S — Aircraft StructuresChapter 6 · 46 practice questions

Chapter 6: Structural Troubleshooting - Fatigue Cracking, Buckling, Improper Repairs, Fastener Failure, Delamination

Includes 6 animated diagrams — view them live in the interactive theory reader.

1. Overview of Structural Troubleshooting

This chapter covers the systematic identification, analysis, and resolution of structural defects in aircraft, including fatigue cracking, buckling, improper repairs, fastener failure, and delamination. Understanding the root causes of these failures is essential for safe and effective maintenance. The material emphasizes that most structural failures are not random events but result from predictable mechanisms such as stress concentrations, cyclic loading, material degradation, and design or installation errors. The goal is to equip the AME with the knowledge to diagnose problems correctly, select appropriate corrective actions, and prevent recurrence.

2. Key Concepts Explained in Detail

2.1 Fatigue Cracking

Fatigue Cracking — Initiation and Propagation Fatigue Cracking — Initiation and Propagation Tension Tension Critical Threshold (ac) 1. INITIATION Stress concentration (Rivet hole, defect) 2. PROPAGATION Progressive growth (Pressurization cycles) 3. FRACTURE Rapid propagation Sudden failure a - N Curve N (Cycles) a ac

Fatigue cracking is the most common service-induced structural failure in metallic aircraft. It results from repeated (cyclic) stresses that are well below the material's ultimate tensile strength.

  • Mechanism: Fatigue cracks initiate at points of high stress concentration, such as sharp corners, fastener holes, weld joints, cutouts (e.g., windows, doors), and changes in cross-section. Each flight cycle (pressurization, maneuver, landing) contributes to incremental crack growth.
  • Common Locations:
  • Fuselage: At corners of window and door cutouts due to pressurization cycles.
  • Wings: In spar caps (perpendicular to spanwise direction) from bending loads; in lower wing skin at rivet holes; at wing root from torsional loads.
  • Engine Mounts: At weld joints, especially if the joint design creates a stress concentration.
  • Control Systems: At bends in push-pull tubes or at fairleads where cables fray.
  • Key Indicators:
  • Cracks that reappear after stop-drilling indicate the underlying stress concentration or fatigue issue has not been resolved.
  • Cracks consistently occurring at the same location (e.g., same weld joint) point to a design or systemic problem, not random material defects.
  • A sudden loss of cabin pressure is often caused by a fatigue crack that has grown to a critical length and propagated rapidly (unstable crack growth).
Diagram — Structural Troubleshooting - Fatigue Cracking, Buckling, Improper Repairs, Fastener Failure, Delamination STRUCTURAL TROUBLESHOOTING — Root Cause Analysis Flow STRUCTURAL DEFECT Visual & NDT Inspection FATIGUE CRACKING Cyclic stress > limit BUCKLING (COMPRESSION) Load > critical stress IMPROPER REPAIRS Deviation from SRM FASTENER FAILURE Shear/tension overload DELAMINATION (COMPOSITE) Interply separation Stress concentration + cyclic loading Slenderness ratio + eccentric load Wrong material/ process deviation Incorrect torque/ hole misalignment Impact damage/ moisture ingress CORRECTIVE ACTION Repair per SRM • Replace component • Modify design PREVENTIVE MAINTENANCE Scheduled inspections • Life limits • Corrosion protection → Detection flow → Root cause link

2.2 Buckling

Buckling — Mechanism and Typical Locations Buckling: Mechanism and Typical Locations Buckling Mechanism Normal State — Compressive Load P P Load P < Critical load No lateral deformation Buckling — Critical Load Exceeded P > P crit P > P crit δ Sudden lateral deformation Loss of geometric stability Not necessarily material failure (elastic instability) If P exceeds the elastic limit: permanent deformation (bending, warping) Typical Locations on Aircraft 1. Wrinkled wing skin between ribs compression compression Repeated compressive loads in flight → progressive skin buckling 2. Tail cone crease after tail strike impact High vertical load → buckling of longerons and frames 3. Buckling of control tubes Normal tube (aligned) Buckled tube Worn rod ends / misalignment → eccentric compressive load Common Causes of Buckling • Overload: hard landing, tail strike, excessive maneuver • Design flaw: missing stiffener, spacing too wide • Corrosion or damage reducing element thickness • Fatigue: repeated compression cycles

Buckling is a sudden, large deformation of a structural member under compressive stress. It is a failure mode distinct from fatigue, though both can interact.

