Welded joints are among the most structurally critical assemblies found in aircraft — from steel tube fuselage frameworks and engine mounts to control system components and exhaust systems. A weld that looks clean on the surface may hide serious flaws that could cause catastrophic failure under flight loads. For the Aviation Maintenance Technician (AMT) working on airframe structures, the ability to identify weld defects, understand why they occur, and apply correct rejection criteria is not merely a certification requirement — it is a fundamental safety responsibility rooted in FAA guidance.
This article covers every major category of weld defect recognized in FAA airframe maintenance standards, explains the metallurgical or procedural root causes, describes how each defect appears visually and under inspection, and defines the conditions under which a weld must be rejected and the repair process begun again. Whether you are preparing for the AMT Airframe knowledge test or refreshing your shop skills, mastering these concepts will make you a more capable and safety-conscious technician.
What Makes a Weld Acceptable
Before cataloging defects, it is worth understanding what an acceptable weld looks like. A sound weld in aircraft-quality steel tubing should show complete fusion of the filler metal with the base metal on both sides of the joint, consistent bead width and height, smooth ripple patterns (in gas welding) without sharp valleys or peaks, no visible surface porosity or cracks, and a gradual transition from the weld bead to the surrounding base metal — not an abrupt or undercut edge. The finished bead should be slightly crowned (convex) but not excessively built up, because excessive reinforcement creates stress concentration at the toes of the weld.
FAA Advisory Circulars and the Aviation Maintenance Technician Handbook — Airframe (FAA-H-8083-31) describe these characteristics and define the defect conditions that require rejection. For certificated aircraft, no weld defect of a structural nature may be accepted without proper engineering approval or repair under an approved data source such as the manufacturer's structural repair manual.
Major Categories of Weld Defects
Porosity
Porosity refers to gas pockets or voids trapped within the solidified weld metal. During welding, gases — primarily hydrogen and oxygen from contaminated base metal, filler rod, or shielding atmosphere — can become entrapped as the molten pool solidifies faster than the gas can escape. Porosity can appear as surface porosity, which is visible to the naked eye as small pits or craters on the bead surface, or as subsurface (internal) porosity, which is detectable only through radiographic or ultrasonic inspection.
In aircraft welding, any significant surface porosity is cause for rejection because each void acts as a stress riser that can initiate fatigue cracking under cyclic loads. The root causes include moisture on the base metal or rod, oil or paint contamination, improper flame adjustment in oxyacetylene welding (a highly oxidizing flame), or inadequate shielding gas coverage in TIG welding. Prevention involves thorough cleaning of all joint surfaces and filler material immediately before welding.
Cracks
Cracks are the most serious weld defects and are cause for immediate rejection in any structural aircraft weld. Cracks can form in the weld metal itself (weld metal cracking) or in the heat-affected zone (HAZ) of the base metal immediately adjacent to the bead (heat-affected zone cracking). They may run longitudinally along the bead centerline, transversely across the bead, or radiate from the weld toe into the base metal.
Hot cracks form during solidification of the weld pool while the metal is still near its melting temperature, often caused by high sulfur or phosphorus content in the base metal or by a joint that restrains the shrinking weld too rigidly. Cold cracks (also called hydrogen-induced cracks or delayed cracking) occur after the weld has cooled, sometimes hours later, and are associated with hydrogen embrittlement in alloy steels. Because cracks can propagate rapidly under load, a cracked weld on any primary structural component must be removed — not repaired by rewelding over the crack — before an approved repair can be made.
Incomplete Fusion and Incomplete Penetration
Incomplete fusion occurs when the filler metal does not bond metallurgically with the base metal or with a previous weld pass. This leaves a planar discontinuity — essentially an internal seam — that provides no load-carrying capacity. It is typically caused by insufficient heat input, moving the torch or electrode too fast, or welding onto a contaminated surface. Incomplete fusion is particularly dangerous because it can be hidden beneath an otherwise smooth-looking bead surface.
Incomplete penetration specifically describes a condition where the weld metal does not fully penetrate to the root of a joint on a butt or groove weld. In a tube cluster weld this can leave an unfused interior gap. Both conditions are rejectable in any primary structure because they significantly reduce the effective throat — the load-carrying cross-section — of the joint.
