S — Aircraft StructuresChapter 2 · 60 practice questions

Chapter 2: Sheet Metal Structures - Rivets, Fasteners, Bend Allowance, Skin Repairs, Doublers, Splices

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Sheet Metal Structures - Rivets, Fasteners, Bend Allowance, Skin Repairs, Doublers, Splices

Overview

This chapter covers the fundamental principles and practices of aircraft sheet metal structures, focusing on the design, installation, inspection, and repair of riveted joints and formed sheet metal components. The material addresses the selection of appropriate fasteners, the calculation of bend allowances for flat pattern development, and the structural considerations for skin repairs, including doublers, splices, and patch configurations. Emphasis is placed on understanding load paths, stress concentrations, and the regulatory requirements for restoring airworthiness.

Diagram — Sheet Metal Structures - Rivets, Fasteners, Bend Allowance, Skin Repairs, Doublers, Splices Sheet Metal Structures — Riveted Joint & Skin Repair LÉGENDE Rivet (AN470 / MS20470) Zone de contrainte Ligne de pliage Doublure (doubler) Joint riveté en recouvrement Tôle extérieure (skin) — 0.040" Al 2024-T3 Doublure (doubler) — 0.063" Al 2024-T3 R R R Charge Pitch = 4D Edge = 2D Développement — Bend Allowance A B R BA = (π/180) × (R + K×T) × θ K = 0.33 (Al 2024-T3, R < 2T) Réparation de peau — Patch riveté Peau d'origine (original skin) Zone endommagée Patch de réparation (0.050" Al 2024-T3) Charge Types de rivets AN470 (tête ronde) Usage général MS20470 (tête fraisée) Surface aérodynamique Règles d'inspection — Joints rivetés Pitch max = 24 × épaisseur peau (max 6") Edge distance min = 2 × diamètre rivet (min 2D) Pas de rivet manquant ou desserré Pas de corrosion entre les tôles TC — Technical Aircraft Maintenance — Sheet Metal Structures

Key Concepts

Rivet Fundamentals

Solid Rivet Installation — From Setting to Bucking Solid Rivet Installation — From Setting to Bucking Installation Sequence and Dimensions Rivet Set Bucking Bar 1.5 D 0.5 D Correct Head Grip Common Defects Mushroom / Cauliflower Excessive hammering Eccentric Head Misaligned tool / Poor hole Undersized Head Insufficient strength Cracks / Cupping Improper hammer setting Spacing Rules Min 2D (Edge Distance) 3D to 5D (Pitch) D = Rivet Diameter

Rivet Types and Materials

Rivets are the primary mechanical fasteners for aircraft sheet metal structures. The selection of rivet material depends on the materials being joined, the required strength, and the installation conditions.

  • 2117-T4 (AD) Rivets: The most common rivet for general aircraft repair. These are driven in the "as received" condition without requiring heat treatment. They offer good corrosion resistance and adequate strength for most aluminum alloy structures.
  • 2024-T4 (DD) Rivets: Higher strength rivets that must be heat-treated and refrigerated before driving. They are used where higher shear strength is required but require careful handling and timing during installation.
  • 1100 Rivets: Pure aluminum rivets that are too soft for structural applications. Used only for non-structural attachments or where minimal strength is needed.
  • 5056 (Monel) Rivets: Used primarily for joining magnesium alloys or corrosion-resistant steel. These are harder to drive and require specialized techniques.

Rivet Dimensions and Grip

The grip of a rivet is defined as the total thickness of all materials the rivet will pass through. This measurement is critical for selecting the correct rivet length. The rivet length must be sufficient to fill the grip and provide enough extra material to form the shop head.

For flush rivets installed in countersunk holes, the rivet length must account for the material removed by the countersink. The formula for determining rivet length is:

> Rivet Length = Grip + Countersink Depth + Shop Head Allowance

Using only the sheet thickness for a countersunk joint would result in a rivet that is too long, causing an excessively large shop head or dimpling.

Rivet Spacing and Edge Distance

Rivet Spacing and Edge Distance Rules Rivet Pitch and Edge Distance Spacing rules based on rivet diameter D — AC 43.13-1B STANDARD PITCH: 3D to 5D ✓ Sheet 3D 5D D = rivet diameter ✓ Uniform shear load distribution ✓ Minimum 3D pitch: prevents sheet weakening 10D PITCH — TOO WIDE ✗ Sheet 10D ✗ Load concentration between rivets ✗ Reduced shear strength 1D–2D PITCH — BUCKLING ✗ Sheet 1D 2D ✗ Holes too close: sheet buckles between rivets ✗ Risk of skin tearing ✗ Significant structural weakening MINIMUM EDGE DISTANCE: 2D ✓ Sheet 2D min Tear-out possible < 2D ✗ ✓ Distance measured from rivet center to edge ✓ 2D minimum: prevents edge tear-out ✓ AC 43.13-1B — acceptable practice D = nominal rivet diameter • Reference: AC 43.13-1B Chapter 2 • AME Training — Transport Canada

Proper rivet spacing and edge distance are essential for joint strength and structural integrity.

