9.3 Welded Steel Tubular Structure: Splices, Sleeves & Engine Mount Repairs

9.3 Welded Steel Tubular Structure: Splices, Sleeves & Engine Mount Repairs

[!WARNING] Every welded repair to primary structure is a major repair. Part 43 Appendix A(b)(1) classifies repairs "involving the strengthening, reinforcing, splicing, and manufacturing of primary structural members or their replacement" as airframe major repairs, and the enumerated list names members of truss-type beams (vi), engine mounts (xii), fuselage longerons (xiii), and members of the side truss, horizontal truss, or bulkheads (xiv). There is no such thing as a "minor weld" on a fuselage truss.

Steel tube structure is not a museum topic. Cubs, Champs, Huskies, Maules, Citabrias, and every certificated helicopter tail boom truss are welded 4130 steel, and the engine mount on a great many modern singles is a welded tube assembly. AC 43.13-1B Chapter 4 covers the accepted methods.

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When Welding Is Acceptable at All

Three questions come before any geometry:

1. Does the manufacturer publish a repair for this member? If a structural repair manual or an FAA-approved repair scheme exists, it governs. AC 43.13-1B is acceptable data, not approved data, and cannot on its own substantiate a major repair (Section 5.1).
2. Is the member repairable, or must it be replaced? Heavily corroded, previously repaired, or heat-damaged tubing is often replaced rather than spliced.
3. Was the original member heat-treated? Welding on heat-treated 4130 changes its properties in the heat-affected zone. Repairs to heat-treated members generally require re-heat-treatment or an engineering-substantiated alternative.

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The Two Splice Geometries

AC 43.13-1B Chapter 4 accepts two families of tube splice, and the exam distinguishes them by how the replacement material relates to the original tube.

Inner Sleeve (Insert Splice)

A tube of the same material, sized to slip inside the original tube, is inserted across the damaged joint and welded through prepared slots or at rosette welds. The outside contour of the member is preserved, which matters where the tube passes through fairings or where clearance is tight.

Outer Sleeve (Telescoped Splice)

A larger diameter replacement tube is telescoped over the stubs of the original member and welded in place. This is the geometry used when the damaged section is cut out entirely and a new center section is installed, and it is the geometry the IAR test asks about.

How is the replacement tube cut? With a fishmouth cut at 30 degrees. The reason is mechanical, not cosmetic:

• A perpendicular (90-degree) cut puts the entire weld on one plane, concentrating the heat-affected zone and the stress riser on a single circumferential line.
• A 45-degree diagonal cut improves that but still terminates in two sharp points.
• The 30-degree fishmouth produces a long, gradually tapering weld line that distributes load transfer along the length of the joint and moves the end of the weld away from the point of maximum bending.

The same reasoning explains the alternative accepted geometry — a scarf cut — and explains why sleeve length matters: a splice sleeve must overlap enough of the parent tube to transfer the load through the weld rather than through the fit.

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Selecting the Replacement Material

The governing standard is § 43.13(b): the repaired member must end up at least equal to its original condition with regard to structural strength. In practice that means:

The exam frequently offers an answer that gets one of these right and another wrong — for instance, an outer sleeve of correct length but of thinner wall than the original. That answer is wrong on the § 43.13(b) standard alone.

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Weld Quality: What the Inspector Looks For

The IA is inspecting someone else's weld, usually after the fact and often after paint. AC 43.13-1B Chapter 4 describes acceptable and unacceptable weld characteristics:

Acceptable weld indications:

• Uniform bead width and height with even ripple spacing
• Good penetration and fusion into the base metal at both toes
• Smooth tapering of the bead into the base metal
• No burning through or undercut of the parent tube

Rejectable weld indications:

• Undercut at the weld toe — a machined-in stress riser
• Lack of penetration or fusion — a joint held together by surface metal
• Excessive buildup with an abrupt transition — the load path steps rather than tapers
• Cracks anywhere in the weld or heat-affected zone
• Oxidation or scaling indicating excessive heat or inadequate shielding
• Evidence of welding over a previous weld without removing the old bead

A special watch item: a tube that has been welded and then filled, primed, and painted can conceal all of the above. Where the records show a welded repair to primary structure and the area has been refinished, the IA should require substantiating documentation of the repair scheme rather than judging the joint through the paint.

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Corrosion in Steel Tube Structure

The failure mode that actually grounds steel tube airframes is not weld failure — it is internal corrosion, which is invisible from outside. Water enters through unsealed joints, drain holes, or cracked fabric and works from the inside out.

• Lower longerons and cluster joints are the classic locations, because water settles there.
• Sounding and probing the tube, inspecting through drain holes with a borescope, and checking for external rust blooms at cluster welds are the standard techniques.
• A tube whose wall has been thinned by internal corrosion is not repairable by adding an external sleeve over the corroded region; the corroded material must be removed.

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Documenting the Repair

Because a welded repair to a longeron, truss member, or engine mount is a major repair:

1. The work must be accomplished using approved data — manufacturer repair data, a DER-approved Form 8110-3, or a field approval (Section 5.1).
2. FAA Form 337 must be executed and disposed of under Part 43 Appendix B(a): duplicate copies, one signed copy to the owner, one to the FAA Aircraft Registration Branch within 48 hours after approval for return to service.
3. Item 8 must describe the repair specifically — which member, which station, what splice geometry, what material and wall thickness, and the exact approved data reference with revision and date.
4. Only an IA, an appropriately rated repair station, or the manufacturer may approve it for return to service.

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High-Yield Exam Traps

• The fishmouth cut is 30 degrees. Not 45, not 90.
• Engine mounts are named in Appendix A(b)(1)(xii) — welding one is unambiguously a major repair.
• A sleeve that is thinner than the original fails § 43.13(b) no matter how well it is welded.
• AC 43.13-1B is acceptable data. A welded major repair still needs approved data and a Form 337.
• Internal corrosion is the real killer in tube structure, and it cannot be sleeved over.