Fuselage skin crack at a lap joint: SRM limits, ferry and widespread damage
During a scheduled A-check at a Part 145 organisation in Christchurch, an inspector working the left side of a Boeing 737-800 (62,400 cycles, 41,200 hours) finds a 22 mm crack running from a fastener hole in the fuselage skin just below the lap joint at stringer S-14, about 40 mm aft of a 2019 blend-out repair. Two adjacent fastener holes show intergranular corrosion, and one of them has a second, shorter crack about 6 mm long. The structural repair manual allows cracks up to 15 mm in that skin bay, provided they are stop-drilled and inspected at 1,200-cycle intervals; anything larger, or any crack in a fastener hole within three pitches of another crack, requires a repair scheme approved by the manufacturer. The aircraft is due for a C-check in 280 cycles and is scheduled for a 90-minute revenue flight that evening. The shift supervisor asks whether the aircraft can be released and what you need before signing anything.
1. Explain how you decide whether this damage is within the SRM allowable limits or requires an OEM-designed repair, and what you would document.
Read the worked answer
Work through the manual in a fixed order rather than jumping to the crack size.
- Identify the structure. Confirm the skin material and thickness, the fastener type and pitch, the bay boundaries, and that the crack is in the zone the SRM figure actually covers. A limit quoted for one skin bay does not automatically apply one frame away.
- Compare against the allowable. At 22 mm the primary crack exceeds the 15 mm allowable, so the SRM disposition stops there. It does not matter that stop-drilling is permitted for smaller cracks.
- Check the exclusion clauses. The 6 mm crack in an adjacent fastener hole lies within three pitches of the main crack. The SRM explicitly excludes multiple cracks in this relationship because they interact and grow faster than a single crack. That clause removes any argument for using the SRM.
- Deal with the corrosion. Intergranular corrosion in the fastener holes has to be removed within blend-out limits; if blending takes the skin below allowable thickness, the repair scheme must restore the structure.
- Route to approved data. A repair design from Boeing or from an approved Part 21 design organisation, with a damage-tolerance justification tied to the remaining cycles. The embodiment is then a Part 145 job against that approved data.
Documentation: a defect/non-conformance record with a dimensioned sketch and photographs, crack map, fastener locations, corrosion depths, the SRM reference and the clause that excluded it, the repair drawing number and revision, a statement of the remaining life assumed, and a Certificate of Release to Service that references the approved data. Also inspect the mirrored right-hand location and the adjacent frame and stringer before anyone talks about returning the aircraft to service.
2. The operator asks whether the aircraft can be ferried to its main base for repair. Outline what would have to be true for a special flight permit and what you would recommend.
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A special flight permit is not a shortcut around the SRM. It is a CAA authorisation to fly an aircraft that does not currently meet its type design, with conditions that make the specific flight safe.
What must be true:
- A structural case for the flight. Someone competent, normally the OEM or an approved design organisation, must show the crack will not reach critical length within the flight. Because this crack is at a lap joint, with a secondary crack nearby and active corrosion, the growth rate cannot be assumed to be that of a single crack.
- Restrictions in the permit. Typically unpressurised or pressurisation-limited, reduced altitude and speed, no passengers, minimum crew, a specified route and duration, and a flight manual supplement. One flight cycle is easier to justify than a series of sectors.
- Growth control. Stop-drilling can arrest growth, but only if it is approved for this configuration and does not conflict with the eventual repair.
- Approval before flight. Permit issued, conditions briefed to the crew, and a release document that states exactly what is and is not permitted.
My recommendation here would be against a pressurised ferry. At 62,400 cycles, with corrosion in the fastener holes and two interacting cracks in a lap joint, the structure is showing the signature of multiple-site damage. I would push for repair on site, or an unpressurised low-altitude ferry only with OEM concurrence and CAA approval, and I would not sign anything until that concurrence is in writing. The cost of a ferry is small compared with the consequence of a lap joint failing at altitude.
3. What features of these findings make you suspect widespread fatigue damage, and what action follows?
Read the worked answer
The concern is not the 22 mm crack by itself; it is the pattern around it.
- Location. A lap joint below the window line is a classic multiple-site damage area, where many small cracks in adjacent fastener holes can link up and turn a slow, inspectable crack into a fast one.
- More than one crack. Two cracks in adjacent holes, and a second crack within three pitches of the first, is the definition of multiple-site damage rather than a single rogue defect.
- Corrosion. Intergranular corrosion in the fastener holes both initiates cracks and hides them from visual inspection, so other holes in the same bay cannot be assumed sound.
- High cycles and a prior repair. 62,400 cycles with a blend-out nearby means the stress field has already been altered once.
Actions:
- Remove fasteners and do a high-frequency eddy current or bolt-hole inspection of every hole in the affected bay and at least one bay either side, not just the visibly cracked ones.
- Check the supplemental structural inspection programme and any applicable airworthiness directives for that lap joint, and verify compliance. A missed or late inspection here is itself a finding.
- Report to the operator and to the CAA; a defect that may affect other aircraft in the fleet is a reportable matter, and the OEM needs the data to decide whether a fleet-wide inspection or directive is warranted.
- Record cycles and hours accurately, because the next aircraft through the hangar may need the same inspection.