Spar Joints

[blakecarl]

Our 1941 BC12-65 Tcraft is being annualed for the first time by the local A&P. He is concerned about joints in the spar. See following photos. Our spars are made from what appear to be three or four individual members glued together. The concern he has is that the joints appear not to have been sanded smooth after assembly. Because of this he is unable to determine if they are still sound. The joints look pretty much the same for the full length of all four spars with one noted exception. In the third photo just under the text “crack parallel to joint” there is just that. This was also observed in the same photo just to the left of the tin piece. This was the only location were this type of crack was observed.

Is a joint like this normal? If not, is there a way to check the integrity of the joint by sanding a spot down, or something else short of taking the wing apart? Ideally if someone is familiar with this and would be willing to have a discussion with our A&P regarding I would be happy to set up the call for this to happen. However, any sharing of experience with a situation like this would be appreciated.

Blake

[Robert Lees]

Re: Spar Joints

Those look perfectly normal joints to me. They are in fact the original factory laminations to make up the spars.

Spars were quite commonly built as laminations back then, laterally, vertically and scarf joints.

As long as there are no visible voids in the joints, they are good.

My BC12D spars have all three of the above laminations in the same spars, and they hasn't failed yet after 65 years.

If your A&P needs further info (or confirmation) of the airworthiness of your spars, let me know...I have photos of mine that look the same.

[crispy critter]

Re: Spar Joints

Rob,I agree with you and all of that does look normal but look at the 3rd picture blown up to full size,that is a little more then just a lamination joint, it is showing signs of a stress crack isn't it?

[Robert Lees]

Re: Spar Joints

A bit difficult to tell. It may be a bit of shrinkage.

[Hank Jarrett]

Re: Spar Joints

You really need to get in there and look really carefully at the "crack". Remember that the middle of the spar (top to bottom) is in shear, not tension or compression. Under "G" loading the top edge of the spar is in compression and the bottom is in tension. The middle has the top and bottom trying to slide past each other. The glue joints between the pieces are just long straight cracks in the wood that are glued together. I would NOT scrap a spar because it had what was effectively a glue bond failure. I would re-glue the failure. If it really is "cracked" it should be repairable, but I wonder if it really is a crack.
Hank

[Robert Lees]

Re: Spar Joints

A thin feeler gauge should determine its depth, and if it is a "crack", then a suitable repair scheme can be developed.

[Hank Jarrett]

Re: Spar Joints

Now if you see the crack turn and go ACROSS the grain, we will all come over for the bonfire.
Hank

[crispy critter]

Re: Spar Joints

Look at the arrow points I have placed on this pic. The crack seems to take its own course in a place or two. On the left side of the nose rib mounting tab is almost certinly a crack branching out from the original lamination joint. The top arrow on the right side couple possibly be just a scratch in the wood but is hard to tell in a picture. Also,that same lamination joint appears like it might be opening up just a bit. This might be repairable by placing a doubler over the area on both sides of the spar but it might be a little tricky because it's so close to a compression strut. Maybe I'm just looking too hard at it but I really believe that is at least one crack and maybe two.. What do ya'll think? You have to click on the picture to enlarge it,then click in it again to enlarge it more. You really have to blow it all the way up to see the suspected areas I'm talking about.

[Hank Jarrett]

Re: Spar Joints

Looks to me like it could be cracks in the grain, or it could just be scratches in the old finish.
How do the old time woodworkers feel about scraping the area to remove the old finish? I wouldn't sand it as I think that could hide the crack if it is really there but doesn't wood scraping like on restoring old furniture only remove the finish and not hide the crack?
Hank

[Robert Lees]

Re: Spar Joints

Hank, when I cleaned the old varnish off my spars, I used a Skarsten scraper. A tip from the yachting fraternity.

[Hank Jarrett]

Re: Spar Joints

It's the best way I know to remove the finish without damage to the wood. The problem is we don't want to hide the crack (if it is really there). I sent a message to Achim Engles in Germany. He is world class restorer of WW-I aircraft and builds some incredible replicas. If he doesn't know if this will work, I am betting he will know a method that will work. I will post as soon as I hear from him.
When you don't know, look for the experts!
Hank

[Hank Jarrett]

Re: Spar Joints

Achim passed my message to his adviser who responded as follows. I don't know what the liability laws are like in Germany so I am leaving off his name and contact information. This kind of advice should be taken as just that, ADVICE. If you want to eliminate all risk, don't fly. Don't even CONSIDER ANY of this advice (from him or me) unless YOU and your IA are ready to take responsibility for your safety! We are in unexplored territory here. Best case is, when you finish looking the crack isn't real or it is something well defined and repairable IAW the FAA approved methods.
I would be willing to make a decision on how I would handle this inspection and repair for MY plane, but my family knows I would rise from the dead to haunt them forever if they ever tried to sue someone for advice I decided to take. The bottom feeder lawyers (there are good lawyers and bad ones) are going to drive us out of the air.
Hank

From: XXXXXXXXX
Date: Friday, August 20, 2010 4:17 PM
To: 'XXXXXXXXX
Subject: RE: Wing spar damage?
Size: 7 KB

It is possible but difficult.

