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When dirt and moisture sit on top of you spar in the wing root, it will form corrosion. It can be of two types, a light pitting or blisters that stand out and are deeper than they are tall.
The general run for pitting is that if any one pit after cleaning out all the corrosion and getting down to new material is to reject the spar if the depth of the pit is more than 15% of the spar wall thickness.
On Cheetahs this is 0.280 inches and on Tigers it is 0.303 inches. So 15% would be 0.042 for an AA5A and 0.045 for a AA5B. Usually one blister will
reject a spar and multiple blisters seal the deal. -
Where is your information for the 15% of the spar wall thickness? Also I’m working on a 73’ traveler with the center spar having one spot of inter granular corrosion. The depth is .040”. What is recommended other than replacement? Also are there any other sources other than fletchair that might have the center spar for replacement?
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This information is not correct. I talked and received emails from Lloyd Montague @ True Flight. He says anything beyond .010” is unairworthy. You can only clean light surface corrosion. The 15% rule from the FAA only pertains to non structural and life limited parts. Theses Spars do not qualify as either. Also spoke with Fletchair and they are saying that .244” is minimum thickness. This is also not true, this is a manufacturing tolerance when they were making these parts. That measurement also pertains to the last 11.6” of the center spar. Grumman or gulfstream never gave tolerances for damage to theses spars and there is no approved repair. Hope this information is useful.
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