Aircraft Materials, Hardware, and Processes
SkyLicense study guide with diagrams.
Chapter: Aircraft Materials, Hardware, and Processes
Overview
This chapter covers the fundamental principles of aircraft materials, hardware selection, and maintenance processes as outlined in FAA Advisory Circular 43.13-1B and related regulations. The content addresses the identification, inspection, repair, and replacement of aircraft structural components, fasteners, and systems. Mastery of these concepts is essential for Aircraft Maintenance Engineers (AMEs) to ensure airworthiness, safety, and regulatory compliance in all maintenance activities.
The chapter integrates knowledge from 14 CFR Part 43 (maintenance regulations), AC 43.13-1B (accepted methods, techniques, and practices), and manufacturer-specific data. Understanding the relationships between material properties, fastener selection, corrosion treatment, and inspection criteria forms the foundation of sound maintenance decision-making.
1. Regulatory Framework and Approved Data
1.1 The Hierarchy of Maintenance Authority
Every maintenance action performed on an aircraft must be justified by approved data. The hierarchy of authority, from highest to lowest priority, is:
Key Principle: When a manufacturer's maintenance manual provides no approved repair procedure for a specific defect, the AME must not improvise a repair. The safe and legal action is to replace the component with a serviceable unit or obtain an FAA-approved repair design. Performing an unapproved repair violates 14 CFR 43.13(a), which requires using methods and practices acceptable to the Administrator.
1.2 Maintenance Records Requirements
Per 14 CFR 43.9, after performing maintenance, the AME must make a logbook entry documenting:
This documentation requirement applies to all maintenance actions, including minor repairs performed under Part 43.
2. Aircraft Materials
2.1 Aluminum Alloys
Aluminum alloys are the primary structural material in most aircraft due to their favorable strength-to-weight ratio and corrosion resistance when properly protected.
2.1.1 2117-T4 Aluminum Alloy
2117-T4 is the standard aluminum alloy rivet material for general airframe repair. Its critical advantage is that it arrives in the as-received (T4) condition and does not require heat treatment before driving. This makes it the default choice for field repairs where heat-treating facilities are unavailable.
Key characteristics:
2.1.2 2024-T4 Aluminum Alloy
2024-T4 offers higher strength than 2117-T4 but requires heat treatment before driving. If 2024-T4 rivets are driven in the wrong condition, they may crack or fail to form proper shop heads. Substituting 2024-T4 for 2117-T4 without approved data is not acceptable because:
2.1.3 5052 Aluminum Alloy
5052 is commonly used for hydraulic and fuel tubing due to its excellent formability and corrosion resistance. It is not a high-strength structural alloy but is ideal for fluid-carrying lines where ductility and corrosion resistance are paramount.
2.2 Steel Alloys
Steel is used in aircraft for high-strength applications such as landing gear components, engine mounts, and control system hardware.
2.2.1 Corrosion Behavior of Steel
Steel fasteners are typically cadmium-plated for corrosion protection. The plating provides sacrificial protection—the cadmium corrodes preferentially, protecting the underlying steel. When the plating is compromised:
Red rust on a steel fastener indicates loss of protective plating and can compromise the fastener's integrity and fit. Corroded or damaged fasteners must be replaced with new, approved parts. Cleaning and reinstalling a corroded fastener is not acceptable because it may hide structural damage and does not restore the original strength or corrosion protection.
2.3 Welded Steel Tube Structures
Welded steel tube frames are common in general aviation aircraft. Cracks in these structures require specific repair procedures:
When welding repairs are performed, the welder must ensure:
3. Fasteners and Hardware
3.1 Rivets
Rivets are the primary permanent fasteners for aircraft sheet metal structures.
3.1.1 Rivet Identification
Rivet part numbers follow military standards that define head type, material, and finish:
| Prefix | Head Type | Material |
|---|---|---|
| MS20426 | Countersunk (100°) | Various |
| MS20470 | Universal | Various |
| AN426 | Countersunk (100°) | Various |
| AN470 | Universal | Various |
Critical Distinction: MS20470 (universal head) and MS20426 (countersunk head) are not interchangeable without engineering approval. Substituting one for the other can affect:
3.1.2 Rivet Material Codes
Rivet part numbers include material designators:
3.1.3 Rivet Length Determination
The correct rivet length is calculated as:
Rivet Length = Total Material Thickness + 1.5 × Rivet Diameter
The 1.5 × diameter allowance provides sufficient material to form a proper shop head. Using the original rivet length may not be correct if the skin thickness has changed.
3.1.4 Rivet Installation Standards
Proper rivet installation requires:
A proud or sunken manufactured head indicates improper installation or incorrect rivet selection, compromising structural integrity and aerodynamic smoothness. Such rivets must be removed and replaced.
3.1.5 Rivet Removal
Proper rivet removal procedure per AC 43.13-1B:
This method prevents damage to the surrounding skin. Chiseling, grinding, or vibrating can damage the structure.