  • Mechanism: Buckling occurs when compressive loads exceed the critical buckling stress of the member. This can happen from overload (e.g., hard landing, tail strike) or from a reduction in stiffness due to corrosion or fatigue.
  • Common Locations:
  • Tail Cone: After a tail strike, longitudinal longerons and transverse frames are prone to buckling due to upward loads.
  • Wing Skin: A wavy appearance between ribs is a classic sign of buckling from repeated compressive loads during flight.
  • Push-Pull Tubes: Can buckle under compressive loads if misaligned, improperly supported, or if there is excessive friction in the system.
  • Key Indicators:
  • A persistent difference in wing tip height (without visible damage) is a strong indicator of structural buckling in the spar, strut, or attachment points.
  • A crease near a gear attachment point after a hard landing suggests significant loads were transmitted through the structure, making hidden damage (cracks in fittings, bulkheads) highly likely.

2.3 Improper Repairs

Improper Repairs — Failure Mechanisms Incorrect Repairs: Causes of Failure Structural Failure Modes — Chapter S-STRUCTURES 6 1. Stress concentration at patch edge Aluminum skin PATCH (stiff) ✕ Cracks initiated at patch edge The stiff patch creates a stiffness discontinuity → high localized stress. ← Cyclic load → ← Cyclic load → 2. Galvanic corrosion steel / aluminum Aluminum structure (anode — corrodes) STEEL PATCH (cathode) H₂O (electrolyte) Electron flow ✕ Galvanic corrosion Dissimilar metals + moisture = electrolytic cell. ⚠ Insulation mandatory (shim, sealant) required 3. Heat affected zone of welds (HAZ) WELD HAZ HAZ HAZ = Heat Affected Zone Microstructure altered by heat → brittle, sensitive to fatigue cracking. ✕ Cracks frequent in HAZ Engine vibrations → Cyclic loads → 4. Incomplete corrosion removal Internal face (not visible) ✕ Residual corrosion Pitting remaining after surface cleaning. Continues to propagate under the paint. ⚠ Requirement: remove ALL corrosion, then apply inhibitor ⚠ SRM: stripping limits — beyond → replace Non-conforming repairs → stress concentration, galvanic corrosion, brittle HAZ, residual corrosion — Ref: CAR 571, FAA AC 43.13-1B, SRM

Improper repairs can create new stress concentrations, introduce galvanic corrosion risks, or fail to restore the original structural strength.

  • Common Issues:
  • Scab Patches: Create a stress concentration at the patch edge due to a change in stiffness, leading to fatigue cracking in the underlying structure.
  • Dissimilar Metals: Using a steel patch on an aluminum structure creates a galvanic cell, causing accelerated corrosion of the aluminum in the presence of moisture.
  • Weld Repairs: Repeated cracking near weld repairs is often due to the heat-affected zone (HAZ), which is harder, more brittle, and susceptible to fatigue. The weld itself may be strong, but the HAZ is the weak point.
  • Incomplete Corrosion Removal: If corrosion is not completely removed (including all pits and corrosion products), it will recur and accelerate.
  • Proud Rivets: Indicate the rivet was not fully driven, meaning the shop head may not be properly formed, and the clamping force is insufficient.
  • Key Indicators:
  • A crack at the edge of a previous repair patch.
  • Loose bolts in a bolted splice, often due to elongated (ovaled) holes from bearing stress.
  • A repair that has been in service for years but uses dissimilar metals—the corrosion risk remains.

