Corrosion Found During a Wing Spar Inspection
During a scheduled inspection, a technician opens an access panel on the lower wing surface and finds white powdery deposit and pitting along the faying surface of a spar cap where it meets the skin. The area is in the wheel well region, exposed to moisture and de-icing fluid residue. The aircraft is 18 years old and operates in a coastal environment. Pitting depth is estimated at 0.3 mm by visual comparison with a corrosion chart, but no depth gauge or eddy current check has been done. The structural repair manual allows blending of corrosion up to 10 percent of the member thickness. The spar cap is 6 mm thick. Maintenance control asks whether the aircraft can be returned to service after cleaning and applying corrosion-preventive compound.
1. Evaluate the proposed return to service and identify what must be established before any blending or release decision.
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Returning to service after cleaning and CPC alone is not acceptable. Cleaning removes the visible product but does not establish the extent of damage. The pitting is on a faying surface of a primary structural member in a wet, contaminated region, which is a classic site for exfoliation and intergranular attack that can be deeper than surface appearance suggests.
Before any decision, the extent must be quantified. Visual comparison with a chart is not a measurement. The technician should use a depth gauge where accessible, and eddy current or ultrasonic inspection to map pit depth and detect subsurface attack, especially under the faying surface where the skin overlaps the cap. Only then can the 10 percent blend limit be applied; on a 6 mm cap that is 0.6 mm, so an estimated 0.3 mm is within limits only if confirmed and if no cracking or exfoliation is present.
If depth exceeds limits, or if cracking is found, engineering disposition is required. Cleaning and CPC are protective follow-up, not a repair.
2. Assuming the corrosion is confirmed within blend limits, describe the corrective and preventive actions and the inspection implications for the fleet.
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Corrective action: blend the corrosion out per the SRM, maintaining smooth contours, respecting the blend limit, and re-protecting the bare surface with the specified primer and CPC. Inspect the adjacent faying surfaces and fastener holes for hidden attack, since corrosion in this region often extends along the joint. Replace any hardware damaged by corrosion.
Preventive action: restore drainage and sealant integrity in the wheel well area, remove accumulated de-icing fluid residue, and ensure the CPC application covers the affected zone per the maintenance program. Consider whether the corrosion-prevention tasks for this area are adequate for a coastal operator.
Fleet implications: a finding in a known susceptible area on an 18-year-old coastal aircraft should prompt a review of other aircraft in the fleet for the same condition, and may justify escalating the inspection interval or adding a targeted task. The finding should be recorded with location, depth, blend amount, and corrective action so trends can be tracked. Any change to inspection intervals must go through the approved maintenance program revision process.