Wing Skin Dent: Within Limits, or a Major Repair?
A Cessna 172 arrives for an annual inspection. On the lower surface of the right wing you find a smooth dent in the 0.032-inch 2024-T3 skin: 1.1 inch diameter, 0.055 inch deep, located 0.8 inch outboard of the nearest rib rivet line and 4 inches aft of the front spar. There is no crease, a dye penetrant check of the dent and the adjacent rivet row is negative, and all nearby rivets are tight and undamaged. The manufacturer's structural repair manual for this zone states that dents up to 0.100 inch deep are acceptable if the dent does not intersect a rivet line, frame, or spar, has no sharp crease or crack, and adjacent structure and fasteners are undamaged; beyond those limits, repair per the SRM is required. The aircraft owner wants the annual signed off today.
1. Is this dent within the manufacturer's limits? Walk through each criterion rather than answering yes or no.
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Evaluate each SRM condition separately:
- Depth: 0.055 in against a 0.100 in limit - passes, with margin.
- Rivet line intersection: the dent radius is about 0.55 in and its edge is 0.8 in from the rivet line, so it does not reach the line - it does not intersect it. It is 4 in from the spar, well clear.
- Crease/crack: no crease; penetrant check negative over the dent and the adjacent fasteners.
- Fasteners and adjacent structure: rivets tight, no working or damage.
One more step before concluding: look inside the wing at the rib and spar flange under the dent for secondary damage, and confirm the zone is actually covered by the SRM figure you are using. Assuming both check out, the dent is within limits, no structural repair is required, and the finding is recorded without a 337.
2. Change the facts: the dent is 0.090 inch deep and it crosses the rib rivet line, with two deformed rivets. What is the disposition, and how does the paperwork change?
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The dent now fails the SRM criteria - it intersects a rivet line and involves damaged fasteners - so it must be repaired using the SRM's repair for that zone (typically a doubler or skin patch with additional fastening, plus replacement of the deformed rivets).
Paperwork implications:
- The SRM repair is FAA-approved data; if the SRM has no applicable repair, you need other approved data (DER repair or field approval). You cannot invent the repair.
- A skin repair of this type, with reinforcement of primary structure, falls within the major repair categories of Part 43, Appendix A, so a Form 337 is required and approval for return to service must come from an IA (14 CFR 43.7).
- Before and after the repair, inspect the rib, flange, and surrounding skin for secondary damage, and record everything per 43.9, including the approved data referenced.
3. The owner says, 'AC 43.13 says dents under 1/8 inch are fine - just sign it off.' How do you respond?
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Two problems with that argument.
- The number fails anyway. One-eighth inch is 0.125 in, which exceeds the SRM's 0.100 in limit. Even if the AC said what he thinks it says, the dent would still be beyond the manufacturer's criterion.
- AC 43.13-1B is not approved data that overrides the SRM. It is a compendium of acceptable methods, techniques, and practices. The structural repair manual is the controlling document for damage limits and repairs on this airframe. Using the AC to sign off damage outside the SRM limits is a deviation from approved data.
The legitimate paths are: show the damage meets the SRM, obtain approved data that covers it, or accomplish the repair. 'Sign it off' is not one of them.