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How deep is too deep?

NorthernRV4

NorthernRV4

Well Known Member
While fitting my RH stab skin tonight I found some fairly deep scratches I hadn't noticed before. My question is how deep a scratch is too deep? How does one decide if they have to replace a skin? I need to evaluate it now before I start drilling it to the skeleton.
 
Scratches

NorthernRV4 said:
While fitting my RH stab skin tonight I found some fairly deep scratches I hadn't noticed before. My question is how deep a scratch is too deep? How does one decide if they have to replace a skin? I need to evaluate it now before I start drilling it to the skeleton.
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If you can feel it with a fingernail, it needs to be buffed out with scotchbrite. Usually a little rubbing with a scotch brite pad will smooth them out. There is probably a rule of thumb for depth. Maybe someone else will comment on deeper scratches.
 
Kind of depends on where they are, how sharp-bottomed they are and what direction they're oriented. Van's would be the definitive authority.

Dave
 
The general rule is 10% of material thickness is the limit for scratches/gouges.
 
Walt said:
The general rule is 10% of material thickness is the limit for scratches/gouges.
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That general rule is for aircraft aluminum tubing...

The sheet stock in question is Alclad, which has a soft outer layer that is about 5% thick on each side.

The tolerance on Alclad sheet should be a bit more than 10%, but it is the designer that gets to define it.
 
az_gila said:
That general rule is for aircraft aluminum tubing...

The sheet stock in question is Alclad, which has a soft outer layer that is about 5% thick on each side.

The tolerance on Alclad sheet should be a bit more than 10%, but it is the designer that gets to define it.
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This is a grey area as there is no airframe repair manual for the RV's, a requirement for certified aircraft.

As Walt suggests, you can find 10% is what many certified airframe repair manuals use, with exception of spars, where there is no repair procedure defined in the airframe repair manual.

It is very difficult to measure the depth of a scratch. A fingernail will pick up as little as one or two thousands of an inch, amazingly sensitive.
My general rule follows AC43 in that the scratch should be faired out, 10:1. I will allow the loss of the Alclad but not much more, which is probably darn close to 10%.
If the scratch is deeper than that, it is a pretty deep. However, depending on the location of the scratch, length, etc.... I would not necessarily trash the part. Repairs of cracks are allowed on most skins, so reasonably one would think a deep scratch may not differ?
 
I support the previous answers. Beneath the dimensions of the scratches it is important to know where the are:
- inner/outer surface of skin
- upper/lower skin
- inboard/outboard
- direction of the scratches

Worst case: scratch on the outer surface and inboard of the upper skin in flight direction, near the spar -> thats where you will have the highest tension stresses.
Best case: scratch on the inner surface and outboard of the lower skin in spanwise direction, best in extremities
 
Thanks guys. The gouges are on the top of the RH stab skin about midway spanwise/cordwise. Just a few that vary around an inch long. They can be seen and easily felt with a finger nail but may have a raised burred edge too. I haven't tried buffing them down yet because I don't want to remove the surrounding Alclad just yet.

10% of the material thickness seems reasonable but how would you measure that with any accuracy? .003" is pretty hard to measure with a caliper.
 
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This brings on another question. I am looking at a 4 that has been sitting around for a number of years in a hangar. It has some corrosion on some of the wing skins outside only and has created some minor pitting. The inside of the skins look perfect. So, what would be the best way to replace a skin and match drill it. Sorry I did not mean to throw this off topic but I kind of figured it was along the same lines.
 
Minor surface pitting I'd be inclined to scotch brite it, fill it, sand it, paint it, keep an eye on it. What's worst case, it cracks in flite? This is 1000 pound rv. Replacing skins seems overboard....any pics?

Or, throw a doubler on it if yr worried. We have double and triple skin "doubler" repairs on the pressurized sections of the fuselage of some of our 'classic' jets at work riveted with rivets the size of your thumb.
 