2.4 Fastener Failure

Fastener Failure — Symptoms and Root Causes Fastener Failure: Symptoms and Root Causes Chapter S-STRUCTURES — Structural Troubleshooting | Transport Canada — AME Training STRETCHED BOLT — Overload LOAD Symptoms: • Lock nut + cotter pin INTACT • Necking of the bolt body • No abnormal play detected Cause: Tensile overload Exceeds the material's elastic limit DIAGNOSIS: Replace — do not reuse a stretched bolt OVALIZED HOLES — Bearing Load BEARING Symptoms: • Deformed hole (oval shape) • Excessive play around the fastener • Fretting marks / burnishing Cause: Bearing load Pressure of the fastener on the hole DIAGNOSIS: Ream to next oversize or repair per SRM SHEARED RIVET HEADS — Vibration Fatigue VIBRATION Symptoms: • Rivet head fallen off or displaced • Clean shear line • Several adjacent rivets affected Cause: Vibration fatigue Cyclic shear stresses DIAGNOSIS: Replace and investigate the source of vibration ROOT CAUSE SEARCH LOGIC 1. INITIAL INSPECTION Identify the failed fastener and failure mode (stretched, sheared, ovalized hole, corrosion) 2. FAILURE MODE ANALYSIS • Overload? → Look for impact, maneuver • Fatigue? → Look for vibration, load cycling 3. INSPECT ADJACENT AREAS • Look for cracks, corrosion, deformation • Check neighboring fasteners (play, condition) 4. DETERMINE ROOT CAUSE Do not replace without understanding WHY Did the fastener fail because it was overloaded, fatigued, or is the structure deformed? AME GOLDEN RULE: A fastener never fails without a reason. Find the cause before replacing. References: CAR 571, FAA AC 43.13-1B, manufacturer's SRM — Chapter S-STRUCTURES 6.2

Fastener failures are rarely random; they usually indicate an underlying systemic issue.

  • Common Causes:
  • Fatigue: Repeated flexing of the skin (e.g., near trailing edge) can cause rivet heads to fail.
  • Overload: Bolts that have stretched (yielded) indicate significant stress. This is a serious finding, especially in critical attachment fittings.
  • Design/Installation Issues: Repeated failures of the same fastener point to misalignment, improper hole preparation, or excessive load due to a structural problem. Simply replacing the fastener without addressing the root cause will lead to further failures.
  • Vibration: A common cause of missing rivet heads, especially in areas subject to aerodynamic flutter or engine vibration.
  • Key Indicators:
  • Loose bolts with castellated nuts and cotter pins suggest the bolts have stretched, not that the safeties failed.
  • Elongated bolt holes in a bolted splice indicate a failing joint.

2.5 Delamination (Composite Structures)

Composite Delamination — Causes and Growth Composite Delamination: Causes and Propagation Healthy Composite Structure Carbon fiber plies BVID Impact (Barely Visible) Minor impact Internal delamination (invisible on surface) BVID Lightning + Freeze-Thaw Cycles Lightning ❄ Freeze-thaw: moisture/air expansion Delamination Detection Tap Test (percussion test) Delamination (dull sound) Hammer Dull sound = delaminated area Ultrasonic Inspection PROBE Echo SCREEN Delamination BVID = Barely Visible Impact Damage — Detection via tap test and ultrasonics

Delamination is the separation of layers in a composite laminate. It is a critical defect that can compromise structural integrity.

  • Mechanism: Delamination can result from impact (e.g., lightning strike, bird strike), manufacturing defects, or moisture ingress. Lightning strikes cause rapid heating and expansion of trapped moisture or air, leading to delamination and disbonding.
  • Primary Concern: Delamination creates a path for moisture ingress. Water can enter the core or between layers, and freeze-thaw cycles can cause the delamination to grow, leading to more extensive damage.
  • Key Indicators:
  • A gel coat crack that does not extend into the underlying layers still requires investigation (e.g., tap test) to ensure no hidden delamination.
  • A soft area in a wet lay-up repair is often due to an incorrect resin-to-hardener ratio or excessive resin (resin-rich area), not necessarily delamination.
  • Cracks at the root end of a composite helicopter blade (chordwise orientation) are critical fatigue concerns.

