Corrosion Find During a Wing Spar Inspection
You are a licensed engineer performing a scheduled 2A-check inspection on a 12-year-old twin-turboprop. The maintenance manual requires a detailed visual inspection of the lower wing spar boom access panels. On removing panel 4, you find white powdery deposits around three fastener heads and a dull, slightly pitted area roughly 20 mm long on the boom surface. The aircraft operates from a coastal base and the operator reports it is washed monthly. The structural repair manual (SRM) gives a permitted blend-out limit of 0.5 mm depth with re-protection, but no NDT is called out unless pitting is suspected. The operator wants the aircraft back on the line tonight for a charter.
1. What corrosion type is most likely, and what immediate technical actions must you take before any decision on return to service?
Read the worked answer
The white powdery deposits and pitting on an aluminium alloy boom in a coastal, moisture-trapping location point to exfoliation or pitting corrosion, likely exacerbated by galvanic action at the steel or cad-plated fasteners.
Before any release decision:
- Clean and strip the area to bare metal per the SRM to reveal the true extent - surface deposits hide depth.
- Measure pit depth with a dial indicator or depth gauge against the 0.5 mm blend-out limit.
- Because the SRM says NDT if pitting is suspected, and the visible pitting plus fastener involvement qualifies, carry out the specified NDT (e.g. eddy current or ultrasonic per the SRM) to confirm no subsurface or intergranular spread.
- Document findings with measurements and photos in the tech log and raise a defect.
Only after the true depth and extent are known can you compare against the SRM allowance. You cannot approve a return to service on visual appearance alone.
2. The measured pit depth is 0.35 mm and NDT is clean. The operator asks you to sign it off tonight. What is your decision and what conditions apply?
Read the worked answer
0.35 mm is within the 0.5 mm blend-out allowance and NDT is clean, so a blend-out repair is permissible - but not a simple 'sign and go'.
Conditions:
- Blend out to the SRM profile (no sharp edges, smooth transition), then re-protect with the specified primer and sealant.
- Replace or re-protect the fasteners showing deposits; the corrosion source must be removed, not painted over.
- Verify the blend depth after blending does not exceed 0.5 mm - blending can deepen the pit.
- Record the repair, measurements and NDT result in the tech log and structural records.
- Consider a corrosion prevention compound treatment and review the wash cycle - monthly washing at a coastal base is marginal; recommend more frequent washing and a CPC application.
- The repair is within my licence scope for a permitted SRM blend-out, so I can certify it, but the aircraft should not depart until re-protection has cured per the manual.
3. Six months later a similar find appears on the same panel. How does this change your airworthiness judgement?
Read the worked answer
A repeat finding changes this from an isolated defect to a systemic trend, which is an airworthiness management issue, not just a task sign-off.
Actions:
- Raise it as a repetitive defect and consider whether the inspection interval is adequate for this operating environment - the coastal base and monthly wash may be insufficient.
- Notify the quality/airworthiness manager so the trend is captured; a repeat structural corrosion finding may warrant a maintenance programme variation (shorter interval, expanded inspection area) or communication to the Type Certificate Holder.
- Expand the inspection to adjacent panels and the opposite wing to establish the true fleet/airframe picture.
- Review whether the previous repair was correctly protected and whether the corrosion source (fastener sealant, drainage) was addressed.
- If the trend suggests the SRM limits are being approached repeatedly, seek engineering/design authority advice rather than continuing to blend out indefinitely - cumulative blending reduces structural margin.