3.1.6 Oversized Holes
If a hole becomes oversized during rivet removal:
3.1.7 Loose Rivets
Loose rivets must be removed and replaced with new rivets of the same size and alloy. Re-bucking is not acceptable because the rivet may be work-hardened or damaged. Applying sealant does not restore the structural integrity of the joint.
3.2 Bolts
3.2.1 AN vs. NAS Bolts
AN (Army-Navy) standard bolts are general-purpose fasteners with:
NAS (National Aerospace Standard) close-tolerance bolts are required in applications subject to:
Using a standard AN bolt in a close-tolerance application is not an approved practice and can lead to premature failure due to improper load distribution and fit.
3.2.2 Bolt Inspection Criteria
Bolts with the following conditions are not acceptable for reuse:
This is especially critical in flight control linkages and other safety-critical systems. Cleaning and reinstalling, even within tolerances, violates standard practice for replacing damaged hardware in critical systems.
3.2.3 Bolt Substitution
For critical structural attachments, only approved fasteners (AN, MS, NAS) are acceptable. Using common hardware store bolts is prohibited because they may not have the required:
3.2.4 Torque and Cotter Pin Alignment
When torqueing a castellated nut:
Loosening the nut is not recommended because it may result in insufficient clamp load. Replacing with a self-locking nut or adding washers is not an approved method.
3.3 Cotter Pins
Cotter pins are single-use items and must be replaced whenever they are removed or disturbed. Reusing a cotter pin that has been bent multiple times risks:
Proper installation procedure:
Safety wire is not an approved substitute for a cotter pin in a castellated nut application. Thread-locking compound does not replace the mechanical locking of a cotter pin.
3.4 Safety Wiring
3.4.1 General Principles
Safety wire provides positive locking of fasteners against vibration-induced loosening. Key requirements:
3.4.2 Figure-Eight Method for Bolt Clusters
For multiple bolts in a cluster, the figure-eight method is used to provide continuous tension that prevents loosening. This method:
The straight-line method is not appropriate for multiple bolts in a cluster configuration.
3.4.3 Turnbuckle Safety Wiring
Turnbuckles in control systems require specific safety wiring methods:
Double-Twist Method (Standard):
Critical Requirements:
3.5 Rod End Bearings
Rod end bearings with threaded shanks are locked by a checknut. The procedure is:
Castellated nuts and self-locking nuts are not typical for rod ends because they do not provide the same locking method.
3.6 Hose Clamps
Hose clamps must be of the type and size specified in the maintenance manual. For example, T-bolt clamps are often required for fuel systems because they:
Using an incorrect clamp can lead to fuel leaks or hose failure. Adding another clamp does not correct the problem, and re-torquing does not address the type of clamp used.
4. Corrosion: Detection, Treatment, and Prevention
4.1 Types of Corrosion
| Type | Description | Typical Location |
|---|---|---|
| Surface | Uniform attack on exposed surface | Skin panels, fittings |
| Pitting | Localized attack forming pits | Fuselage skin, fasteners |
| Galvanic | Dissimilar metal contact | Joints, fasteners |
| Intergranular | Attack along grain boundaries | Heat-treated aluminum |
| Exfoliation | Layer-by-layer attack | Extrusions, forgings |
| Stress corrosion | Combined stress and corrosive environment | Highly stressed areas |
4.2 Corrosion Treatment Process
The corrosion treatment process follows a specific sequence:
Step 1: Neutralization (for Battery Corrosion)
For corrosion from lead-acid batteries (acidic), neutralize with a baking soda solution before any mechanical treatment. This stops the active corrosion process.
Step 2: Complete Removal of Corrosion Products
All corrosion products must be completely removed before any chemical treatment or protective coating is applied. Methods include:
Applying conversion coating or primer over active corrosion will not stop the process.
Step 3: Surface Evaluation
After corrosion removal, evaluate the remaining material thickness:
Step 4: Chemical Treatment
Apply corrosion inhibitor and protective finish per approved procedures.
Step 5: Documentation
Record the maintenance action per 14 CFR 43.9.
4.3 Corrosion on Fasteners
4.3.1 Steel Fasteners
Minor corrosion on a steel bolt may be reworked if it remains within dimensional limits and strength requirements, but any doubt requires replacement. The evaluation must consider:
4.3.2 Aluminum Fasteners
Corrosion on aluminum rivets or fasteners typically requires replacement, as the corrosion may have compromised the material's structural integrity.
4.4 Corrosion in Primary Structures
Cracks or significant corrosion in primary structural components must be repaired using approved methods, which typically involve:
Drilling stop holes is a temporary measure, not an approved permanent repair. Welding without proper engineering approval can alter material properties and is not acceptable.
5. Structural Inspection and Repair
5.1 Damage Evaluation
All damage must be evaluated against the manufacturer's structural repair manual (SRM). The evaluation considers:
5.2 Dents and Minor Damage
5.2.1 Non-Structural Components
Minor dents in non-structural components (e.g., wing tips, fairings) that are:
may be left as is or repaired cosmetically. However, cracks in non-structural aluminum fairings should be repaired by replacement, as stop-drilling is not an approved permanent repair.
