Lap-Joint Corrosion and Fastener-Hole Crack at a 737-800 C-Check
During a C-check on a 737-800 with 42,000 flight cycles, an inspector finds a 180 mm long, 0.35 mm deep corrosion pit in the fuselage skin at a lap joint, station 727, left side, between frames 24 and 25. The SRM allows blend-out to 0.20 mm below nominal skin thickness, provided the blend is within a 300 mm circle and no more than two adjacent fasteners are affected. Nominal skin is 1.6 mm. The blend would remove 0.35 mm, leaving 1.25 mm. A second area 150 mm away shows a 12 mm crack at a fastener hole. The aircraft is due for release in 18 hours; the operator wants a ferry flight to a base 2 hours away with 30 passengers. Outside temperature is 38°C. The structural repair engineer must decide.
1. Evaluate the corrosion and the crack against the stated SRM limits. Can the corrosion be blended out and the aircraft released under the SRM?
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The blend would remove 0.35 mm, while the SRM allows only 0.20 mm below nominal. Since 0.35 > 0.20, the corrosion exceeds the blend-out depth allowance, so a simple blend-and-release is not permitted. The 12 mm crack at a fastener hole 150 mm away is a separate finding. A crack of that length at a lap-joint fastener is not covered by the stated blend allowance and falls in the range where a damage-tolerance assessment and multi-site damage inspection are needed. The two-adjacent-fastener limit and the 300 mm circle are not the controlling issues here; the depth and the crack are. The aircraft should not be released under the SRM. The corrosion needs an approved repair such as a doubler or splice, or an OEM engineering disposition, and the crack needs its own assessment. Before repair, inspect the full lap joint with high-frequency eddy current to map any adjacent cracking.
2. Can the operator ferry the aircraft with 30 passengers as proposed? Identify the approvals and restrictions you would require.
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No. A passenger ferry with known damage that exceeds SRM limits is a revenue flight, not a ferry, and I would not authorize it. A one-time ferry would need a special flight permit or equivalent authorization from the airworthiness authority, plus an engineering assessment showing the damage is safe for the specific flight. I would require: passengers prohibited; pressurization limited, likely unpressurized or at reduced differential; reduced speed and altitude; avoidance of turbulence; inspection before and after the flight; and a route that keeps the aircraft near a suitable airport. The 38°C outside temperature affects takeoff performance and engine cooling, but it is not the main reason to refuse; cyclic pressurization and gust loads on a cracked lap joint are. If the crack cannot be shown to be within damage-tolerance limits for the planned flight, no ferry should be approved. The 18-hour release deadline does not change the airworthiness decision.
3. What NDT and documentation should accompany the repair and return to service?
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After blending or repair, inspect the area with high-frequency eddy current for cracks at fastener holes and with fluorescent penetrant for surface cracks on any blended surface. Eddy current is preferred over visual inspection because lap-joint cracks often start under the fastener head or between skins. Sample fasteners may need removal to inspect the underlying skin. Record the pre-repair map, blend depth, repair drawing, fastener type, and post-repair inspection results. If the repair is outside the SRM, treat it as a major repair or major modification and obtain design-approval approval and a release document signed by authorized personnel. Include a damage-tolerance or inspection program if the repair affects inspection intervals. The return-to-service entry should reference the approved data, not just say repaired.