Inspect for loose or missing rib nails per Service Letter C-139. If loose or missing nails are found, inspect spar by the rib for damage resulting from the rib rubbing against the spar. If the friction between the rib and spar has broken the wood grain completely across the spar and more than 1/16" deep, this is cause for rejection. Also inspect for nail hole elongation. Rejection must also occur if the nail holes have elongated such that wood grains are broken more than 1/16".
Inspection: The only area where it is possible to positively identify a compression crack is on (Bottom/Top) the top and bottom surfaces of the spar. Both front and rear spars need to be inspected. The key areas to be concerned with are shown in figure 1. Additional inspection holes may be necessary to do a thorough inspection. Service Letter 417, Revision C, may be used as a guide for installing additional inspection holes.
Warning :Compression failures are often difficult to detect with the unaided eye. Do not expect an open crack or gap. Compression failures start as barely visible, minute, jagged series of lines running cross grain on the top or bottom of the spar. Important : The initial inspection described below does not guarantee adequate access to complete the inspection and, is not recommended for inspectors without wood compression failure experience. Also, this method is inadequate for aircraft with wing damage history since last spar inspection.
For initial inspection of the spar top, a high intensity flexible light (for example: Bend-A-Light) and small inspection mirror can be used in conjunction with a small wooden wedge. First temporarily push the leading edge skin away from the front spar cap gently with the wooden wedge. After closely illuminating the area of concern with the Bend-A-Light, inspect, as closely as possible, with the small inspection mirror. This method is useful at the ends of the doubler plates on top of the front spar. This is considered an initial inspection only. If there are any questionable findings, further inspection through additional inspection holes is required.
For initial inspection of the spar bottom, a high intensity flexible light and two mirrors can be used. After resting one mirror on the fabric under the area of concern and placing the Bend-A-Light adjacent to the area, inspect with the other mirror, using the reflection off of the former mirror. Again, this is considered an initial inspection only. If there are any questionable findings, further inspection through additional inspection holes is required.
Note : It may be helpful during compression failure inspection to apply upward or downward force at the wing tip.
Alternatively, the top and bottom of the front and rear spar may be inspected through inspection holes on the bottom of the wing using a flexible probe borescope.
Instructions: If any damage was found in above inspections, it requires that the spar be repaired or replaced. Minor chafing and small indentations are not cause of rejection. Spar repair shall be accomplished in accordance with AC 43.13-1A Acceptable Methods, Techniques and Practices or other Federal Aviation Administration approved data.
Figure 1 Crack Location
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