Aft Pressure Bulkhead Corrosion at Annual Inspection
During an annual inspection of a 1978 pressurized twin used under Part 91, you find white powdery corrosion around a stringer attachment on the aft pressure bulkhead. Eddy-current inspection indicates about 0.020 in of material loss. The SRM allows a maximum of 0.010 in loss in that area and identifies the fastener row as fatigue-critical, requiring engineering disposition for any repair. The aircraft owner wants the annual signed off because the next inspection is 12 months away and the aircraft is only flown 80 hours per year. The bulkhead is accessible only after interior panel removal.
1. What is the airworthiness determination, and can you sign off the annual inspection?
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The aircraft is not airworthy with respect to this finding. The measured material loss of 0.020 in is twice the 0.010 in SRM limit, and the area is identified as fatigue-critical. The annual inspection cannot be approved as satisfactory until the discrepancy is corrected or an approved disposition is obtained.
You may complete the inspection and document the finding, but you cannot sign the annual as closed or return the aircraft to service. The owner's low annual utilization does not change the allowable damage limit.
2. What repair or approval path is appropriate?
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Because the loss exceeds the SRM limit and the fastener row is fatigue-critical, the repair requires engineering disposition. Appropriate paths include:
- OEM engineering authorization or a DER repair that addresses the bulkhead, stringer attachment, and fatigue-critical fastener row.
- Replacement of the affected structure if the approved repair cannot restore the required strength.
- Inspection of adjacent stringers, frames, and fastener holes to define the full extent of corrosion.
A field repair based only on the mechanic's judgment is not acceptable. The approved repair data must be followed, and the repair must be documented as a major repair if applicable.
3. How should you respond if the owner asks to defer the corrosion as a minor maintenance item?
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Do not defer it as minor maintenance. The corrosion exceeds the SRM allowable and affects a fatigue-critical pressure-boundary structure. Deferring it would leave the aircraft unairworthy and could allow the damage to grow.
Explain that the annual cannot be signed off with this discrepancy open, that the aircraft must remain out of service until an approved repair is accomplished, and that the finding and measurements will be recorded in the maintenance records. If the owner disputes the finding, the proper route is a second evaluation or engineering review, not a deferral.