Wing Spar Fastener-Hole Corrosion and Crack Indication
During a scheduled C-check on a 19-year-old twin-turboprop, eddy-current inspection of the lower wing spar cap finds corrosion in three adjacent fastener holes near station 142. The worst hole shows a corrosion depth of 0.006 in in a spar cap that is 0.250 in thick. The structural repair manual permits local blend-out to 10% of nominal thickness, provided the blend-out length-to-depth ratio is at least 20:1 and fastener edge distance remains within limits. While cleaning the middle hole, the technician also obtains a 0.030-in-long indication oriented along the spar cap grain. The aircraft is in a heavy-maintenance hangar with limited NDT support; the operator wants the aircraft back in service for a charter the next morning. The fastener holes are 0.250 in diameter, and the nearest edge distance after initial cleaning is 0.62 in.
1. Evaluate whether local blend-out of the corrosion is acceptable and identify the additional inspection needed before any return-to-service decision.
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Blend-out is not automatically acceptable just because 0.006 in is less than the 10% limit. The 10% limit equals 0.025 in, so the measured depth is about 2.4% of cap thickness and is within the depth criterion. The blend-out ratio requirement is also likely satisfiable: a 0.006-in-deep blend-out needs a length of at least 20 × 0.006 = 0.120 in, which is practical in a 0.250-in-thick cap. Edge distance after cleaning must be compared with the SRM limit; with a 0.250-in hole and 0.62 in remaining edge distance, the ratio is about 2.48D, which is normally acceptable but must be confirmed against the specific SRM table.
The controlling finding is the 0.030-in indication. A crack indication in a primary wing spar cap is not dispositioned by blend-out alone. The area must be re-inspected with a method capable of characterizing the indication, such as high-resolution eddy current or ultrasonic inspection, and the SRM or OEM must be consulted. If the indication is confirmed as a crack, the spar cap is not airworthy under the blend-out allowance; repair or replacement is required. Do not release the aircraft for the charter until the indication is evaluated and the SRM/OEM disposition is completed.
2. Assuming the indication is confirmed as a crack, outline the structural disposition and return-to-service requirements.
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A confirmed crack in a wing spar cap is a primary-structure defect and must be treated as a structural repair, not a cosmetic blend-out. The technician should stop work, preserve the inspection record, and contact the OEM or a designated engineering representative because the SRM may not cover this crack length at this location. Likely options are replacement of the affected spar cap section or installation of an approved repair strap or splice with substantiated load paths. Any repair must restore fatigue and static strength and must be inspected per the approved instructions. Return to service requires an approved repair scheme, correct fastener installation, protective coating of bare metal, and a documented inspection of all adjacent holes. The aircraft should not be released on a temporary blend-out or speed tape. The operator’s charter schedule is not a factor in the airworthiness decision.
3. Describe the corrosion-prevention and inspection steps after an approved blend-out in the unaffected holes.
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After an approved blend-out, the blended surface must be smooth and free of sharp steps, with the blend-out length-to-depth ratio and edge distance verified. The area should be cleaned and protected with the specified conversion coating and primer; bare aluminium must not be left unprotected. Fasteners should be reinstalled with the specified interference fit or sealant, and the hole should be inspected for scoring or galling. Because blend-out removes material, the area should be recorded in the aircraft maintenance records and added to the corrosion inspection program at a shorter interval if the SRM requires it. A follow-up eddy-current inspection after a defined flight-hour interval, or at the next C-check if the SRM specifies that, is appropriate. The decision should be based on the SRM limits and the operator’s corrosion-control program, not simply on the fact that the aircraft is needed for a charter.