1.4 Repair & Inspection Procedures for Ferrous Structures

Why the Syllabus Adds a Repair Sub-Topic

Sub-module 6.1(c) — "Repair and inspection procedures for ferrous materials, structures, and airframes" is examined at level 2 for B1 and B3 and level 1 for B2. It is the point where the metallurgy of Sections 1.1 and 1.2 stops being theory: you are expected to know how a damaged steel structure is inspected, what limits decide repair versus scrap, which repair schemes are acceptable, and which actions are absolutely forbidden.

The classic ferrous airframe is the welded 4130 chrome-molybdenum steel-tube truss — the fuselage of most light aeroplanes, the engine mount of nearly every piston aircraft, most seat frames, many tail-plane structures and virtually all light-aircraft undercarriage legs. The figures quoted below come from FAA AC 43.13-1B, Chapter 4 (Metal Structure, Welding and Brazing), which National Aviation Authorities and Part-145 organisations across Europe accept as acceptable industry practice where the Structural Repair Manual (SRM) is silent.

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Inspecting Welded Steel-Tube Structure

Where the Damage Actually Is

A steel tube is a closed vessel. The dangerous corrosion is the corrosion you cannot see:

Internal protection is a maintenance action in its own right: after inspection, tubes are re-protected by flushing with hot linseed oil or an approved corrosion-inhibiting compound and re-sealing, with the drain holes left open at the lowest point of each bay.

Techniques

Because 4130, 4140 and 4340 are ferromagnetic, the primary crack-detection method is magnetic particle inspection. Wet fluorescent MPI is used for the highest-stress items (landing-gear legs, engine-mount fittings, undercarriage clevises). Remember the two directions: circular magnetisation (prod or central conductor) finds longitudinal cracks; longitudinal magnetisation (encircling coil or cable wrap) finds transverse cracks. Every MPI must be followed by demagnetisation, otherwise residual magnetism attracts steel debris and corrupts compass swings.

Liquid penetrant works on steel but will not find sub-surface flaws and is defeated by paint. Ultrasonic thickness measurement is the standard method for quantifying wall loss on tubing without cutting it open. Eddy current is used around fastener holes in steel fittings.

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Damage Classification and the Approved-Data Rule

Before touching the structure the damage is classified against the SRM, the Aircraft Maintenance Manual or AC 43.13-1B:

1. Negligible damage — within published allowable limits; requires only clean-up and re-protection.
2. Damage repairable by patch or splice — an approved repair scheme restores the original strength.
3. Damage requiring replacement of the member — beyond repairable limits, or in a location where repair is prohibited.

Anything outside published data is a major repair requiring approved data. This is the single most examinable idea in 6.1(c): the technician's job is to classify the damage and apply the published scheme, not to invent one.

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Repair Scheme 1 — Welded Split-Sleeve Reinforcement

Used for a tube dented, bent or cracked within a bay (i.e. away from a cluster joint).

• Carefully straighten the member first. For a crack, drill a No. 40 (0.098 in) stop hole at each end of the crack to arrest propagation.
• Select a sleeve of the same material and at least the same wall thickness, with an inside diameter approximately equal to the outside diameter of the damaged tube.
• Cut both ends of the sleeve diagonally at 30°, sized so the sleeve extends not less than 1½ tube diameters beyond the edge of the crack or dent.
• Split the sleeve lengthwise into two half-shells, clamp them over the damage, then weld along both longitudinal seams and around both ends.

Absolute rule: filling a dent or a crack with welding rod instead of reinforcing the member is not acceptable. Weld metal deposited into a crack simply re-cracks under load.

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Repair Scheme 2 — Welded Patch (Dents and Punctures)

A patch of the same material, one gauge thicker, may be welded over a dent or a hole, but only inside strict limits:

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Repair Scheme 3 — Inner-Sleeve Splice

Used when part of the tube must be replaced and a smooth external surface is required (for example inside a fairing or where fabric is stretched over the tube).

• Remove the damaged length with a diagonal cut and deburr.
• Cut a replacement tube of the same material, same diameter and at least the same wall thickness, also diagonally, leaving a deliberate 1/8-inch gap at each end between the replacement and the original stubs.
• Select inner-sleeve stock whose outside diameter equals the inside diameter of the damaged tube, fitting snugly with a maximum diameter difference of 1/16 inch.
• Cut two inner sleeves long enough that each sleeve end sits at least 1½ tube diameters from the nearest end of the diagonal cut.
• If the sleeve is very tight, chill it with dry ice or cold water, or polish the diameter down with emery cloth — never force it.
• Tack the outer and inner tubes with rosette welds (plug welds through pre-drilled holes), then weld the inner sleeve to each tube stub through the 1/8-inch gap, forming a weld bead over the gap.

