M — Airframe (Cellule)Chapter 1 · 161 practice questions

Chapter 1: Aircraft Structures — Sheet Metal & Composites

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Chapter: Aircraft Structures — Sheet Metal & Composites

Overview

This chapter covers the fundamental principles of aircraft structural maintenance, focusing on sheet metal repair techniques, composite material handling, and the regulatory framework governing structural repairs. The material addresses the knowledge required for aircraft maintenance engineers to inspect, evaluate, and repair aircraft structures while complying with Canadian Aviation Regulations (CARs) and industry standards. Key areas include damage assessment, repair design principles, material selection, and the classification of repairs as minor or major modifications.

Diagram — Aircraft Structures — Sheet Metal & Composites AIRCRAFT STRUCTURES — SHEET METAL & COMPOSITES Structural Repair Decision Flowchart — CARs & Industry Standards DAMAGE DETECTION Visual inspection Tap testing / NDT methods DAMAGE ASSESSMENT Measure depth / extent Compare to SRM limits DECISION Within limits? Repair or replace? YES NO MINOR REPAIR (CAR 571) Doubler patch — flush rivets — cold bond — approved data required MAJOR REPAIR / REPLACEMENT (CAR 571) Scarf repair — stepped lap — hot bond — engineering approval required MATERIAL TYPE Sheet Metal 2024-T3 Alclad 7075-T6 high strength Composites Carbon/epoxy prepreg Glass / Kevlar laminates LEGEND: Inspection/NDT Minor repair path Major repair path Material selection SRM = Structural Repair Manual CAR 571 = Canadian Aviation Regulations

Key Concepts

Sheet Metal Structures

Stress distribution in a riveted joint Stress Distribution in a Riveted Joint Pitch and Edge Distance — Definitions PITCH (rivet center spacing) EDGE DISTANCE (distance to edge) Rivet Line Top plate Bottom plate Rivet line (longitudinal axis) Rivets aligned on the same axis Failure Modes of a Riveted Joint 1. Rivet Shear ✂ Opposing forces on the plates The rivet yields under the shear force 2. Plate Tension CRACKS The plate tears between rivets or toward the edge 3. Bearing Pressure Ovalized hole The rivet ovalizes the hole under load → loose rivet Correction: ream to next larger Ø Maximum stress concentration at rivet/plate contact points
Sheet-metal load path Sheet Metal Load Path SKIN — Tension and Shear 2024-T3 Sheet 0.040 in TENSION TENSION SHEAR The skin transfers aerodynamic loads to the frames and stringers. Tension stresses stretch the metal; shear deforms it. FRAMES AND RIBS — Compression Formed frame 7075-T6 Aluminum COMPRESSION Frames maintain the fuselage shape. Ribs support the wing skin. They resist compression and distribute loads across the structure. STRINGERS — Tension and Compression Main stringer C-section profile TENSION TENSION COMPRESSION Stringers are the main beams of the wing and fuselage. They carry flight loads: tension at the bottom, compression at the top. RIVETS AND TORSION Load transmission through rivets AN470 (round head) — AN426 (countersunk head) Loose rivet = ovalized hole = replacement TORSION Torsional moment SUMMARY — Load Path Frames Stringers Rivets Aerodynamic loads are absorbed by the skin, transmitted to the frames and ribs, then to the stringers. Rivets ensure structural continuity. Any failure of one element compromises the load path.
Aircraft Structure — Sheet Metal vs Composites Aircraft Structure — Sheet Metal vs Composites 1. Semi-Monocoque (Metal) Doubler Stressed Skin (Alclad 2024-T3) Frame Stringer Rivets Key Characteristics: • Inspection: Easy visual (loose rivets, corrosion, cracks). • Repair: Standardized (SRM), riveting, reinforcement plates (doublers). 2. Composite (Sandwich) Repair Skin (Laminate) Fibers + Resin Core Honeycomb (Lightens structure) Matrix & Reinforcement Key Characteristics: • Weight: Excellent strength-to-weight ratio. • Inspection: Difficult (internal damage invisible, requires NDT). • Repair: Complex (Curing, vacuum, precise temp & resin mix control).

Material Properties and Identification

Aircraft sheet metal structures commonly use aluminum alloys, with 2024-T3 being one of the most prevalent for skin panels. This alloy offers excellent strength-to-weight ratio and fatigue resistance. When inspecting sheet metal, engineers must understand that dents, cracks, and corrosion each require specific evaluation criteria.

