Wing Spar Web Corrosion at a Fastener Row
During a C-check on an 18-year-old Part 25 twin-engine airplane with 22,000 flight cycles, eddy-current inspection finds corrosion on the lower wing spar web. Nominal web thickness is 0.250 in. The structural repair manual (SRM) sets a minimum allowable thickness of 0.225 in and permits blend-out only if the blended area is at least 1.0 in from any fastener row. The deepest corrosion measures 0.238 in remaining thickness and lies 0.75 in from the nearest fastener row. The adjacent fastener hole shows 0.008 in elongation; the SRM hole limit is 0.010 in. The aircraft is scheduled for return to service in 36 hours.
1. Is blend-out per the SRM acceptable? State the disposition and the reasoning.
Read the worked answer
No. The remaining thickness of 0.238 in is above the 0.225 in minimum, so thickness alone would allow blend-out. However, the corrosion is only 0.75 in from the nearest fastener row, and the SRM requires at least 1.0 in clearance. The proximity limit is violated, so blend-out under the SRM is not permitted.
Disposition: do not blend the area. The damage must be evaluated by engineering or repaired with approved data. Because no approved blend-out or repair is available on-site, the aircraft cannot be returned to service on the current schedule.
2. How should the adjacent fastener hole affect the decision, and what inspection follow-up is needed?
Read the worked answer
The 0.008 in hole elongation is within the 0.010 in SRM limit, but it is in the same fatigue-critical area as the corrosion. A single within-limit finding does not close the issue because the corrosion disposition already requires engineering.
Follow-up:
- Inspect the entire fastener row and adjacent rows for elongation, cracking, and corrosion.
- Record hole sizes and locations so engineering can evaluate the combined effect.
- If the approved repair requires fastener removal, ream or oversize only per the approved repair data.
- Establish repetitive inspection requirements if engineering directs them.
3. What documentation and approval steps are required before return to service?
Read the worked answer
The repair will be a major repair because it affects a primary structural element and requires engineering disposition. Before return to service:
- Obtain approved repair data, such as an STC, DER repair, or OEM engineering authorization.
- Record the corrosion findings, measurements, and inspection method in the maintenance records.
- Complete FAA Form 337 if the repair is a major repair or alteration.
- Have the repair inspected and approved as required by the applicable maintenance and inspection program.
- Return to service only after the approved repair is complete and all required inspections are signed off.