e?

[icepac]

Fuel is tightly controlled by the FAA regulations so you should never run into any fuel that is "corrosive".

Most issues with either bladder or true wet wings are with water intrusion.........and it's usually the fuel caps.

A bladder is best left full of fuel or it may wrinkle and possibly get brittle but they are made by super exacting standards and usually last at least 12 to 15 years with some lasting over 30.

With wet wings, you will see one plane that never leaks and maybe an identical plane parked next to it that is always having leaks fixed.

Sealant is only as good as the preparation and care in putting it on.

Things to weigh are how prices for labor and parts are changing relative to each other which could make one option better than the other when before it was the opposite.

[zack1234]

I knew a aircraft engineer who mentioned that Middle Eastern sanitary standards on the planes toilets were causing structural issues in the planes structures.

Apparently getting to these compartments in the plane was impossible.

[Maverick]

The Piper Comanche I had for over 10 years had "wet" wings. The fuel tanks were flexible bladders installed inside. When I got it there was always the odor of fuel in the cockpit. The mechanic I had said the bladders had gone porous over the years due to having been left sitting with less than full tanks. We pulled the bladders and I sent them off to a repair station and got a set of replacements. AFAIK they rebuild (re vulcanize?) them and use them again. In the next 9+ years of ownership I never had a fuel odor or leak issue from them. I ALWAYS kept them full. As soon as I landed at the end of the flight day I had them topped off.

Installing them is kind of tedious as there are clips built into loops on the bladders that fit into slots in the wing structure to hold the shape. You also need to install fresh chafe tape in areas that tend to abrade the bladders or you will be doing the job again sooner than you need to otherwise.

[Vraciu]

The biggest problems with wet wings are microbials.   Jet fuel has living organisms in it and if you don't keep up with it the stuff will eat the airplane.  True story.

Sealing is pretty straightforward.     Also, maintenance schedules have items like draining water and running Biobore at intervals to address these issues.   

If your maintenance is sound you'll be fine.    For a non-combat airplane it's the way to go.

[colmbo]

The biggest problems with wet wings are microbials.   Jet fuel has living organisms in it and if you don't keep up with it the stuff will eat the airplane.  True story.

Avgas as well.  We found a 180 with a big wrinkle in the bladder trapping some water.  The accumulated dirt and algae in the "pond" looked like a garden.

[nrshida]

Ah this is perfect information. Exactly what I was looking for.

Many thanks all 

[DaveBB]

Petrochemical inspector here.  Yes there are microorganisms that do metabolize oil and various types of fuel.  Once a tank or pipeline gets colonized with these microorganisms, they never go away.  Even with biocides. Most likely all fuel storage tanks for avgas are colonized with these microbes.  The most harmful type are the sulfur-reducing bacteria (to metal).  They create distinctive cup shaped pits.

[Vraciu]

Petrochemical inspector here.  Yes there are microorganisms that do metabolize oil and various types of fuel.  Once a tank or pipeline gets colonized with these microorganisms, they never go away.  Even with biocides. Most likely all fuel storage tanks for avgas are colonized with these microbes.  The most harmful type are the sulfur-reducing bacteria (to metal).  They create distinctive cup shaped pits.

Yes, but left untreated they  will eat the airplane.  I saw the inside of a Falcon or Citation where  it was damaged badly enough it  had to be scrapped. 

[Vraciu]

Avgas as well.  We found a 180 with a big wrinkle in the bladder trapping some water.  The accumulated dirt and algae in the "pond" looked like a garden.

Ewww.

[colmbo]

Ewww.

It was a good thing really.  At least it wasn't sloshing out and going through the engine.  Had that happen on takeoff one fine day at about 50'.  Luckily the Continental was able to choke it all down before we hit the trees.

[colmbo]

I don't think a true wet wing has a bladder though does it? 

[Vraciu]

It was a good thing really.  At least it wasn't sloshing out and going through the engine.  Had that happen on takeoff one fine day at about 50'.  Luckily the Continental was able to choke it all down before we hit the trees.

Yikes.  I have been supremely lucky in that regard.   I count my blessings for it.   

[Vraciu]

I don't think a true wet wing has a bladder though does it?

The jets I have seen in MX do not have bladders in the wings (or belly tanks best I can remember).  They have numerous sealed access panels on the underside of the wing and sides of the belly tanks.  They're designed in such a way that they're pretty easy to seal so no leaks occur.   I had my engineers (mechanics, techs) show me once and it was a pretty nifty design. 

One of the guys I knew used to work on BIIIIIIIIG jets.    He had to go up inside the fuel tank with a respirator to do work in it.   Said it scared the hell out of him.   Lol

[saggs]

I'm an A&P who works on Douglas DC-6 and Curtiss C-46 aircraft most of the time.  I'll use the DC-6 as an example because it has a combination of wet wing and bladders.

The DC-6 has a combination of wet wing, and bladders in the wing.  The main tanks are wet wing between the front and center spar, with bladders behind the center spar.  The outboard alt tanks are the same, and the inboard alt tanks are all bladders.  However some in our fleet we have deleted the alt tanks, and some we have even deleted the main tank bladders.

As far as wet wing vs bladders when it comes to maintenance, I'll take wet wing any day.  When there are damaged stringers, or corrosion or popped rivets or anything that needs fixed in the wet wing, it's drain the tank open up a couple panels, ventilate for several hours, get in and fix what needs fixed. (you may need to scrape away some sealant depending on what needs fixed.)

With the bladders, it's drain the tank, open panels, remove the bladder cap panels (a PITA), ventilate for several hours, get in and disconnect all the bladder interconnect points(another PITA), unsnap and remove the bladder (being very careful not to crease or damage it) then you have a phenolic liner underneath the bladder that protects it from sharp edges on rivet shop heads and stringers that you have to un-tape the seams of, then unscrew and remove... then you are finally down to the structure where you can repair.

It takes probably 5-10 times as long to gain access for repairs when we have to pull a bladder vs. just the wet wing.  As for leaks, the wet wing does tend to leak more.  But with Avgas small leaks evaporate away so quick we don't even worry about them (just leaves a small blue stain) For bigger leaks that we do have to fix it is much easier to fix then with a leaky bladder.  Just get in and reseal wherever it's leaking from, vs. tearing out bladders and replacing gaskets.

You asked about sealant.  Wet wings (AVgas wet wings anyway) are sealed wherever there are rivets or bolts through the skin with PRC (commonly called Proseal) and then top-coated with a fuel resistant product called Scotchweld EC-776. Any rivets in the wet wing are shot "wet" (meaning coated in proseal before they are shot) to eliminate leaks.

Now the Curtiss is different altogether.  It has actual aluminum tanks built inside the wings, and they are pretty much maintenance-free.  For other repairs on the wings you just have to work around them. The downside is they do not fill all of the available space with fuel like with a wet wing.

I don't think a true wet wing has a bladder though does it? 

This is correct. "Wet wing" refers to the fuel actually filling the structure of the wing, nothing but a skin panel between you and the fuel, vs having a bladder or welded tank or something else as the container inside the wing.