My first approach would be a microscopic inspection, using a 30x pocket
microscope, National FF-393E Light Scope, as I used on the D.VII fuselage
test last year. I would use the scope to check without removing the finish,
to determine if the "crack" is in the finish or is showing an underlying
crack in the wood. The results of this would then suggest if the finish
should be removed. The challenge here would be, in removing the finish would
the "crack" be filled? If the surface is accessible, then slicing off the
finish with a sharp knife, scalpel, may be the best way to remove it without
filling the crack. Re-inspect with the scope and decide.

Second, is a bit more uncertain, to use a wet dye test. This has the
potential to damage the wood and or the local structures. Not recommended as a non-destructive test. The number of variables on this type of test are
high. The ability of the finish to repel water, the absorption rate and
retention rate of the finish and the wood. It would require a lot of
pre-testing on sample material to determine if this test would work. Not one
I've tried at all.

Better option could be the use of ultrasonic's, but these would come at a
high cost and require specialists to make test pieces for testing the
soundness of the material against.

A challenge for sure.

Source for pocket microscope, 100X but same design, UK:

http://www.amazon.co.uk/Portable-Pocket-Magnification-Illuminated-Microscope

/dp/B001HKTGRA/ref=sr_1_3?ie=UTF8&s=toys&qid=1282335231&sr=8-3

Source for 30X, same as mine, USA:

http://www.amazon.com/Edmund-Scientifics-Illuminated-Pocket-Microscope/dp/B0

00L2LGH8/ref=sr_1_2?ie=UTF8&s=office-products&qid=1282335368&sr=8-2

I modified my scope by removing the clear plastic ring, to allow me to get
into corners more easily.

Hope this helps.
XXXXXX

Hank back talking now,
I have to agree with Achims expert on almost every thing he has said. I have a set of Jewelers Loops I have used for looking at cracks that are very similar to what he describes. I usually start with he 10 power and then step up to 30 once I am familiar with what I am looking at.
I also agree that the dye will be a real problem. I don't know of any of my NDI co-workers that have ever done it. Might have to go as far as the Smithsonian to even find someone who knows about it.
From what he is saying it sounds like a scraper will remove the finish without damage to the crack, AFTER careful inspection with the Loops.
As for ultrasonic inspection, if this was aluminum I would jump on it, but in 35 years I have never used an ultrasonic probe on wood. I wouldn't know where to start. To do ultrasonic inspections you need a standard. Where do we find a standard for decades old wood with unknown density, grain structure, hardness....... If the standard doesn't match the test part, you get garbage from the test. Maybe it is possible, but like I said, I never even thought about ultrasonics on wood before. I really would have to think about that one.
Don't know if we have added any clarity yet. I sure hope the crack isn't really there under 30 power.
Hank

[Ron Coleman]

Re: Spar Joints

The photo's are not high enough resolution to discern the nature of the joints but you can clearly see a crack (at the point of the arrow) crossing the grain. I think I would do a twist test to see if the joint or crack opens up. I would also use a good macro lens, a spot light at a sharp angle and perhaps in one shot wipe the joint with a slightly damp cloth, to clean the surface and produce a little more contrast in the grain.

Good luck with this one.

RonC

[crispy critter]

Re: Spar Joints

Another spot that I just noticed is the bottom of the spar at the butt end. Look in this picture at full size. I hate to support bad findings but from what I can see I think your local mechanic might have some reason to be concerned. Of course it's very hard to make an accurate decision without being able to look,clean,inspect,twist,gauge,etc in person.

[Hank Jarrett]

Re: Spar Joints

OK Guys, NO WAY would I make a decision based on these photos. It "could" be cracks, but I have seen an awful lot of wood with marks like the photos that was NOT cracked. You need a good wood worker to look at the real thing with a magnifying glass at AT LEAST 10 power and I really like the idea of 30 power.
I would NOT fly the spar unless i was convinced the wood was safe. I would also NOT scrap the spar unless I was sure it was actually cracked and unrepairable.
This is one I REALLY want to see the outcome on. I wish I was close to look at it in person.
Hank