Damage Assessment

The Structural Repair Manual (SRM) is the primary reference for determining whether damage is within allowable limits. For dents in non-damage-tolerant areas, the SRM provides specific depth and diameter limits based on material thickness and alloy. For 2024-T3 skin of 0.040 inch thickness, a dent of 0.5 inch depth and 2 inches diameter may be within allowable limits if no cracking is present. When damage is within limits, the area should be lightly filed to smooth the contour and prevent stress risers before returning to service.

Riveted Joints

Riveted joint repair sequence Riveted Joint Repair Sequence STEP 1 — LOOSE RIVET Ovalization of hole ⚠ LOOSE RIVET Cause: Play between the rivet and the hole wall. The rivet can move by hand. Consequence: Loss of structural integrity, fretting, and potential crack propagation. Top view STEP 2 — DRILLING & REAMING 2a — Drill out the rivet 2b — Ream to next size up +1/32" The hole is enlarged to the next nominal size up. STEP 3 — LARGER RIVET 3a — Countersink the edges Countersink 3b — Insert the rivet Insertion Larger Ø 3c — Buck the tail Fills the hole

Rivets are the primary fasteners for sheet metal repairs. Key principles include:

  • Loose rivets indicate worn holes and require drilling out, reaming to the next larger size, and installing the next larger diameter rivet
  • Rivet holes must be deburred and the rivet must completely fill the hole after driving
  • The manufactured head is typically placed on the external surface, with the shop head formed on the interior
  • Rivet sets must match the shape of the rivet head

Doubler Plate Design

Doubler plate design rules Reinforcement Plate Design (Doubler) Plan View — Damaged Sheet with Reinforcement Plate Crack Reinforcement Plate Rounded corner (avoids cracks) Compatible material: aluminum alloy (e.g., 2024-T3) Sized per repair design criteria Reinforcement plate (doubler) Rivets Crack in sheet metal Cross-Section View — Reinforcement Plate Repair Original sheet (parent structure) Reinforcement plate Sized thickness Original sheet Load transfer Reinforcement Plate Design Principles Stress concentration Reduced stress 1. Rounded corners 2024-T3 2024-T3 ✓ Same alloy ✓ Same treatment 2. Compatible material Sized per design criteria ✗ Not "as large as possible" 3. Sizing

When repairing damaged sheet metal with a doubler plate:

  • The doubler must be of similar or compatible material to the parent structure
  • All corners must be rounded to reduce stress concentration and prevent crack initiation
  • The doubler must be sized according to the repair design, not simply made as large as possible

Control Cables

7x19 control cable construction and damage limits Construction of a 7x19 Control Cable Strand/wire structure — Inspection criteria — Critical zones 7×19 Structure Strand zoom: 19 individual wires (steel) twisted together Strand (group of wires) Individual wire (steel) Inspection limit 10 feet Rule: Maximum 2 broken wires within a 10-foot length ✓ Acceptable outside critical zone ✗ Replace if in critical zone Critical zones ⚠ Critical zone — Pulley ⚠ Critical zone — Guide Any broken wire = replacement (even a single wire) Control cable inspection procedure 1. Visual inspection • Scan the entire length of the cable • Look for broken wires, corrosion, wear, deformation, kinks • Pay particular attention to areas passing over pulleys and through guides (Use a magnifying glass if necessary) 2. Counting broken wires • Count broken wires over each 10-foot length • Note their position • If ≤ 2 broken wires and outside critical zone → acceptable • If ≥ 3 broken wires or in critical zone → replacement required 3. Decision • Outside critical zone, ≤ 2 broken wires per 10 ft → keep in service and monitor • Critical zone (pulley, guide) with 1 broken wire → REPLACE the cable Reference: airworthiness standards — flight control cables — inspection criteria and wear limits

For 7x19 cable construction, AC 43.13-1B specifies a maximum of 2 broken wires in any 10-foot length, provided the broken wires are not in critical areas such as over pulleys or fairleads.

Push-Pull Tubes

Push-pull tubes in flight control systems are critical structural components. Any bend is unacceptable as it indicates overload and potential fatigue. The tube must be replaced; straightening is not permitted.

