2.4 Repair & Inspection Procedures for Non-Ferrous Structures

Scope of Sub-Module 6.2(c)

6.2(c) — "Repair and inspection procedures for non-ferrous materials, structures, and airframes" is a level 2 requirement for B1 and B3 and level 1 for B2. In practice it means the repair of aluminium alloy semi-monocoque structure: skins, stringers, frames, ribs, spar caps and fittings — plus the special handling rules for magnesium and titanium introduced in Section 2.3.

The governing principle, stated in every SRM, is simple to say and hard to apply: a repair must restore the strength, stiffness, fatigue life, corrosion protection and, where relevant, the aerodynamic contour of the original structure. Restoring static strength alone is not a repair — a stiff patch that shifts the load path can create a new fatigue origin one rivet row away.

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Inspection Before Repair

Aluminium is non-magnetic, so magnetic particle inspection is useless. The non-ferrous NDI toolkit is:

Eddy-current conductivity measurement (in %IACS) is the standard field method for deciding whether a suspect panel has been over-heated in a fire or lightning strike: heat damage changes the temper and therefore the conductivity, without changing the appearance.

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Classifying the Damage

The SRM divides damage into the same three classes as ferrous structure — negligible, repairable, and replace — but adds an aluminium-specific pair of limits you must be able to apply:

• Blend-out (clean-up) depth. Corrosion or a nick is removed by blending a smooth, faired depression. SRMs typically allow removal of up to about 10 % of the local material thickness as negligible damage, with a 20:1 blend ratio (the blend-out is at least 20 times longer than it is deep) so no stress riser is created. Always read the actual figure from the SRM — it varies by structure and by aircraft.
• Residual thickness. After blending, measure with an ultrasonic thickness gauge or a depth micrometer. If the remaining thickness is below the SRM minimum, the damage becomes repairable-by-patch or replace.

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Crack Repair in Sheet Metal

1. Find both crack tips, using penetrant or eddy current — a crack rarely ends where the eye says it does.
2. Stop-drill. Drill a small hole of 3/32 inch (or 1/8 inch) at the extreme end of the crack, centred exactly on the tip. A stop-hole that misses the tip does nothing; a hole drilled short of the tip leaves the crack free to run.
3. Add reinforcement to carry the stress across the damaged portion and stiffen the joint. Stop-drilling alone is a temporary measure to prevent growth until the structural repair is embodied — it is not a repair.
4. Understand why the crack appeared: cracks form where a stress concentration (a nick, a tool mark, a scratch, a forming or heat-treatment residual stress, a design detail) coincides with repeated stress. Increasing sheet thickness alone is usually beneficial but does not necessarily remove the condition that caused the cracking.

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Patch and Splice Repair

Material Selection

Repairs to aluminium alloy members are made with the same material, or with suitable material of higher strength. 7075 has greater tensile strength than 2014 or 2024, but it is notch-sensitive and has poorer fracture toughness — which is why 2024-T3 is the standard skin repair material even where 7075 would be "stronger". If a thinner but stronger alloy is substituted, the area must still be substantiated.

Where cladding is cut through, the exposed core alloy loses its sacrificial protection. Every cut edge, every drilled hole and every blended area must be re-protected with chromate conversion coating and primer, and structural fasteners should be wet-installed in sealant or primer.

Rivet Pattern Rules

The number of rivets is calculated so that the splice carries the design tensile load of the member without failing the member at the outermost hole. AC 43.13-1B Tables 4-9 to 4-11 give the rivets required per inch of width for the common alloys; the splice bar is tapered at its ends and the outer holes are made small and staggered back, so that load is fed in gradually and the joint does not fail progressively rivet by rivet.

Skin Repairs

Where skin is extensively damaged, the correct repair is to replace an entire panel from one structural member to the next, with the repair seams lying along stiffening members or bulkheads. Each new seam must reproduce the manufactured seam exactly in rivet size, splicing and pattern; if the two adjacent manufactured seams differ, the stronger one is copied.

Flush (scarf or plug) patches are used where aerodynamic smoothness matters; overlay (external) patches are used elsewhere. If adjacent stringers are spliced, stagger the splices so a single cross-section does not contain every joint.

Tubular Members

Round or streamline aluminium tubular members are spliced with an internal or external sleeve. Two rules are examinable: splices in struts that overlap fittings are not acceptable, and where a solid rivet passes completely through a hollow tube its diameter must be at least one-eighth of the outside diameter of the outer tube. Rivets loaded in shear through tubing are hammered only enough to form a small head — driving a full round head buckles the rivet inside the tube.

Forming Rules That Constrain the Repair

• Minimum bend radius depends on alloy, temper and thickness. 2024-T3 has a far larger minimum bend radius than 2024-O or 5052; bending a T3 skin to a soft-alloy radius cracks the outer fibre.
• Setback for a 90° bend equals the bend radius plus the metal thickness; other angles use the K-chart. Getting setback wrong makes the formed part the wrong length.
• Do not form heat-treated material cold beyond its limits. Where a complex form is needed, the part is formed in the annealed (O) condition and then heat treated — which brings back all the quench-delay and distortion constraints from Section 2.2.

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Prohibitions and Special-Metal Rules

• Do not fusion weld 2024 or 7075. Both crack severely from welding heat. 2024 is used for wing and fuselage skins and most structural parts; 7075 for machined fittings such as wing-spar and landing-gear attachments. Repairs to these alloys are mechanically fastened, not welded.
• Damaged castings are replaced, not repaired, unless the repair is specifically approved.
• Elongated bolt holes in fittings designed without bushings are not reamed oversize and are never filled with welding rod; the fitting is replaced unless an approved repair exists.
• Magnesium. Never repair by welding without approved data — magnesium ignites and burns; never use steel wool or a steel wire brush on it; strip and re-apply the chromate (Dow-type) treatment plus a full paint scheme after any blend-out, because bare magnesium corrodes rapidly. Keep a Class D dry-powder extinguishant available; water, foam and CO₂ are prohibited on burning magnesium.
• Titanium. Never mark it with a lead pencil; never allow chlorinated solvent residue; never use cadmium-plated tools or hardware against hot titanium. Grinding overheats the surface easily — use light cuts, generous coolant and sharp abrasives, and inspect for grinding burns and alpha-case contamination afterwards.

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Restoring Fatigue Life

Two processes appear regularly in exam questions because they improve on the original structure:

• Cold-hole expansion (cold working). An oversized mandrel is drawn through the fastener hole, leaving a ring of residual compressive stress that dramatically slows fatigue crack initiation. It is used at the wing-skin fastener holes of transport aircraft during heavy maintenance.
• Shot peening. Uniform compressive residual stress in the surface layer of a machined fitting or a spring, again suppressing fatigue crack initiation.

Both rely on the same physics as Section 8.2: fatigue cracks start at a tensile surface, so pre-loading that surface in compression buys life.

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Exam Traps

• Stop-drill sizes differ by structure: 3/32 or 1/8 inch for cracked aluminium members, No. 40 (0.098 in) for welded steel tube. Both are "small hole at the extreme end of the crack".
• "Same material or suitable material of higher strength" — the permitted substitution is upward, never downward.
• The repair must be as strong as the original, not stronger everywhere: over-stiffening moves the failure to the edge of the doubler.
• MPI is not an option on aluminium, magnesium or titanium. If a question offers magnetic particle inspection for an aluminium skin crack, it is the distractor.
• A stop-hole is a crack-arrest measure, not a repair. A question asking "what is the repair?" wants the reinforcement.