Pre-buy missed that one...

[bluerooster]

Inter-grainual corrosion usually starts below the surface.

That's usually due to faulty alloy, in the manufacturing process of the aluminum that the part is made from. Not from exposure to corrosive agents.

Oh, btw, it's intergranular.

 

[azure]

Hmmm. Given that, how do you know the mouse didn't move in between your prebuy and your 12-month-later annual?

Paul

I don't know for 100% certain, but I discussed this at length with my mechanic. He said that based on the condition of the carry-through, and of the nest material, and of the mouse turds (?!), the nest had to have been there for a lot longer than a year.

Moreover, there was never any evidence of mice in the hangar, neither during that year, nor in the three years following that I hangared my plane there. Seems unlikely that mice would enter my plane in my hangar, build a nest, do their mousey business unbeknownst to me (and I flew that plane a heck of a lot that first year) without leaving any evidence anywhere else in the plane or the hangar, and then go off and die somewhere else.

 

[Cap'n Jack]

There are several rules in welding that prevents this from being welded. You first must know the alloy, and Scott is no longer around to tell us.
Then you must know if it was heat treated, and Scott is no longer around to tell us.
There are ways to figure all this out, but the FAA would never approve with out OEM data.
Getting that would be near impossible. just like your hinge.

Determining the alloy is pretty easy now. X-ray diffraction and elemental analysis in a hand-held device.
https://www.bruker.com/products/x-r...xrf/applications/pmi/xrf-aluminum-alloys.html

There are many manufacturers for these devices.

 

[Tom-D]

And thank God they weren't Tom, otherwise nothing would ever get done.

When it's done right I have no problems with it.
Remember I'm the one who built two Fairchild wing spars from scratch.
Bring me proper engineering data, or go away

 

[Tarheelpilot]

There is the key.
Which noose are they willing to put their head in, If your hard earned ratings are at stake, or your house would you get into that liability loop?

Like I've said certification of owner produced parts if "IFFY" if you can get some one take the responsibility for them.

Remember it's not simply what the FAA will except, we must consider what the liability responsibility is.

The only way not to have liability is to do nothing.

 

[Fearless Tower]

The only way not to have liability is to do nothing.

Which is why so many IAs and FSDOs won’t sign off on repairs that seem simple or common sense to aircraft owners.

 

[Tarheelpilot]

Which is why so many IAs and FSDOs won’t sign off on repairs that seem simple or common sense to aircraft owners.

Yeah. It’s retarded.

 

[Tom-D]

Which is why so many IAs and FSDOs won’t sign off on repairs that seem simple or common sense to aircraft owners.

When you have a manufacturer that will supply new parts like a tail wheel casting, It makes absolutely no sense to manufacture your own.
When you have a antique it is the only way.
And that is what the owner produce part is all about, building stuff that can't be found anywhere.

 

[Tom-D]

The only way not to have liability is to do nothing.

But, you must lower the risk, anyway you can, and building parts that are available from the manufacturer, ain't the way to do that.

 

[JAWS]

But, you must lower the risk, anyway you can, and building parts that are available from the manufacturer, ain't the way to do that.

Depends on the part in question. Build a magneto? No. Make a new baffle plate? Sure.

 

[Tom-D]

Depends on the part in question. Build a magneto? No. Make a new baffle plate? Sure.

No, It's a matter of how much liability you want to take.

 

[JAWS]

No, It's a matter of how much liability you want to take.

I don't think it is so black and white. I have replaced all sorts of parts with shop made - baffles, wire harnesses, fuselage skins, windows. The liability on parts like these is pretty low in my opinion. I can cut and shape and fit and install a new skin far cheaper than paying for Cessna. And it can be done legally and often better than factory.

Start getting into compound curves and different heat treatments and truly specialized stuff, that is a different kettle of fish.

 

[Tom-D]

I don't think it is so black and white. I have replaced all sorts of parts with shop made - baffles, wire harnesses, fuselage skins, windows. The liability on parts like these is pretty low in my opinion. I can cut and shape and fit and install a new skin far cheaper than paying for Cessna. And it can be done legally and often better than factory.

Start getting into compound curves and different heat treatments and truly specialized stuff, that is a different kettle of fish.

Like we have said many times Repairs are regulated under a different set of rules, Most aircraft have a structural repair manual, making a repair by building a new skin is covered in this manual

 

[JAWS]

Like we have said many times Repairs are regulated under a different set of rules, Most aircraft have a structural repair manual, making a repair by building a new skin is covered in this manual

Skins have part numbers and are usually found in the IPC. Sometimes you can specify that they be undrilled, which will be a slightly different number, much better for fitting. Or you can make your own depending on the part and your skill level. Whatever floats your boat.

 

[Timbeck2]

And thank God they weren't Tom, otherwise nothing would ever get done.

