Main Wing Spar Scratch

[NavyS3BNFO]

While inspecting my main wing spars, I noticed this scratch on the right spar near the tip. It is about 8 inches long. The anodized coating is not scratched off or through so it was there prior to the spar being anodized.

Obviously concerned I emailed Vans with pictures. Here's the response I got:

"I would leave the longer scratch in the second photo alone. That appears to have been there before it was anodized, and does not look deep enough to be a concern. Some minor scratches like this are a little too deep to be removed by the sanding process on the web, but they?re not worth removing entirely."

The response was a little confusing to me. If it was too deep to polish out, why ship it? I'm surprised it passed inspection. Shouldn't it have been corrected prior to shipping?

So I'm a little worried about leaving the scratch due to the risk of future cracking. There are several other small scratches that aren't of a concern.

I wanted to get some other opinions. What would you do? Would you be comfortable leaving it as is as Vans suggests?

[StressedOut]

There are several factors that come into play here. Location of the scratch, orientation, length, depth, accessibility after construction, etc. I'll give you some things to think about to help you make your decision here.

Location:
You mentioned it's near the tip. As you get closer to the tip the bending moment drops significantly. However, so does the cross section (usually) so the stress isn't necessarily lower as you go outboard.

You didn't say if it was on the upper or lower part of the spar. The upper surface of the spar is in compression and the lower surface is in tension (unless you're in level inverted flight). It's more critical if the crack is near the lower surface because that tension will work over time to deepen the scratch into a thru-thickness crack.

Orientation:
The worst orientation for a crack is perpendicular to the principal stress. Principal stress is a combination of tension and shear. Since you're in the web, the shear stress may be significant. The fact the scratch is running spanwise is a good thing.

Length and Depth:
The deeper the initial scratch, the faster it will grow to a through thickness crack. Same goes for length.

Accessibility after construction:
Assuming you decide to use it as is, do you have any way to periodically inspect the spar? If you discover it has become a crack, at least you know and can repair it. Cracks generally give you plenty of time to discover and fix them before they become a catastrophic failure.

On the other hand, if you can't get into that area for inspection you'll never know what's going on.

If you decide to repair, then here's what we do at the big airplane company that sounds like a bouncing spring.

1. Measure the depth. Let's say you get 0.005 in.
2. Blend out the scratch with a minimum 10 to 1 aspect ratio (20 to 1 is better, but not required). For example, with a 5 mil deep scratch the width of the blend out area is 0.005 X 10 = 0.050 in.
3. Reapply a chemical conversion coating to the repaired area (i.e. alodine).

If this were my spar, I would send it back. You paid good money for undamaged parts. It would bother me knowing it existed if I didn't repair it.

[lr172]

Originally Posted by NavyS3BNFO

The response was a little confusing to me. If it was too deep to polish out, why ship it? I'm surprised it passed inspection. Shouldn't it have been corrected prior to shipping?

I suspect that QA felt the same as the tech you communicated with - The scratch does not impact the structural integrity of part. The scratch is anodized, so no concern over corrosion. Is this on the 1" thick aluminum piece? I can't imagine that a scratch a few mills deep could have any impact on the structural integrity of that part.

Larry

[NavyS3BNFO]

Larry,

It's on the relatively thin web near the tip - not the thick aluminum parts.

I really want to believe Vans that it's fine but something keeps bugging me about it... At least I have some time to make a decision while I'm working on nutplates and ribs...

What's interesting, and not to read too much into this, is the other (left) spar has a bunch of 'smudges' where small scratches were polished out prior to anodizing. I've read several other builders had the same thing so that was fine.

The spar with the big scratch had no such 'smudges' and it didn't look like any scratches were polished out at all. Makes you wonder if it just missed that step...

Feel free to let me know if I'm overthinking this...

[NavyS3BNFO]

Thanks for sharing your expertise Art. All good things to think about.

I think I like the idea of blending it out the best. I don't know how I could reliably measure the depth but I'm guessing it can't be more than a few mils deep and anything I'd use to blend it out (sandpaper/scotch bright) will easily cover the aspect ratio you mentioned.

Originally Posted by StressedOut

There are several factors that come into play here. Location of the scratch, orientation, length, depth, accessibility after construction, etc. I'll give you some things to think about to help you make your decision here.

Location:
You mentioned it's near the tip. As you get closer to the tip the bending moment drops significantly. However, so does the cross section (usually) so the stress isn't necessarily lower as you go outboard.

You didn't say if it was on the upper or lower part of the spar. The upper surface of the spar is in compression and the lower surface is in tension (unless you're in level inverted flight). It's more critical if the crack is near the lower surface because that tension will work over time to deepen the scratch into a thru-thickness crack.

