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Tubing Corrosion

CaptFox

CaptFox

Registered User
Justin, TX
When doing a complete rebuild of a Fuselage, after stripping down and sandblasting the frame and locating all the obvious damage and corrosion, what is or is there a good way to check the structural integrity and internal corrosion of the frame if is not apparent from the outside.

Thanks.

David.
 
Probably lots of very expensive ways to check interior of tubes. New fuselages are so inexpensive it makes little sense to do such testing. I have converted several fuselages into scrap tube, and have been impressed with how clean the interior walls are, even on the most corroded tubes. One was from 1939; had lain around for a decade, and was simply too corroded on the outside to consider for use, yet the inside of all tubes I checked looked brand new! Really bad spots make themselves known with pinholes all the way through.
Good luck! bob
 
I have to agree with Bob. All of the frames I have rebuilt have corroded from the outside. After blasting and finding lots of pits I start cutting out tubes. Most of the tubing is still oily on the inside. Most of the bad tubing is wear it is wrapped in sheetmetal or was stored on the floor. I have been painting the tubes and sheetmetal with 3M weldable primer, welding the sheetmetal to the tubing, blasting and then epoxy primer and polyurethane top coat. Also keep the inside of the fuselage as clean as you can. It is amazing how much crap comes out from between the tubing and the fabric when you cut it off. This stuff allows moisture to sit on the tubing and eventually rust away. After blasting I very rarely find any bad tubing by poking with a sharp awl. It is mostly evident by the pitting on the outside.
 
icepick and hammer. Hit it hard. it the pick goes in its bad, if the pick bends its good.
 
Steve,
Can you enlighten me on "weldable primer"? I'm about to add on the sheetmetal for the door frames on a 2+2, and I am concerned about how to protect the tubes that are covered by the metal.
Laz
 
The "ice pick approach" can be a good method but can also be a bad thing in the hands of an inexperienced person. Nearly all of the original tubing in an older cub was 1020 mild steel and much of it is only .028 thick. I can guarantee you that being too aggressive searching for corrosion can damage a perfectly good tube. Just because you can drive a hole into it doesn't necessarily mean that it is corroded.
The cub that I am now restoring had several "pre purchase" inspections by others and had "ice pick tracks" all over it. They were sure that they would find corrosion I guess because of some light surface rust. Some tubes had been damaged to where they required replacement. :( Oil had leaked out from one of the holes that was left. I am replacing all lower tubing to 4130 and in the process have found no internal corrosion, just lots of preservative oil residue.
Wonder how many people selling their airplanes have had this kind of damage done during "pre buys" to perfectly good tubing? Think that I would be rather nervous and would want to be there to observe.
 
Also something to keep in mind that tubing can be perfectly good in one section, and the same piece can be corroded less than inches away from the good section. So be sure to check the WHOLE tube. I had a friend who
was covering a J-3 and had just finished covering the interior and just before slipping on the fuse. cover discovered a bad section of one of the top longerons. The corroded area was very close to the interior fabric where a new piece had to be rosseted in. So look your tubing over good before you prime or cover it.

Another good method of checking the tubing that I learned from Wayne Mackey just last week is to firmly but gently squeeze the tube with a pliers to check it's solidity.

Ryan
 
I've cut out tubes that look perfect on the outside, yet were rotted internally. This usually shows up in the sandblast as the tube will dent in or pin hole if it is thin. Sometimes you could ice pick right next to the pin holes and not find anything, so that is not a full proof method.

Worst case, check the low spots in the frame for bulged or split tubes. This comes from the tubes being full of water and freezing.
 
Tubes

We replaced almost half of the tubes on my 11. We were very liberal in our selection. Anything that we didn't like (either from the accident, signs or corrosion, or both) we sacked.

Now it looks great, is straight as can be, and will give many, many years of excellent service.

It bears repeating--you will be flying in your plane. Take the time and money to make it right. There is no substitute.

