Wing Skin Corrosion Beyond Blend-Out Limits
During an annual inspection, a mechanic finds pitting corrosion around steel fasteners on the lower wing skin of a 1970s trainer. The skin is 2024-T3 aluminum, 0.032 inch thick. The manufacturer’s structural repair manual allows local blend-out to a maximum depth of 0.004 inch; deeper damage requires a repair scheme or engineering approval. The mechanic measures a pit depth of 0.006 inch over an area about 2 by 3 inches. He proposes to sand the area with 80-grit paper, apply alodine, and prime it. The aircraft is otherwise airworthy. The owner wants the aircraft back in service quickly because a student has a checkride scheduled.
1. Determine whether the corrosion can be blended out under the SRM limit. Show the thickness calculation and the decision.
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SRM blend-out limit = 0.004 in. Measured pit depth = 0.006 in. 0.004 / 0.032 = 12.5% allowable. 0.006 / 0.032 = 18.75% measured. If 0.006 in were removed, remaining thickness = 0.032 - 0.006 = 0.026 in.
The measured depth is 0.002 in beyond the SRM blend-out limit, so the area cannot be simply sanded and primed under that allowance. It needs an SRM-approved repair scheme or engineering or DER approval. A cosmetic blend would remove structural material without substantiation.
2. What corrosion removal and protective finish steps are acceptable if the damage had been within limits?
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Use a non-aggressive removal method approved for aluminum, such as fine abrasive paper or an approved nonwoven pad, and blend smoothly without gouging. Stay within the SRM limits. Clean per the manufacturer, apply the specified conversion coating (for example alodine per MIL-DTL-5541), then the specified primer (for example MIL-PRF-23377 epoxy primer) and topcoat or sealant. Replace or treat corroded fasteners and install them with approved primer or sealant to break galvanic contact. Do not use steel wool; it leaves iron particles that promote corrosion.
3. Is this repair a major repair? What records and approvals are required?
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A skin repair of this depth around fastener holes likely affects structural strength, so treat it as a major repair unless the SRM explicitly provides a minor repair procedure. Major repair requires approved data, FAA Form 337, and approval for return to service by an IA or repair station. Record the corrosion found, the blend or repair method, materials used, and the approved data reference in the maintenance record. If the SRM does not cover the damage, obtain engineering approval before proceeding; do not return to service on the owner’s schedule alone.