Corrosion Found During a Corrosion Prevention Programme Check
During a scheduled corrosion prevention and control programme (CPCP) inspection on a regional turboprop, you find intergranular corrosion on the wing spar lower cap in the fuel tank area. The corrosion exceeds the CPCP baseline limit (level 2). The aircraft has 28,000 flight hours and 22,000 cycles. The structural repair manual gives a blend-out limit, but the measured depth is 0.4 mm against a limit of 0.3 mm. The operator wants the aircraft back in service within 48 hours for a charter contract.
1. What does exceeding the CPCP level 2 limit mean, and what is the correct course of action?
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Exceeding the CPCP baseline limit (level 2) means the corrosion is beyond the level anticipated by the programme and requires evaluation and corrective action before further flight. The correct course is:
- Do not release the aircraft for service.
- Report the finding through the organisation's quality system and to the operator's CAMO.
- Evaluate the corrosion against the SRM blend-out limits and, if exceeded, obtain an approved repair scheme from the TC holder or design authority.
- Assess whether the finding indicates a need to escalate the CPCP (level 2 findings may require programme review).
- Record the finding, measurement, and disposition.
A blend-out beyond the SRM limit cannot be accomplished under the SRM alone; it needs approved data.
2. How do you handle the operator's 48-hour deadline?
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The deadline is commercial, not airworthiness, and cannot override the structural finding. Options to present:
- Expedite design approval - request an approved repair scheme from the TC holder/design authority as a priority.
- Temporary repair - only if an approved temporary scheme exists and is permitted.
- Ferry - only under a specific NAA ferry authorisation with engineering justification and flight restrictions.
- Substitute aircraft - advise the operator to source alternative capacity.
You must not sign a CRS until the structure is restored to an approved condition. Document the pressure and the decision.
3. What records must accompany the repair, and why does traceability matter here?
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Records must include:
- the finding (location, measurement, corrosion type);
- the approved data used (SRM reference or repair scheme number);
- the materials and fasteners used, with batch/lot traceability and certificates of conformity;
- the process (blend-out profile, surface protection, sealant, torque values);
- the certifying engineer's signature and date;
- the post-repair inspection results.
Traceability matters because the repair affects primary structure; without it, subsequent inspections and continued airworthiness cannot be assured, and the aircraft's maintenance history would be incomplete.