Composite Structures

Composite layup and fibre orientation Composite Stacking and Fiber Orientation Load paths according to orientation — Typical aircraft skin laminate Typical Stacking (Laminate) 0° (longitudinal axis) +45° (shear) 90° (circumferential) −45° (reverse shear) 90° (circumferential) +45° (shear) 0° (longitudinal axis) Stacking sequence: [0/45/90/−45/90/45/0] Matrix: epoxy resin — Reinforcement: continuous fibers Matrix (resin): transfers loads between fibers Fiber Orientation and Load Paths 90° 0° 0° 90° +45° −45° Tensile load (carried by 0° fibers) Shear (carried by ±45° fibers) Role of each layer: • 0° → carries longitudinal tensile/compressive loads (wing bending, fuselage) • 90° → maintains shape, carries circumferential loads (cabin pressure, buckling) • ±45° → carries shear loads (wing torsion, control torque) 0° +45° 90° −45° Load

Repair Principles

Pre-preg cure cycle control Prepreg Cure Cycle Control °F 350 300 250 200 150 0 15 30 45 60 75 90 105 Time (minutes) Ramp 2-5°F/min Cure dwell 250°F ± 10°F Cooling Deviation 20°F Deviation detected → Stop Legend: TC1 — Thermocouple 1 (correct mat placement) TC2 — Thermocouple 2 (incorrect mat placement) ✕ Stop cure if deviation ≥ 20°F Safety instruction: A 20°F deviation between TCs indicates incorrect heating mat placement. TC1 TC2 Heating mat (cross-section) Prepreg laminate TC1 TC2 i Typical cycle: temperature ramp → dwell → cooling Total duration: ~105 min | Dwell temperature: 250°F

Composite repairs require precise control of environmental conditions and material properties. For pre-preg materials, the cure cycle (temperature and time) specified by the manufacturer must be strictly followed. Temperature differentials across the repair area should be minimized; a 20°F difference indicates improper heat blanket placement and requires aborting the cure.

Wet Lay-Up Repairs

For honeycomb sandwich structures with face sheet damage but no core damage:

  • The resin-to-hardener ratio is the most critical factor determining chemical cure and final mechanical properties
  • Incorrect ratio can result in incomplete cure, weak bond, or exothermic reaction
  • Vacuum bagging and heat may be required depending on the resin system

Corrosion in Composite Structures

Filiform corrosion treatment chain Filiform Corrosion Treatment Under Paint Aluminum alloy skin — Complete treatment sequence 4-STEP TREATMENT SEQUENCE 1. DETECTION Vermicular traces under the paint Paint Corrosion Aluminum 2024-T3 2. PAINT REMOVAL stripper Remove paint chemical stripper Inspect full extent 3. REMOVAL abrasive Remove mechanically sanding / grinding Do not exceed SRM limits 4. INHIBITOR + COATING Inhibitor Primer Paint 5. QUALITY CONTROL ✓ Verify absence of residual corrosion

Filiform corrosion under paint on aluminum alloy skins, while appearing cosmetic, can lead to structural damage. The correct action is to remove paint, mechanically remove corrosion, treat with corrosion inhibitor, and repaint.

Welding in Aircraft Structures

4130 Steel Tube Structures

TIG (Tungsten Inert Gas) welding is the preferred method for welding 4130 steel in aircraft structures because it provides precise control, reduces the heat-affected zone, and produces high-quality welds. Oxy-acetylene welding is used for some applications but is less precise.

Regulatory Requirements

Under Standard 573.02(8)(c), an AMO performing weld repairs on tubular structures must hold both the Structure (Tubular) and Welding ratings. Specialized maintenance such as welding must be performed by an AMO with the appropriate category for that process.

Important Regulations, Formulas, and Procedures

Canadian Aviation Regulations (CARs)

Maintenance Data Requirements (CAR 571.02)

  • Persons performing maintenance must follow manufacturer's recommendations
  • When aircraft manufacturer recommendations conflict with engine or propeller manufacturer recommendations, aircraft manufacturer recommendations take precedence
  • If no manufacturer recommendations exist, standard industry practices may be used
  • When using standards other than manufacturer's recommendations, the technical record entry must reference the standard used

Maintenance Release Requirements (CAR 571.11)

  • For transport category aeroplanes and turbine-powered helicopters, the AME must have completed an applicable approved type course
  • The AME must hold a licence with a rating appropriate to the work
  • The AME signing the release does not need to be employed by the AMO that performed the work

Major Repairs and Modifications (CAR 571.12)

  • Major repairs and major modifications must be reported to Transport Canada
  • A major modification is an alteration to the type design that has other than a negligible effect on weight, balance, structural strength, performance, etc.
  • STC installations requiring structural changes are major modifications

Acceptable Data (Standard 571.06)

  • Includes advisory documents issued by foreign airworthiness authorities with whom Canada has entered into airworthiness agreements
  • FAA Advisory Circular 43.13-1 and -2 are acceptable data for use in Canada
  • Repairs can be performed using acceptable data such as manufacturer's instructions or AC 43.13

Specialized Maintenance (CAR 571.04)

  • Includes welding, non-destructive testing (NDT), and overhaul of components like engines and propellers
  • Must be performed by an AMO with the appropriate category for that process
  • Specific NDT tasks in Appendix K can be performed without an AMO NDT category if the person has completed specific training