  • Pitch (Spacing): The distance between rivet centers in a row. The standard pitch for single-row riveted joints is 3D to 5D, where D is the rivet diameter.
  • A pitch of 10D is too wide, concentrating loads on fewer rivets and allowing skin buckling or separation between rivets.
  • A pitch of 1D to 2D can cause inter-rivet buckling of the sheet.
  • Edge Distance: The minimum distance from the center of the rivet hole to the edge of the sheet is 2D. This prevents the rivet from tearing out under load. While 2.5D is often recommended for improved strength, 2D is the absolute minimum acceptable limit.

Rivet Installation Defects

Proper Shop Head Dimensions

Rivet Shop Head Defects — Comparison Rivet Bucking Head Defects Correct head (reference) 1.5D 0.5D Width: 1.5 × diameter Height: 0.5 × diameter Bucking bar held straight Flat head (under-driven) ✗ Head too flattened: Insufficient clamping force Reduced tensile strength Cone (angled bucking bar) ✗ Bucking bar held at an angle Asymmetric head Mushroom / cauliflower ✗ Rivet over-driven Edges raised into mushroom or cauliflower Eccentric head ✗ Rivet not centered on hole Hammer held at an angle or hole misaligned Collar around the head ✗ Shank too large for hole or hole too small Material forced into collar Transport Canada — AME Training — Structures S — Chapter 2: Rivets and fasteners

A properly formed shop head should be approximately:

  • Width: 1.5 times the rivet shank diameter
  • Height: 0.5 times the rivet shank diameter

This provides the necessary strength and clamping force. A flat shop head indicates under-driving, while a cone shape indicates the bucking bar was held at an angle.

Bend Allowance and Flat Pattern Development

Bend Allowance — Sheet Metal Flat Pattern Development Bend Allowance — Flat Pattern Development of Sheet Metal Mold Point Tangent Line Inner Radius (R) Setback Setback Material (Sheet) Neutral Axis (Neither stretched nor compressed) Section 1 Allowance (BA) Section 2 Total Flat Pattern Length Formulas and Definitions Bend Allowance (BA): BA = A × (π/180) × (R + K×T) A=Angle, R=Radius, T=Thickness, K≈0.33 Flat Pattern Length: L = Σ(Straight Sections) + Σ(BA) Sum of flat parts + allowances Key Concepts: Neutral Axis: Stress-free zone. Its length remains constant during bending. Setback: Distance from mold point to tangent line. Subtracted to obtain actual straight sections. Min Radius (2024-T3): 3 × Thickness (3T) Development

Bend Radius

The bend radius is always specified as the inside radius of the bend. This is the radius of the die or form block used to create the bend. The outside radius will be larger by the material thickness.

Minimum Bend Radius

For 2024-T3 aluminum sheet, the recommended minimum bend radius is 3 times the material thickness (3T). Bending to a tighter radius (1T or 2T) is likely to cause cracking in this alloy.

Neutral Axis

The neutral axis is a theoretical plane within the material where the stresses are zero during bending. The material on one side is in compression, and on the other side in tension. The neutral axis does not change length, which is the basis for bend allowance calculations.

Bend Allowance Calculation

The bend allowance (BA) is the amount of material in the bend. The formula using the K-factor method is:

> BA = A × (π/180) × (R + K × T)

Where:

  • A = Bend angle in degrees
  • R = Inside bend radius
  • T = Material thickness
  • K = K-factor (typically 0.33 for aluminum)

Flat Pattern Length

The flat pattern length is the sum of the straight sections plus the sum of the bend allowances:

> Flat Pattern Length = Sum of Straight Sections + Total Bend Allowance

Setback

The setback is the distance from the bend tangent line to the mold point (the intersection of the two outside surfaces of the bend). It is used in calculating the flat pattern length but is not directly added to the flat length.

Skin Repairs and Structural Considerations

Repair vs. Modification

  • Repair: Intended to restore an aeronautical product to its original airworthy condition, or as close to it as possible.
  • Modification: An alteration to the type design that changes the product from its original configuration.

Under CARs Standard 571, both repairs and modifications require acceptable or approved data, depending on their classification (major or minor).