Undercutting
Undercutting is a groove or channel burned into the base metal at the toes of the weld bead, running parallel to the weld along one or both edges. It appears as a visible notch or depression where the bead meets the surrounding metal. Undercutting is caused by excessive heat input, incorrect torch angle, or too-high amperage in electric arc processes. Even a shallow undercut of only a few thousandths of an inch dramatically increases stress concentration at that point, which in cyclic-load environments like aircraft structures can initiate fatigue cracks at stress levels well below the material's ultimate strength. Any measurable undercut in a structural aircraft weld is generally cause for rejection.
Overlap (Cold Lap)
Overlap, sometimes called cold lap, is the opposite problem from undercutting: the filler metal flows over the base metal surface without fusing to it, creating a notch on the other side of that overlap. This happens when heat input is too low — the weld pool is too cold to achieve fusion at the edges. The resulting lapped bead edge creates a sharp geometric notch that promotes cracking just as undercutting does, even though the surface may appear as though the bead is well built up.
Burn-Through and Excessive Melt-Through
Burn-through occurs when excessive heat input melts completely through the base metal, creating a hole or a very thin, overheated zone. In aircraft tubing structures this is a particularly common error because the thin-wall chrome-moly (4130) steel tubing used in fuselage construction has relatively little thermal mass. A burn-through must be rejected; attempts to fill a burn-through hole by continued welding almost always produce a weak, porous, distorted repair that is structurally inferior to cutting out the damaged section and re-welding.
Distortion and Warping
While distortion is not a defect in the weld metal itself, excessive warping of a welded assembly can indicate that residual stresses are severe enough to cause dimensional nonconformance or that the heat input was so poorly controlled that the weld quality is suspect. A structure that has warped significantly from its design dimensions may not carry loads as designed, and in extreme cases the residual tensile stresses can promote cold cracking in alloy steels.
Why Weld Defect Recognition Matters
Aircraft primary structures are designed to specific load factors with safety margins that assume sound material properties. Every weld defect reduces the actual strength of the joint below the designed value. Because many defects — particularly internal porosity, incomplete fusion, and cold cracks — are not visible without nondestructive testing (NDT) such as radiography (X-ray), dye penetrant, or magnetic particle inspection, the visual quality of the bead surface provides only a first-line check. A visually clean weld that still contains internal voids may pass casual inspection and fail in service. This is why FAA-certificated welders on primary structure must demonstrate proficiency through test coupons and why repair welding on certificated aircraft must follow approved data.
Key Numbers and Rules
- Any crack in a structural weld — regardless of length — is cause for rejection with no exceptions.
- Porosity: Surface porosity that is visible to the unaided eye is rejectable on primary structure. Scattered fine porosity may be evaluated against the applicable standard, but any porosity forming a linear cluster (aligned voids) is always rejectable.
- Undercutting: Any continuous undercut deeper than approximately 1/32 inch (or as defined by the applicable manufacturer's data) on a structural weld is rejectable.
- Incomplete fusion or penetration is always a rejection condition on structural joints — there is no acceptable minimum.
- Overlap: Any overlap on a primary structural weld is rejectable.
- Repair welding over a defective area (without removing the defect first) is not an acceptable repair method for structural components.
- 14 CFR Part 43 governs maintenance and alteration of certificated aircraft; all weld repairs must meet the standards of the applicable approved data (manufacturer's SRM, FAA-approved repair data, or AC 43.13-1B for standard practices where applicable).
Common Test Traps
- Undercutting vs. overlap confusion: Test questions often flip these. Remember — undercutting removes base metal (a notch at the bead edge); overlap deposits metal over the base metal without fusion (a cold lap creating a notch on the other side).
- Assuming a smooth surface means a sound weld: Internal porosity and incomplete fusion can exist beneath a visually smooth bead. The test may ask which NDT method detects internal defects — only volumetric methods like radiography or ultrasonic testing reveal subsurface voids; dye penetrant finds only surface-breaking defects.
- Rewelding over cracks: A common distractor answer suggests grinding a crack surface clean and rewelding over it. FAA standards require complete removal of the cracked material before repair welding.
- Hot crack vs. cold crack timing: Hot cracks form during solidification (immediately); cold cracks (hydrogen-induced) can appear hours or even days after welding. A weld that looks fine immediately after completion can still develop a cold crack later — this is especially relevant for alloy steels.
- Burn-through repair: Test questions sometimes suggest that a burn-through can simply be filled with additional weld passes. The correct answer is that burn-through requires cutting out and replacing the affected section, not filling in place.