The Structural Threats Hidden Beneath Fresh Paint
A glossy polyurethane topcoat often provides a false sense of security. In the Midwest, where humid summer conditions give way to freezing winter hangar storage, condensation collects inside enclosed wing cavities.
Without routine boroscopic inspection and proper drainage paths, simple surface oxidation quickly progresses into intergranular and exfoliation corrosion, directly threatening spar cap margins and wing attach fittings.
Three Visual Indicators Every Aircraft Owner Should Monitor
Pillowing Around Rivet Rows: When oxidation forms between skin sheets and internal rib flanges, the expanding aluminum oxide forces the metal outward. This creates small blisters or "smoking" dark streaks around rivet heads, indicating loss of rivet clamping force.
Fastener Exfoliation on Spar Caps: Many general aviation airframes are subject to Airworthiness Directives (ADs) requiring structural inspections (such as eddy current tests on lower spar caps). Delaminating layers of aluminum around primary attach bolts require immediate grounded intervention.
Lap-Joint Crevice Attack: Capillary action draws trapped moisture into the overlap where upper and lower wing skins meet. If the protective primer barrier degrades, corrosion spreads horizontally along the joint.
Approved Structural Repair Standards (FAA AC 43.13-1B)
Correcting structural wing damage requires strict alignment with FAA Advisory Circular 43.13-1B:
Mechanical Dressing: Minor surface corrosion must be carefully removed within manufacturer skin thickness tolerances using non-metallic Scotch-Brite pads or aluminum oxide abrasives. Steel wire brushes are strictly prohibited, as they embed ferrous particles that accelerate galvanic corrosion.
Chemical Conversion (Alodine MIL-DTL-5541): After mechanical cleaning, bare 2024-T3 or 6061-T6 aluminum must be acid-etched and treated with a chromate conversion coating before applying an epoxy primer.
Engineered Splice Patches: If pitting or metal removal exceeds 10% of nominal skin thickness, the area requires an engineered flush or lap splice. Technicians must calculate proper rivet spacing, minimum edge distance (at least $2\times$ the hole diameter), and uniform shear distribution to prevent localized stress fractures.
Concerned About Wing Corrosion or Due for an Annual Inspection?
Greater Cincinnati Airmen provides eddy-current testing, structural sheet metal repair, and corrosion mitigation right here at Lunken Airport. Book your consult now by clicking below.