Patch Types

Scab vs Flush vs Scarf Patch — Load Path Comparison Types of Repair Patches: Scab, Flush, Scarf SCAB PATCH Patch applied over the skin Original skin Scab patch Eccentric load path → bending stresses ⚠ Premature fatigue • Eccentric load creates bending moment • Stress concentration at rivets • Cracking risk under repeated load FLUSH PATCH Smooth aerodynamic surface Flush patch Backing plate (reinforcement) Smooth surface ✓ Advantages • Smooth aerodynamic surface • Minimized drag • More direct load path • Backing plate distributes loads SCARF JOINT Gradual stiffness transition Scarf patch Bevel Bevel Role of joggle • Small offset (step) formed in the repair patch • Allows overlapping while maintaining an exterior surface that is flush Joggle detail Joggle AC43.13-1B — Skin repairs: choice depends on location, loads, and aerodynamic requirements Scab = temporary | Flush = aerodynamic | Scarf = optimal stiffness transition
  • Scab Patch: A patch placed on top of the skin without a doubler. This creates an eccentric load path because the load is transferred from the skin, through the rivets, into the patch, and back into the skin. This offset creates bending stresses in the rivets and the skin, leading to premature fatigue failure.
  • Flush Patch: Designed to be flush with the original skin contour, providing a smooth aerodynamic surface. This minimizes drag and is critical on high-speed or flight-critical surfaces.
  • Scarf Joint: A patch where both the patch and parent material are tapered (scarfed). This provides a gradual change in stiffness from the repair to the original structure, avoiding sudden changes in load path that would create severe stress concentrations.

Doublers

A doubler is a reinforcing plate used to distribute loads from the repair patch to the existing structure. It is typically placed on the back side of the skin to strengthen the area around the damage. The doubler helps prevent the repair from creating a new stress concentration and ensures the load is spread over a larger area.

Joggles

A joggle is a small offset or step formed in the repair patch. Its purpose is to allow the patch to overlap the existing skin while maintaining a flush outer surface. Without a joggle, the patch would sit on top of the skin, creating a raised area.

Patch Thickness Considerations

The thickness of a repair patch must be determined by engineering analysis or data from the manufacturer's Structural Repair Manual (SRM). Simply matching or exceeding the skin thickness does not guarantee a proper repair. A patch that is too thick can create a stress concentration, change the load paths, and cause failure in the surrounding structure.

Stop-Drill Holes

When repairing a crack, stop-drill holes are drilled at each end of the crack to reduce the stress concentration at the crack tip. According to AC43.13-1B, the minimum acceptable diameter for a stop-drill hole is 1/8 inch (0.125 inches).

A stop-drill rivet installed at the end of a crack serves to reduce the stress concentration and mechanically prevent the crack from growing further. Its primary function is crack arrestment.

Oil Canning

Oil canning is a loose, unstable area in a skin panel that pops in and out. It is typically caused by a loss of tension or stiffness in the skin panel, resulting from improper riveting (rivets too tight or too loose), impact damage, or fatigue.

Fastener Systems

Solid Rivets

Solid rivets require access to both sides of the joint for installation: one side for the rivet gun and the other for the bucking bar. The rivet set (attached to the rivet gun) has a concave cup that fits over the manufactured head and hammers it, forming the head and driving the rivet into the hole. The bucking bar provides the reaction force that causes the shank to upset and form the shop head.

Blind Rivets

Blind rivets (e.g., Cherrymax, Huck, Avex) can be installed from one side only, making them suitable for locations where access to the blind side is not available.

  • Standard Pop Rivets: Generally have lower shear and tensile strength than solid rivets of the same diameter because the hollow mandrel leaves a less than solid cross-section. Typically used for non-structural or light-load applications.
  • Structural Blind Rivets (Cherrymax, Huck): Designed to overcome the limitations of standard pop rivets. They are installed by pulling the stem (mandrel) with a special tool until the stem breaks at a predetermined point, expanding the sleeve on the blind side.

For a Cherrymax rivet, a properly set rivet will have the stem break flush with or slightly below the rivet head.