I don't think you are being very fair. If you notice, not a lot of IA's are jumping on the band wagon when it comes to disagreeing with Tom. An IA with a stroke of a pen is telling the world that they have inspected an aircraft and deem it airworthy. When an incident happens it always comes back to the person responsible, the IA. IA's know a lot of stuff but they don't know everything and can't possibly be expected to know the metallurgy of every part of every aircraft, therefore it is best and safest to err on the side of caution. I want to get my A&P license but I have no desire to be an IA at this point. Tom has helped me out with suggestions of repair to my Cherokee; he isn't the person depicted as someone who doesn't take any risk. In fact, he is a pretty nice guy who is more than willing to help anyone out with suggestions on what they need to do in order to get their airplane back in the air.

A lot of people have different opinions when the repair is on their aircraft and they're responsible vs being responsible for repairs on other people's aircraft. I had some hangar fairies weld a cracked bracket which holds my wheel pant on long before I knew what I know now about aircraft maintenance. As an IA or even an A&P would I authorize that on someone else's aircraft? Probably not.

 

[EdFred]

When you have a manufacturer that will supply new parts like a tail wheel casting, It makes absolutely no sense to manufacture your own.

Unless of course the tailwheel casting is some ridiculous amount of money, and you can have someone machine one out of a billet of the same material for 1/10th the cost.

 

[Tom-D]

Unless of course the tailwheel casting is some ridiculous amount of money, and you can have someone machine one out of a billet of the same material for 1/10th the cost.

The FAA doesn't make any distinction about money in the rules.
Now explain how you would prove to the FAA that the part your super machinist made is a part that is an exact duplicate of the OEM, With out any OEM data.
Remember if it isn't an exact duplicate, it is a modification requiring a Pre-approved field approval on a 337 with block 4 signed by a FSDO inspector.

 

[Tom-D]

Skins have part numbers and are usually found in the IPC. Sometimes you can specify that they be undrilled, which will be a slightly different number, much better for fitting. Or you can make your own depending on the part and your skill level. Whatever floats your boat.

When you are doing repairs that require skins, you are making major repairs and that requires documentation on a 337.
This is a different subject than owner produced parts.

 

[bluerooster]

The FAA doesn't make any distinction about money in the rules.
Now explain how you would prove to the FAA that the part your super machinist made is a part that is an exact duplicate of the OEM, With out any OEM data.
Remember if it isn't an exact duplicate, it is a modification requiring a Pre-approved field approval on a 337 with block 4 signed by a FSDO inspector.

Hand them the old part, and the new part, and a micrometer. I would think that the old part would provide all the OEM data needed to produce one identical.

 

[EdFred]

Hand them the old part, and the new part, and a micrometer. I would think that the old part would provide all the OEM data needed to produce one identical.

Pretty much this.

 

[JAWS]

When you are doing repairs that require skins, you are making major repairs and that requires documentation on a 337.
This is a different subject than owner produced parts.

Or is it "parts replacement"?

 

[Tom-D]

Or is it "parts replacement"?

no it is a simple major repair. Are you not working on the larger assembly?

 

[Tom-D]

Pretty much this.

Nope,,,, when the FAA puts it under the micro scope they will ask, how did you know what the original was with out OEM data. They believe you must know what you are building prior to duplicating it.
How would you know what hardness it was, or how it was hardened ?
How would know how it was finished.

 

[EdFred]

Nope,,,, when the FAA puts it under the micro scope they will ask, how did you know what the original was with out OEM data. They believe you must know what you are building prior to duplicating it.
How would you know what hardness it was, or how it was hardened ?
How would know how it was finished.

Maybe I'm a metallurgist with testing equipment.

 

[Tom-D]

When producing owner parts a blue print is "Gold"

 

[Tom-D]

Maybe I'm a metallurgist with testing equipment.

And maybe they will tell you to buy a new one.

 

[JAWS]

Hand them the old part, and the new part, and a micrometer. I would think that the old part would provide all the OEM data needed to produce one identical.

What about base material alloy? Heat treatment? Post manufacture corrosion treatment?

Relatively simple parts is easy. Skins, wire harnesses, etc. The material is often specified

A tail wheel, I wouldn't sign off on it. Too complex and too much liability.

 

[nrpetersen]

Try to touch it with a sharp file to see if it might be just gray cast iron. Then try to drill a small hole in it. That'll tell you if its a 4140 heat treated casting (or similar) alloy real quick

 

[EdFred]

And maybe they will tell you to buy a new one.

Then "they" can cough up the difference in price. Plus as an owner producing the part I wouldn't involve the FAA. Just as I haven't with other things I have fabricated and had fabricated for my plane

 

[Tom-D]

Then "they" can cough up the difference in price. Plus as an owner producing the part I wouldn't involve the FAA. Just as I haven't with other things I have fabricated and had fabricated for my plane

That's the funnest thing you have written.
The FAA doesn't cough up a red cent, ever.
You need never worry, they will get involved when they think they should, And when they do it is not pretty.
When you want to get under that microscope you go right ahead.

