Corrosion in a Lap Joint and a Repair Decision
During a heavy check on a transport-category aircraft, an inspector finds intergranular-looking corrosion and blistering in a fuselage lap joint around a row of rivets, in the bilge area below the cabin floor. The affected area is about 150 mm long. The structural repair manual gives a permitted blend-out limit for this area and requires eddy-current inspection if blending exceeds a stated depth. The inspector proposes to blend out the corrosion, apply conversion coating and primer, and release the aircraft, stating that 'the blend is within limits'. No depth measurement has been recorded, and no eddy-current inspection has been performed. The aircraft is due for return to service in 12 hours.
1. Why is the inspector's statement insufficient, and what must be established before any blend-out?
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The statement 'within limits' is an assertion without evidence. The SRM limit is a depth limit, and no depth has been measured, so compliance cannot be demonstrated.
Must establish before blending:
- Measure the actual depth of corrosion/blend-out (e.g. with a depth gauge or by the method the SRM specifies) and compare with the permitted limit.
- If blending would exceed the stated depth, the SRM requires eddy-current inspection; that inspection must be performed to detect cracks or further exfoliation before deciding the repair.
- Confirm the corrosion type - intergranular/exfoliation corrosion in a lap joint is structurally significant and can hide beneath the joint, so the visible extent may understate the damage.
- Record all measurements and inspection results.
Without a measured depth and, where triggered, an eddy-current result, there is no approved basis for release.
2. The corrosion is in a lap joint in the bilge area. What additional structural and environmental considerations apply?
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Lap joints concentrate moisture and are classic sites for exfoliation and crevice corrosion; the bilge is a wet, contaminated area (lavatory/ galley spillage, condensation) so recurrence is likely.
Considerations:
- The joint is a load path; material removal reduces section thickness and can affect fatigue life and static strength, so the SRM's limits exist for structural reasons, not cosmetic ones.
- Corrosion products can force the joint apart (pillowing/blistering), which may indicate damage extending beyond the visible area; the joint may need to be opened or inspected by NDT.
- The source of moisture must be identified and corrected (drainage, sealant, spillage), otherwise the repair will fail again.
- Correct surface treatment after blending (removal of corrosion products, conversion coating, primer, sealant) is part of the repair, not an optional finish.
Judgement: a 'within limits' blend in a lap joint is only valid if depth, extent and the triggering NDT are all established and the moisture source is addressed.
3. The 12-hour return-to-service deadline is tight. How should the deadline influence the technical decision?
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It should not change the technical decision; it changes only the planning. Airworthiness requirements are not negotiable against schedule.
Appropriate response:
- State clearly what evidence is missing (depth measurement, eddy-current if triggered) and that release cannot occur until it exists.
- Estimate the time needed for measurement and NDT and communicate it to the operator so they can plan (e.g. delay, substitute aircraft).
- If the inspection reveals damage beyond limits, escalate to engineering for a repair scheme under approved data.
- Document the decision and the reason for any delay.
The human-factors point is that a deadline is a pressure source that can push a mechanic toward an unsupported sign-off; the correct behaviour is to protect the certification by insisting on evidence, while helping the operator manage the consequence.