Jeff
 
I have access to a portable ultrasound. Has anybody ever tried to use one to check tubing? It would only show a variation in thikness.
 
corrosion

My dad was flying his cub about a month ago, pushed the stick forward to go down to look at some cows, went to pull back up, nothing happened!!!!! The smaller tube thats inside the larger tube connecting front and back sticks had corroded from the inside out! (That'll make ya pucker) He managed to get it on the ground safely. He said as long as you'd keep the airspeed above 60 the nose would'nt slump down. Our mechanic said he'd never seen or heard of that tube corroding in half before. The outside of the tube looks perfect.
 
cubpilot2 said:
The "ice pick approach" can be a good method but can also be a bad thing in the hands of an inexperienced person. Nearly all of the original tubing in an older cub was 1020 mild steel and much of it is only .028 thick. I can guarantee you that being too aggressive searching for corrosion can damage a perfectly good tube. Just because you can drive a hole into it doesn't necessarily mean that it is corroded.
Click to expand...
that be a good way to find all that soft stuff and then you'd have to replace it I guess. Had a fuselage in one time a guy wanted me to fix up. I could drive an ice pick thru it about anywhere with a light blow but the tubes looked ok. Finally found out it had been in a hangar fire. Not sure why the tubes were so soft. I thought that would be the same as normalizing it after welding.
I would assume somebody that is a real pro could take a light hammer and go along tapping the tubes listening to the ring and be able to tell if the tube is good.

I guess if someone drove a pick thru a good tube, you could call them pre-oil holes !! :bad-words:
 
Connector Tube Corrosion

With regard to the above on connector tube corrosion (i.e. the thingamabob in the torque tube), see my post under that very subject.
 
x-ray

I found an outfit that will x-ray an 18 I'm looking at buying. Seems like a good idea for a pre-purchase inspection. What do you think?
 
How much will they X-Ray and from how many angles? It is hard to predict all the places that may be corroded, although there are some common problem areas. How much will they charge? I would contact someone in the piping industry who could tell you how effective X-Raying would be in finding corrosion in thin wall tubing. Maybe someone on this site has that background. I talked to one AI here in Alaska who told me about someone who had X-Rays taken of their fuselage including the vertical tube below the rear spar fitting. Unfortunately, shortly after their rebuild, they heavily damaged the plane. This tube failed and they found it was heavily corroded. It never showed up in the X-Rays. The damage to the tube was a result of the incident and was not a contributing factor. But it was an eye opener, they thought they had a sound fuselage after the all inspections and the cost of the rebuild.
 
On Monday it's time for glass blasting and powder coating (incl Zinc primer) of my L-18C. At the same time we'll blast and powder coat my friend's PA-14.

One thing we discussed yesterday: How are the different tubes "internally connected"? I.e. where must we drill holes and spray oil (for corrosion protection)?

/Mattias
 
I have repaired many Piper fuselages and have never found any holes at the cluster. Air Tractor drills a #30 hole at each cluster with a plug in the front at the firewall. When the fuselage comes out of the jig it is pressure checked with nitrogen and then pumped full of hot linseed oil. I drill the bottom longerons and put oil in and then use a solid pop rivet with PRC on it. Most of the frames I have repaired show no internal corrosion unless there was a hole somewhere in the tube. Cut a landing gear/strut carry-thru out of a 1949 Clipper this week and it was as clean as the day it was new. Same at the end of the lower longerons. The outside of the tubing was pitted between the rudder post and the first diagonal tube coming down and forward of the rudder post. I cut a section out and found it had a tube inside a tube and was still oily inside. The fuselage drawing did not show this tube inside a tube. Have since found it on other fuselages.
 
sekps said:
On Monday it's time for glass blasting and powder coating (incl Zinc primer) of my L-18C. At the same time we'll blast and powder coat my friend's PA-14.

One thing we discussed yesterday: How are the different tubes "internally connected"? I.e. where must we drill holes and spray oil (for corrosion protection)?

/Mattias
Click to expand...



How are you going to bead (glass) blast and transport your Fuselage to avoid surface corrosion forming between the time of blasting and powdercoating??

We are going to powder coat our entire fuselage and the powdercoating firm here wants to drop the entire frame in an acid bath. They just asked us to remove all the paint and leave bare metal when we take it to them.

ON the Tube Oil, is there anything worng with filling the tube with oil prior to powdercoating and welding shut the drilled hole?? or, given that we will acid dip, should we fill the tube oil after powdercoating in case any acid "leaks" into the tube!! (Should not have any pin holes I know but hard to find them if they ARE there)

Regards
 
RPURCELL said:
How much will they X-Ray and from how many angles? It is hard to predict all the places that may be corroded, although there are some common problem areas.
Click to expand...