Technical Records (Standard 573)

  • Records must be retained for the period the aeronautical product is in service plus one year
  • All maintenance and elementary work must be recorded in the journey log
  • Compliance with Airworthiness Directives must be recorded including AD number, method of compliance, and date

Recency Requirements (Standard 566.05)

  • To exercise AME licence privileges, the holder must have performed at least 6 months of aircraft maintenance in the preceding 2 years
  • This is a general recency requirement, not specific to an aircraft type

Quality Assurance Program (Standard 573.09)

  • Must include a system for conducting internal audits
  • Audits must be conducted at least once every 12 months
  • Auditors must be independent and not audit their own work

Torque Calculations

When using calibrated tools with specified tolerances:

  • Acceptable range = Specified value ± (Specified value × Tolerance percentage)
  • Example: 100 inch-pounds with ±3% tolerance = 100 ± 3 = 97 to 103 inch-pounds

Life-Limited Parts (Standard 571.09)

When a life-limited part reaches its life limit or is removed, it must be destroyed to prevent inadvertent re-installation.

Undocumented Parts (Standard 571, Appendix H)

Undocumented parts must be quarantined and subjected to inspection and testing to ensure conformance to type design before acceptance.

Common Relationships Between Concepts

Troubleshooting Methodology

Systematic troubleshooting follows a logical progression:

  1. Verify all inputs are present (electrical, hydraulic, mechanical) before replacing components
  2. Begin with non-invasive functional tests as specified in maintenance manuals
  3. Check for simple causes first (loose connections, damaged wiring, corrosion)
  4. Use fault codes to narrow the problem area

Hydraulic System Relationships

  • Low fluid level always indicates a leak that must be found and rectified
  • Spongy brakes indicate air in the system
  • Fluctuating pressure under load indicates air in the system
  • Slow operation with pressure drop indicates restriction in the line

Engine Performance Relationships

  • Retarded ignition timing causes power loss and lower cylinder head temperatures
  • High EGT with normal fuel flow indicates turbine blade damage or inefficiency
  • Low compressor discharge pressure for given RPM indicates compressor fouling or damage
  • High EGT at idle returning to normal at higher power indicates stuck-open bleed valve

Fuel System Relationships

  • Fuel heaters prevent ice crystal formation at high altitudes
  • Low refrigerant level indicates a leak in the system
  • Erratic fuel quantity readings suggest faulty probes or wiring issues

Structural Repair Classification

Structural repair classification Classification of Structural Repairs MINOR/MAJOR DECISION Does the repair affect structural strength? YES MAJOR REPAIR CAR 571.06 — Non-negligible effect on integrity NO MINOR REPAIR Negligible effect on weight, CG, strength EXAMPLES • Major: 4-inch crack on pressurized fuselage • Minor: non-critical dent REQUIRED DATA 1. MANUFACTURER'S RECOMMENDATIONS SRM / Repair manual 2. ACCEPTABLE STANDARDS AC 43.13-1B (FAA) Airworthiness agreements 3. CONFLICT? CAR 571.02 — The aircraft manufacturer prevails CAR 571.03 RECORDING All maintenance must be logged in technical records AMO / AME AND RELEASE AFTER REPAIR AMO Approved Maintenance Organization — Appropriate category Specialized maintenance: welding, NDT AME Aircraft Maintenance Engineer certified Recency: 6 months / 2 years (Std 566.05) MAINTENANCE RELEASE CAR 571.11 — Signed by the AME Transport category: type course required Elementary work: recording only CAR 571.12 REPORTING Major repairs and major modifications → Transport Canada SPECIAL CASES • Undocumented parts (Appendix H): quarantine, inspection, testing • Life-limited parts (Std 571.09): destruction after removal • Technical records: product life + 1 year (Std 573.15) DECISION FLOW: Assess damage → Consult SRM / manufacturer data → Classify minor/major → Perform repair → Release → Report if major

The classification of repairs (minor vs. major) depends on:

  • Effect on weight, balance, structural strength, performance, etc.
  • Whether the repair deviates from the type design
  • Location (pressurized vs. non-pressurized areas)
  • Whether the repair affects the pressure vessel integrity

AMO Ratings and Capabilities

The relationship between AMO ratings and maintenance capabilities:

  • Engine category allows overhaul but not specialized processes like welding
  • Welding requires separate AMO category
  • Line maintenance rating excludes scheduled checks with segmented portions of higher-level checks
  • Aircraft operated under Part IV (FTU) or Part VII must have maintenance performed under AMO control

Diagram

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