Thorp Air Command - T18.net

Hey guys,

I was inspecting the wings today and found a crack about 1 1/2" long on front face of inboard main spar fitting between the top and bottom main spar pins. It was fairly straight and in the middle of the fitting (not starting from an edge like you would expect).





The crack stopped about 3/4 inch above the bottom pin and goes half way up to the other spar pin hole. It was definitely noticeable and went through the primer and felt like into the metal. I was hoping it just went into the cadmium plating and no deeper, so I sanded off the plating in the cracked area. I can no longer find any evidence of a crack. I am going to dye penetratant check it tomorrow to be sure. Has anyone else had the cadmium plating crack in this way on a steel part? I am fairly certain it was just the plating cracked as it is not in an area you would expect a crack to form.

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Bernie Fried
N18XS

Is this an S wing Bernie, I dont see the crack but I would sure check it out..Dan

Bernie: I did not see a crack either in the area that you sanded on the inboard main spar fitting.

Since you are going to dye penetratant check the fitting may I also suggest that you extend you check for cracks to the attaching hardware about 1/2 a inch above the lower pin hole. Cracks can some times develop in areas where attaching hardware is located.

In case you do not know how to perform the dye penantant check or LPI (Liquid penantant Inspection) here are a couple references:

http://en.wikipedia.org/wiki/Dye_penetrant_inspection
http://www.twi.co.uk/content/ksijm001.html

My recommendation for dwell time is 20-30 minutes as small cracks (shallow cracks that do not penitrate the suface very deep) require longer penatation time. (longer time for the penatrate to be absorbed into the crack) Also as the article states cleaning preparation is key to a successful test.

Hopefully all you have is damage in the plating and not the underlying material.
Cheers Vic

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Victor J Thompson
C-GIRQ
"In Memory Of Dad"

Here is an update on what I found. Unfortunately I did not take a before picture. I discovered the crack 2 days before I was planning on leaving for Oshkosh and was a bit rushed, so didn't have time to post my findings till now.

The fitting is plated with something (doesn't look like cadmium) and the plating is very thick and quite rough (orange peel texture). I removed the primer and the crack was still quite evident. I sanded the area over the crack and eventually got down to the base metal. As I hit the base metal the crack disappeared. I then dye penetrant checked the fitting and no cracks were evident. I let the dye set about 25 minutes for a good test. I removed the steel spar catch plate that sits between the two legs of the inboard spar fitting to inspect the back side for cracks and none were evident. The plating on the fitting was about the thickness of very thick paint.

The picture posted shows the metal after the sanding was completed. The crack was lined up with the inboard edge of the steel piece that sits just behind the forward spar fitting that is there to hold the outer wing when the pins are removed. My theory as to why the crack occured in this low stress area of the fitting is that when the slide hammer is used to insert the spar pins, it introduces a slight vibration or ringing to the fitting as the pin is seated. It wouldn't seem like this is enough to flex the forward spar fitting about the steel piece sitting behind it, but is my best guess.

Whatever the cause, good news it didn't affect the fitting! Flew to Oshkosh and back and all is well.

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Bernie Fried
N18XS

I forgot to add that it is a folding wing with original airfoil and I use pins(not bolts)on the main spar.

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Bernie Fried
N18XS