3. Important Formulas, Regulations, and Procedures

3.1 Key Regulations and Standards

  • FAA AC 43.13-1B: Acceptable Methods, Techniques, and Practices – Aircraft Inspection and Repair. This is the primary reference for standard maintenance practices.
  • CAR 571 (Canadian Aviation Regulations): Governs maintenance and manufacturing. Relevant sections include:
  • 571.02: General maintenance standards (e.g., corrosion treatment, crack assessment).
  • 571.05: Damage tolerance and fatigue evaluation (inspection intervals based on flight cycles).
  • 571.06: Repairs and modifications (permanent repairs require approved data).
  • SRM (Structural Repair Manual): The aircraft-specific document that defines allowable damage limits, repair procedures, and inspection intervals. Always consult the SRM first when assessing damage.

3.2 Critical Procedures

  1. Crack Assessment:
  • First Step: Consult the SRM to determine if the crack is within allowable limits.
  • Within Limits: May be stop-drilled or left alone per the SRM.
  • Exceeds Limits: Requires a repair designed with approved data (SRM, STC, or repair design approval). Stop-drilling is a temporary measure; repeated cracking requires a permanent solution.
  1. Corrosion Treatment:
  • First Step: Determine the extent of damage using NDT and visual inspection. Compare findings to SRM limits.
  • Critical Rule: All corrosion products and pits must be completely removed. Incomplete removal leads to accelerated recurrence.
  • Exfoliation Corrosion: A serious form that can significantly reduce strength. Requires thorough assessment before any repair or replacement.
  1. Post-Incident Inspection:
  • Hard Landing/Tail Strike: A crease or buckle near a load-bearing point (e.g., gear attachment, tail cone) mandates a thorough inspection of the entire load path for hidden damage before any repair.
  • Lightning Strike: On aluminum, look for burn marks or pits. On composites, look for delamination and disbonding.
  1. Rigging and Alignment:
  • Landing Gear Doors / Cargo Doors: If actuators and hydraulics are functional, the most likely cause of improper closure is misalignment (out of rig). Adjust per the maintenance manual.
  • Flight Control Surfaces: If cables and pulleys are good, binding is often due to corroded or unlubricated hinges.

3.3 Troubleshooting Logic

  • Systemic vs. Random: Repeated failures at the same location (cracks, fastener failures) indicate a systemic problem (design, installation, or load path issue), not a random material defect.
  • Root Cause vs. Symptom: Frayed cables at a fairlead are a symptom; the root cause is a worn, misaligned, or too-hard fairlead. Loose bolts are a symptom; the root cause may be bolt stretching or hole elongation.
  • Hidden Damage: A visible crease or buckle after an overload event (hard landing) is a strong indicator of hidden damage in the underlying structure. Inspect the load path before repairing the visible damage.

4. Common Relationships Between Concepts

  • Stress Concentrations and Fatigue: All structural failures (cracks, buckling, fastener failure) are initiated or exacerbated by stress concentrations. Cutouts, sharp corners, weld joints, fastener holes, and repair patch edges are all stress raisers.
  • Vibration and Fatigue: Vibration is a common root cause of fatigue cracking in engine mounts, welded tubular structures, control system components (push-pull tubes, cables), and trailing edge skin. It also causes rivet head failure and bearing wear.
  • Corrosion and Structural Integrity: Corrosion (especially pitting, exfoliation, and galvanic) reduces the effective cross-sectional area of a structural member, making it more susceptible to fatigue and buckling. Incomplete corrosion removal guarantees recurrence.
  • Repairs and New Stress Concentrations: A repair, while intended to restore strength, often creates new stress concentrations (patch edges, weld HAZ, dissimilar metal interfaces) that can become the site of future failures if not properly designed.
  • Design vs. Maintenance Issues: A crack that consistently appears at the same weld joint is a design issue (poor joint configuration). A crack that appears randomly at different welds is more likely a maintenance or material issue.

Diagram

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