They say can x-ray any angle I want, of the entire fuselage, but generally a profile is all they need. Are there "problem areas" beside the lower longerons? The x-ray examples they've shown me are very clear. Has anyone done this?
 
I have watched this thread with some interest, Im curious on if the Xray can determine wall thickness on the various parts of the fuselage. Some of the large auto racing sanctioning bodies use a sonic test to check wall thickness on roll cages and frame members in tech inspections. I have seen it done at auto racing engine shops on cylinder blocks to determine if the block would accept an overbore. I would assume this type of test would work for a fuselage.
 
The ultrasonic test can now be used for the AD note on life struts. It should work on tubes also.
 
Mark C.
Two thoughts on the oiling. When my fuselage was done they baked it after blasting the fuselage. That way any place there was a hole would show up as the oil oozed out of any pinhole, allowing a fix before the acid dip and powdercoating.

Literally filling the tubes with oil sounds like a bad idea too, I'd think that coating the inside would be the goal. Full of oil would be heavy and heating the tubes up, any tube 100% full could see pretty high pressure as the temperature rises.
klm
 
Sonic Testing Tubes

I would like to add something to the X-ray or Sonic testing of tubes. I own a Boiler Shop in the Seattle area, and we test pressure vessels and tubes as well as community water tanks by Sonic testing or X-ray (same thing)
What this does is tell you how thick the existing metal is in a dime size area. What you do is lightly scrape or sand the spot you want to test to bare metal, then wipe some salve on this spot, put the tester on it a push the button. You get a instant read out on that spot only. In the case of our boilers the tubes might be .105 wall thickness when new. Due to rust and pitting ect. you can tell when you getting thin right away. We make a map of the tank and mark every test site so we can see a pattern. In the case of the supercub you might notice its near the tail section thats thin. We have worked with companys with very exspensive equipment and just about all of them end up using this method. Its only random checks but will give you some idea. The test equipment is small enough to fit in your pocket and cost might be around $1000. Or hire those guys to come in and test it using their own equipment, its not that exspensive. Look in the yellow pages under Sonic Test, Metal Testing labs ect. I wish that I had a rusted old Supercub to work on you guys are lucky. :wink:
 
markc said:
How are you going to bead (glass) blast and transport your Fuselage to avoid surface corrosion forming between the time of blasting and powdercoating??

We are going to powder coat our entire fuselage and the powdercoating firm here wants to drop the entire frame in an acid bath. They just asked us to remove all the paint and leave bare metal when we take it to them.

ON the Tube Oil, is there anything worng with filling the tube with oil prior to powdercoating and welding shut the drilled hole?? or, given that we will acid dip, should we fill the tube oil after powdercoating in case any acid "leaks" into the tube!! (Should not have any pin holes I know but hard to find them if they ARE there)

Regards
Click to expand...

It's the same company that will blast and powder coat the fuselages. They have done several fuselages (Cubs, Austers, Moths etc.) and they have also made a lot of work for SAAB (when they were producing the SAAB 340 and SAAB 2000).

Blasting is made in one box and then the fuselage (and all other small parts) is directly moved (on a conveyer) to the paint box where a 97% Zinc coat is applied. Time between blasting and Zinc is max 10 minutes and it's a controlled/dry environment. The Zinc coat dries in a controlled environment before the powder is applied.

Then on the same conveyer directly over to the oven. Boxes and oven are 7.5x2.5x2.5 metres so they can handle quite large parts. Some pictures at
http://www.ahnelovsindustrimalning.se/pulverline.htm

The reason to oil the tubes after coating is that the surface must be absolutely clean under the Zinc and powder. The 170°C in the oven will also make the Tectyl (which I'm planning to use) lose some of it's good properties.

/Mattias
 
I would oil after powder coating because of the pressure build up from the heat. The oil is put in, fuselage rotated to coat the inside and then drained out.
 
Cost of a new fuselage

I see a lot of people talking about buying a new fuselage. Is this really affordable? How much, generally, do they cost?
 
Roughly 9 to 14K. If all you get is a basic rectangle 8 or 9K If you get gear etc up to 12 or 14 K.

Bill
 
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