Orientation:
The worst orientation for a crack is perpendicular to the principal stress. Principal stress is a combination of tension and shear. Since you're in the web, the shear stress may be significant. The fact the scratch is running spanwise is a good thing.

Length and Depth:
The deeper the initial scratch, the faster it will grow to a through thickness crack. Same goes for length.

Accessibility after construction:
Assuming you decide to use it as is, do you have any way to periodically inspect the spar? If you discover it has become a crack, at least you know and can repair it. Cracks generally give you plenty of time to discover and fix them before they become a catastrophic failure.

On the other hand, if you can't get into that area for inspection you'll never know what's going on.

If you decide to repair, then here's what we do at the big airplane company that sounds like a bouncing spring.

1. Measure the depth. Let's say you get 0.005 in.
2. Blend out the scratch with a minimum 10 to 1 aspect ratio (20 to 1 is better, but not required). For example, with a 5 mil deep scratch the width of the blend out area is 0.005 X 10 = 0.050 in.
3. Reapply a chemical conversion coating to the repaired area (i.e. alodine).

If this were my spar, I would send it back. You paid good money for undamaged parts. It would bother me knowing it existed if I didn't repair it.

[StressedOut]

Originally Posted by NavyS3BNFO

Thanks for sharing your expertise Art. All good things to think about.

I think I like the idea of blending it out the best. I don't know how I could reliably measure the depth but I'm guessing it can't be more than a few mils deep and anything I'd use to blend it out (sandpaper/scotch bright) will easily cover the aspect ratio you mentioned.

One way you can measure the depth is to take some clay or maybe plumber's putty and work it into the crack. Peel it off and try to carefully measure the height of the protrusions with a dial caliper (the opposite end, not the caliper end).

I'll do some research and see if I can find a better way.

Oh I forgot to mention you should not attempt a blend out repair unless the scratch is less than 0.1 X T (10% of the thickness). Any more than 10% deep and you need to go to a doubler repair.

[Rallylancer122]

I understand Vans point, but ultimately its YOUR airplane. If you aren't happy with it, and you are going to worry about it every time you fly, send them back now and ask for new ones. May cost you a few bucks, but better than going through all the work of building your plane and then not trusting it.

DEM

[Ironflight]

You asked for opinions, so I’ll just say that you’ve asked the factory - the guys responsible for the design - and their answer is that it is fine as is. If you don’t trust them on this, how can you trust them on anything else in the aircraft? They’ve got a pretty good track record with 10,000 flying airplanes, and I can tell you from having looked at lots and lots of them, very few are without blemishes.

Ultimately, do what you are comfortable with, but remember that better is the enemy of good, and oftentimes doing “nothing” is better than doing “something” when you are just guessing.

[lr172]

Originally Posted by NavyS3BNFO

Larry,

It's on the relatively thin web near the tip - not the thick aluminum parts.

I really want to believe Vans that it's fine but something keeps bugging me about it... At least I have some time to make a decision while I'm working on nutplates and ribs...

What's interesting, and not to read too much into this, is the other (left) spar has a bunch of 'smudges' where small scratches were polished out prior to anodizing. I've read several other builders had the same thing so that was fine.

The spar with the big scratch had no such 'smudges' and it didn't look like any scratches were polished out at all. Makes you wonder if it just missed that step...

Feel free to let me know if I'm overthinking this...

You should assume that it was not dressed because that would compromise the part more than the scratch. A scratch is one thing, removing 10% of the thickness at 10 times the area is another all together. I am with Paul, you will likely create more problems fixing it. I certainly would not indiscriminately remove .006" from an .063 part for a scratch on my wing spar.

No sense calling Van's for advice if you don't trust them. Better to get your advice from the Internet anyways (Sarcasm, of course).

We should remember, this is Van's, not Toyota. They don't use robotic mfg tooling and if they threw out every part with a cosmetic, non-structural blemish, costs would certainly be higher. We should trust their judgement on this, as they bear significant liability if a shipped part does not meet their structural specification, especially when you have asked to to confirm the integrity.

Larry

[StressedOut]

Originally Posted by Ironflight

You asked for opinions, so I?ll just say that you?ve asked the factory - the guys responsible for the design - and their answer is that it is fine as is. If you don?t trust them on this, how can you trust them on anything else in the aircraft? They?ve got a pretty good track record with 10,000 flying airplanes, and I can tell you from having looked at lots and lots of them, very few are without blemishes.

Ultimately, do what you are comfortable with, but remember that better is the enemy of good, and oftentimes doing ?nothing? is better than doing ?something? when you are just guessing.

Paul I agree with everything you said here. My original reply was really a listing of what the variables are to consider, but ultimately an analysis would be the deciding factor. The formation of an initial crack and subsequent crack growth may be so slow that it's of no practical concern.

Having said that I would still like to inspect the spar periodically.