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.
Key Concepts
Sheet Metal Structures
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
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
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
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
Repair Principles
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 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:
98.Verify all inputs are present (electrical, hydraulic, mechanical) before replacing components
99.Begin with non-invasive functional tests as specified in maintenance manuals
100.Check for simple causes first (loose connections, damaged wiring, corrosion)
101.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
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