Hi-Lok Fasteners

Blind Rivets and Hi-Lok Fasteners — Installation Sequence Blind Rivets and Hi-Lok Fasteners STANDARD BLIND RIVET (POP) Clamped sheets Pre-formed head Upset (low strength) Stem pull LOWER STRENGTH Reduced shear and tension STRUCTURAL RIVET (CHERRYMAX / HUCK) Flush stem break Mechanical locking STRUCTURAL STRENGTH Equivalent to solid rivets HI-LOK FASTENER — COLLAR TORQUED TO RUPTURE 1. Insertion 2. Collar torquing Torque predetermined 3. Hex section break-off Ejected fragment RESULT Consistent and controlled clamping each time Transport Canada — AME Training — Sheet Metal Structures — Chapter 2

Hi-Lok fasteners consist of a threaded pin and a threaded collar. The collar is installed on the pin, and a special tool engages both the hex on the collar and the hex on the pin. The tool tightens the collar until a predetermined torque is reached, and then the hexagonal driving section of the collar breaks off. The collar is torqued, not swaged, providing consistent, controlled clamp-up.

Corrosion Considerations

Galvanic Corrosion

Galvanic corrosion occurs when two dissimilar metals are in contact in the presence of an electrolyte. The more anodic (active) metal corrodes preferentially.

  • Magnesium Alloys: Magnesium is the most anodic structural metal. Fasteners that are more cathodic (noble), such as cadmium-plated steel, will cause severe galvanic corrosion of the magnesium. 5056 aluminum and monel are commonly used with magnesium because they are closer in the galvanic series.
  • Aluminum Alloys: 2024-T4 aluminum rivets are compatible with 7075-T6 aluminum and will not cause significant galvanic corrosion.
  • Float Plane Operations: The presence of salt water as an electrolyte accelerates galvanic corrosion between incompatible metals.

Structural Repair Manual (SRM) and Regulatory Compliance

The Structural Repair Manual (SRM) is the primary source for approved repair procedures. The first step in any repair is to consult the SRM, which provides:

  • Allowable damage limits (e.g., maximum dent depth and diameter for a given skin thickness)
  • Approved repair procedures for damage exceeding allowable limits
  • Material specifications and fastener requirements

AC43.13-1B (Acceptable Methods, Techniques, and Practices - Aircraft Inspection and Repair) provides general guidelines for common repair practices, but the manufacturer's SRM takes precedence.

Important Formulas, Regulations, and Procedures

Key Formulas

Critical Procedures

  1. Rivet Removal: Drill through the center of the manufactured head with a drill bit slightly smaller than the rivet shank diameter. Remove the head, then drive out the remaining shank with a punch.
  2. Deburring: Drill holes must be deburred on both sides to remove sharp edges that act as stress risers and can initiate cracks under fatigue loading.
  3. Drilling Aluminum: Use a slow drill speed with firm, steady feed pressure to avoid work hardening the material.
  4. Honeycomb Repair: Ensure the core is properly sealed to prevent moisture ingress, which can freeze, expand, and delaminate the skin from the core.

Common Relationships Between Concepts

  • Rivet Spacing and Load Distribution: Uneven rivet spacing leads to uneven load distribution. Rivets that are closer together carry a higher proportion of the load, potentially causing them to fail before the rest of the joint.
  • Patch Thickness and Stress Concentration: A patch that is too thick creates a stress concentration and changes load paths, potentially causing failure in the surrounding structure.
  • Dimpling vs. Countersinking: On thin skins (less than 0.040 inches), dimpling is preferred because it displaces material rather than removing it, maintaining the full thickness and providing a stronger joint.
Dimpling vs Countersinking — Thin Skin Comparison Countersinking vs Dimpling COUNTERSINKING Original thickness Weakened zone Material removed t < 0.040 in ⚠ WEAKENS THE SHEET Removes material — reduces strength Characteristics: Beveled cut (cone) machined into the sheet Rivet head flush with the surface DIMPLING Thickness preserved Material displaced t < 0.040 in ✓ PREFERRED FOR THIN SHEETS Displaces material — preserves thickness Characteristics: Boss formed by die (male + female) Rivet head flush — stronger joint SELECTION RULE — THIN SHEETS (< 0.040 in / 1.0 mm) Sheet thickness t < 0.040 in? DIMPLING Preserves thickness COUNTERSINKING (not recommended if t < 0.040 in) Why dimpling is preferred: • Countersinking removes material — weakens the sheet • Dimpling displaces material — maintains thickness • Stronger joint and better fatigue performance • Smooth aerodynamic surface maintained
  • Scab Patch and Eccentric Loading: A scab patch creates an eccentric load path, inducing bending stresses in rivets and skin that can lead to premature fatigue failure.
  • Bend Radius and Material Condition: The minimum bend radius depends on the material and its temper condition. 2024-T3 requires a minimum bend radius of 3T to avoid cracking.
  • Sealant Function: In fuel tanks, sealant prevents fuel leakage. In pressurized fuselages, sealant maintains cabin pressure by preventing air leaks.

Diagram

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