The 1/8-inch gap exists precisely so the weld bead can penetrate to the inner sleeve; close the gap up and you get a cosmetic bead with no root fusion.

Repair Scheme 4 — Outer-Sleeve Splice

• Remove the damaged section with a 90° cut.
• The replacement tube (same material, diameter and at least the same wall) must bear against the original stubs with a total tolerance not exceeding 1/32 inch.
• Outer-sleeve stock must be the same material and at least the same wall thickness, with an inside-diameter to outside-diameter clearance not exceeding 1/16 inch.
• Cut the two sleeves diagonally or, preferably, fishmouth them; each sleeve must extend at least 1½ tube diameters from the cut. Use a fishmouth sleeve wherever possible — the tapered fingers spread the load transfer and avoid a hard stress step.
• Tack each sleeve in two places, weld both ends of one sleeve, allow it to cool, then weld the second sleeve. Welding both without cooling warps the member.

Because the outer-sleeve splice requires the greatest amount of welding, it is used only when the inner-sleeve or reinforcement methods are unsuitable.

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Damage at a Cluster Joint — The Finger Patch Plate

Dents at a cluster (where several tubes meet) are repaired with a formed steel finger patch plate:

• Cut it from sheet of the same material and thickness as the heaviest damaged tube in the cluster.
• Trim the fingers so each extends over its tube by a minimum of 1.5 times that tube's diameter.
• Strip all existing finish from the area to be covered.
• Form the plate hot or cold so that the gap between the plate and the joint contour is no more than 1/16 inch, tack-weld it, then weld all plate edges to the cluster.

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Absolute Prohibitions

These are high-yield exam items because they are stated as flat bans:

• Do not weld parts whose strength depends on cold working — streamlined brace wire and cables.
• Do not weld brazed or soldered parts; the brazing alloy or solder penetrates the hot steel and embrittles it.
• Do not weld heat-treated alloy-steel parts such as aircraft bolts and turnbuckle ends — welding destroys the heat treatment that gives them their rating.
• Do not use bolted-sleeve repairs on welded steel-tube structure unless specifically authorised by the manufacturer or the authority; the bolt holes remove tube area at the very section that is already critical.
• Do not ream elongated or worn bolt holes oversize in fittings that were designed without bushings — replace the fitting unless an approved repair exists. Do not fill worn holes with welding rod.
• Damaged castings are replaced, not repaired, unless the manufacturer or authority has specifically approved the repair.

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After the Repair: Metallurgy Comes Back

1. Weld filler. Use 4130 rod (AMS 6457 / AWS A5.18) for mild steel and 4130; 4140 rod (AMS 6452) for 4140. Matching the filler to the parent alloy keeps the joint's strength and hardenability consistent.
2. Post-weld condition. A weld in 4130 quenches itself in still air and can leave brittle untempered martensite in the heat-affected zone. Normalising or stress-relieving the assembly after welding restores toughness where the design or repair data calls for it.
3. Grinding and re-plating. Chromium plating and selective (brush) plating are used to restore dimensions on worn landing-gear journals. Grinding a high-strength steel too aggressively burns the surface and can produce grinding cracks — always inspect by MPI after grinding.
4. Hydrogen embrittlement relief. Any high-strength steel part (roughly above 1,000 MPa tensile) that has been acid-cleaned, pickled or electroplated must be baked at 190 °C–205 °C to drive out absorbed atomic hydrogen. Skip the bake on a landing-gear component and you invite a delayed brittle fracture days or weeks later, with no warning.
5. Re-protection. Restore the paint scheme and internal corrosion protection. Bare 4130 will rust visibly within days in a humid hangar.

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Exam Traps

• "1½ tube diameters" is the recurring number — it applies to the split-sleeve reinforcement, the inner-sleeve splice and the outer-sleeve splice. The finger patch plate uses 1.5 tube diameters for its fingers. Learn it once.
• The inner-sleeve splice uses a diagonal cut plus a 1/8-inch gap; the outer-sleeve splice uses a 90° cut with the stubs butting to 1/32 inch. Candidates routinely swap these.
• Stop-drill size for steel tube cracks is No. 40 (0.098 in) — not the 3/32/1/8-inch holes used on cracked sheet-metal members.
• "One gauge thicker" applies to the welded patch, not the sleeves; sleeves are "at least the same wall thickness".
• Do not confuse bolted sleeve (prohibited without specific authorisation) with welded sleeve (a standard scheme).