 

[azure]

It's not rare. The very first Cardinal I inspected had a corroded spar carrythrough that required a lot of work to salvage. And it doesn't take seaside air or mouse nesting to corrode it. There are a couple of CAT hoses in the ceiling that duct fresh air from the inlets to the overhead vents; they are against the spar and they vibrate and wear through, and the steel wire comes into contact with the aluminum spar. The moisture comes fom the breath of the occupants; it gets into the ceiling and condenses on the cold aluminum in flight, and corrosion of various sorts sets in. The real dangers is that the aluminum can appear mostly OK but have great craters beneath the surface with only a pinhole at the surface to indicate a problem. It takes a thorough cleaning, an eddy-current test and ultrasonic thickness measuring to determine the true state of the thing. Cessna demands extensive data that they then analyze and approve or reject the spar. Long process. A simple prebuy on a Cardinal or 210 is insanely insufficient.

Dan, sorry to revive this discussion so late in the game, but I just had an interesting email exchange with one of the CFO mods that got me to thinking, and a couple of your statements now seem a bit confusing to me.

(1) How does this internal corrosion "with only a pinhole at the surface to indicate a problem" develop, undetectable except by high tech testing, in a part that supposedly began its life by passing Cessna's rigorous internal testing? The corrosion on my spar was perfectly obvious once the headliner was removed, and though in places it appeared to be intergranular, again this could be determined by careful observation and sanding away the surface corrosion to reveal... more, and equally nasty, corrosion underneath.

(2) Why do you need ultrasound to determine whether the spar thickness is within spec? My mechanic just used a caliper. Low tech, effective, and no need to send the spar out to a lab. Or is there another reason?

I hope you're still reading this thread.

 

[Dan Thomas]

Dan, sorry to revive this discussion so late in the game, but I just had an interesting email exchange with one of the CFO mods that got me to thinking, and a couple of your statements now seem a bit confusing to me.

(1) How does this internal corrosion "with only a pinhole at the surface to indicate a problem" develop, undetectable except by high tech testing, in a part that supposedly began its life by passing Cessna's rigorous internal testing? The corrosion on my spar was perfectly obvious once the headliner was removed, and though in places it appeared to be intergranular, again this could be determined by careful observation and sanding away the surface corrosion to reveal... more, and equally nasty, corrosion underneath.

(2) Why do you need ultrasound to determine whether the spar thickness is within spec? My mechanic just used a caliper. Low tech, effective, and no need to send the spar out to a lab. Or is there another reason?

I hope you're still reading this thread.

I'm still here. The problem is one of certification of the repair. Cessna has spent some time determining how much metal can safely be removed, and no doubt they have a conservative buffer built into their minima. If your mechanic grinds away at the carrythrough until the corrosion is gone, then signs it off, he's on his own hook. If it fails in flight, he's in trouble. Besides, where did he get the dimensions to use for determining minimum thickness at any given point?

We worked with Cessna on the deal. We buffed out corroded areas as recommended. We had the NDT guy come with his UTM and eddy current machinery, and make hundreds of measurements in the affected areas as per Cessna. That involved drawing grids of quarter-inch squares in all the areas that we had removed metal, and getting a thickness at each one.They were satisfied so we primed the whole thing with the zinc-chromate primer specified by them, and they gave us a certificate of conformity.

Cessna made that part, or had it made, using the casting technology in place at the time. It would have been free of any defects of consequence. Corrosion is a large and complex subject when it comes to various aluminum alloys, and the pinholing phenomenon is out there and has to be addressed. When Luscombe and some others made their formed aluminum spar carrythroughs (as opposed to cast), they were also in dandy shape. But age and contaminants and some heat-treating issues led to some serious corrosion problems in some airplanes, and sometimes ADs were issued.

Nobody expected these airplanes to be still flying 50 and 60 years after they left the factory. The manufacturers were building thousands of them every year and some owners regularly upgraded. But now they're so expensive that we keep fixing the old ones, which just get older and older. We shouldn't be surprised that they're rotting away.

 

[azure]

Okay Dan, thanks for the information. Just to answer your question...

I'm still here. The problem is one of certification of the repair. Cessna has spent some time determining how much metal can safely be removed, and no doubt they have a conservative buffer built into their minima. If your mechanic grinds away at the carrythrough until the corrosion is gone, then signs it off, he's on his own hook. If it fails in flight, he's in trouble. Besides, where did he get the dimensions to use for determining minimum thickness at any given point?

My guy said he used Cessna's spec, so apparently it's available. In fact I'm sure it is, as he told me the values. And as I said, the spar was out of spec after removing the corrosion, so it was all for naught. That was the only part of the repair labor that I paid for, though I don't think he really charged me for ALL of the hours he spent on it. That is the kind of person he is.

The new spar was tested at a lab before the install.