5.2.2 Structural Components
Dents in structural components must be evaluated per the SRM. If within allowable limits, they may be left as is. If beyond limits, repair or replacement is required.
5.3 Cracks
5.3.1 Crack Evaluation
Cracks in primary structural components require immediate action:
5.3.2 Crack Repair in Welded Structures
Small cracks in welded steel tube structures may be repaired by welding if:
5.4 Control Cables
5.4.1 Broken Wire Criteria
Control cables must be replaced when broken wires exceed allowable limits:
| Cable Diameter | Allowable Broken Wires per Lay |
|---|---|
| ≤ 5/16 inch | Any broken wire requires replacement |
| > 5/16 inch | More than one broken wire in any lay |
Example: A 3/16-inch cable with 7 broken wires within one lay exceeds the allowable limit (which is 0 for this diameter) and must be replaced.
5.4.2 Cable Inspection
Inspection of control cables includes:
Splicing control cables is generally not an approved repair method; cables must be replaced when damaged beyond limits.
6. Fluid Lines and Fittings
6.1 Tubing Inspection Criteria
6.1.1 Dents
Dents in rigid tubing are acceptable if:
Dents in bends are more prone to cracking and require replacement. Dents in straight sections away from fittings are generally not a structural concern.
6.1.2 Other Defects
Tubing must be inspected for:
6.2 Flared Fittings
6.2.1 Leak Troubleshooting
A leak at a flared fitting with proper torque indicates:
Correct action: Inspect the flare; if damaged, re-flare the tube with the correct tool.
Incorrect actions:
6.2.2 Flare Inspection
The flare must be inspected for:
6.3 Line Replacement
When replacing fluid lines:
7. Propeller Maintenance
7.1 Minor Damage Repair
Minor nicks and dents on propeller blades within allowable limits should be:
Leaving nicks as-is may allow cracks to develop. Replacing the propeller is not necessary for minor damage. Drilling holes is not an approved repair for nicks.
7.2 Damage Evaluation
Propeller damage evaluation considers:
8. Electrical Wiring
8.1 Wire Inspection
Electrical wiring must be inspected for:
8.2 Wire Support Requirements
Wiring must be properly supported with clamps that have a cushion to prevent chafing. Plastic cable ties are not approved for permanent support because they can:
8.3 Wire Replacement
Damaged wiring must be replaced, not repaired with tape or varnish. The replacement must:
9. Measurement and Inspection Tools
9.1 Micrometer Usage
Proper micrometer procedure ensures accurate measurements:
The ratchet stop is critical because it provides:
9.2 Other Measurement Tools
10. Engine Maintenance
10.1 Compression Testing
When a compression test reveals low compression in a cylinder, the next step is a differential pressure check to determine whether the leak is past:
This diagnostic step identifies the specific cause before any repair is performed. Replacing the cylinder without diagnosis is not cost-effective and may not solve the problem.
11. Common Relationships and Principles
11.1 The Relationship Between Material Condition and Repair Method
The condition of the material determines the repair method:
| Condition | Repair Method |
|---|---|
| Surface corrosion (within limits) | Remove corrosion, treat, protect |
| Pitting (within limits) | Blend out, verify thickness, protect |
| Beyond allowable limits | Replace component |
| Crack in primary structure | Replace or engineering-approved repair |
| Crack in non-critical structure | Repair per approved data |
11.2 The Relationship Between Fastener Type and Application
| Application | Fastener Type | Rationale |
|---|---|---|
| General sheet metal | MS20470AD (universal head) | Standard installation |
| Countersunk applications | MS20426 (countersunk head) | Flush surface required |
| Shear-critical applications | NAS close-tolerance bolts | Precise fit required |
| General structural | AN bolts | Standard strength |
| Safety-critical | Castellated nut + cotter pin | Positive mechanical locking |
11.3 The Relationship Between Corrosion and Structural Integrity
Corrosion affects structural integrity by:
The evaluation of corrosion damage must consider all these factors to determine whether repair or replacement is appropriate.
11.4 The Relationship Between Approved Data and Maintenance Actions
Every maintenance action must be traceable to approved data:
| Maintenance Action | Required Data |
|---|---|
| Rivet replacement | AC 43.13-1B, SRM, or MM |
| Bolt replacement | IPC, MM, or AC 43.13-1B |
| Corrosion treatment | AC 43.13-1B Chapter 6 |
| Welding repair | Approved welding procedures |
| Cable replacement | AC 43.13-1B Chapter 7 |
12. Summary of Critical Standards
12.1 Rivet Standards
12.2 Bolt Standards
12.3 Safety Wire Standards
12.4 Corrosion Treatment Standards
12.5 Tubing Standards
13. Conclusion
The maintenance of aircraft materials, hardware, and processes requires a thorough understanding of:
The AME must always work within the framework of approved data, using sound judgment to determine when repair is appropriate and when replacement is required. Safety is paramount—when in doubt, replace the component and document the action properly.
This chapter provides the foundational knowledge necessary for the FAA General examination and for practical application in aircraft maintenance. Mastery of these concepts ensures that maintenance actions preserve the airworthiness of the aircraft and comply with all applicable regulations.
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