Minor surface pitting I'd be inclined to scotch brite it, fill it, sand it, paint it, keep an eye on it. What's worst case, it cracks in flite? This is 1000 pound rv. Replacing skins seems overboard....any pics?

Or, throw a doubler on it if yr worried. We have double and triple skin "doubler" repairs on the pressurized sections of the fuselage of some of our 'classic' jets at work riveted with rivets the size of your thumb.
 
NorthernRV4 said:
Thanks guys. The gouges are on the top of the RH stab skin about midway spanwise/cordwise. Just a few that vary around an inch long. They can be seen and easily felt with a finger nail but may have a raised burred edge too. I haven't tried buffing them down yet because I don't want to remove the surrounding Alclad just yet.

10% of the material thickness seems reasonable but how would you measure that with any accuracy? .003" is pretty hard to measure with a caliper.
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Unless you have a surface gauge or can get a micrometer around it you can't. What I accept in my own work is the scratch should be able to be faired out without going too much past the Alclad. The Alclad coating is typically 5% nominal thickness. If you polish through the Alclad and the scratch is gone or almost gone, you are good. You still have that much depth to go before you hit 10%.
 
Jstov said:
This brings on another question. I am looking at a 4 that has been sitting around for a number of years in a hangar. It has some corrosion on some of the wing skins outside only and has created some minor pitting. The inside of the skins look perfect. So, what would be the best way to replace a skin and match drill it. Sorry I did not mean to throw this off topic but I kind of figured it was along the same lines.
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Same answer as above. Polish out the pits up to the point you are past the Alclad.
You might consider treating the areas with an acid etch to stop the cathodic process. You would then need to do a conversion coating and prime. (Acid etch and Alodine is the most common process.)
AC-43 has an excellent chapter dedicated to corrosion repairs. You can view it online.
If you lost a wing skin to the point it was not repairable, I would have major concerns about the rest of the structure.
 
NorthernRV4 said:
10% of the material thickness seems reasonable but how would you measure that with any accuracy? .003" is pretty hard to measure with a caliper.
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Get a 10x magnifying glass and a set of feeler gauges. Put a feeler gauge next to the scratch. Using the magnifier, compare the thickness of the feeler gauges, one by one, with the depth of the scratch, until you've got a good idea of the depth. I'll bet you can get within .001 inch or so this way.

A piece of paper is about .003 inches thick if you haven't any feeler gauges handy.

Dave
 
You guys are great, always have the answers. That makes me feel better. I have not purchased plane yet but things are looking good. If I go ahead with the purchase I am sure I will have a lot more questions. I will post pics if the bird becomes mine!
 
All great response. Replacing a skin on a non-match drilled kit sounds next to impossible but I haven't been faced with that. I've gone over that in my mind a few times and if I couldn't get access to the rear side I'd use the old skin as a drilling template to carefully replicate the old one. That's just a guess though.

I did polish out one test scratch with 400grit then maroon scotchbrite. I don't think I had to go too far but how do you know if you penetrated the Alclad layer? The colour didn't really change, just got to be a more dull light grayish.


Untitled by jim_soutar, on Flickr
 
NorthernRV4 said:
All great response. Replacing a skin on a non-match drilled kit sounds next to impossible but I haven't been faced with that. I've gone over that in my mind a few times and if I couldn't get access to the rear side I'd use the old skin as a drilling template to carefully replicate the old one. That's just a guess though.

I did polish out one test scratch with 400grit then maroon scotchbrite. I don't think I had to go too far but how do you know if you penetrated the Alclad layer? The colour didn't really change, just got to be a more dull light grayish.


Untitled by jim_soutar, on Flickr
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Take a piece of scrap and go at it. You should see the difference when you get past the Alclad. It will be a darker gray look.
Do not use the tape on the edges or square off the "dish out" of the repair. It should be an oval shape tapering out from the center of the damage, blending into the surrounding sheet.
Frankly, from the pics, these are hardly even scratches. They are beauty marks! I don't think you have anything to worry about.....
 
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