Chapter 5: Aircraft Materials, Hardware, and Processes

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:

1. Type Certificate Data Sheet (TCDS) – Defines the basic design and limitations
2. Manufacturer's Maintenance Manual (MM) and Structural Repair Manual (SRM) – Provide specific procedures and limits
3. Airworthiness Directives (ADs) – Mandatory actions for safety
4. AC 43.13-1B – Accepted methods, techniques, and practices when manufacturer data is unavailable
5. Other FAA-approved data – Engineering orders, service bulletins, supplemental type certificates

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:

• A description of the work performed
• The date of completion
• The AME's signature, certificate number, and type of certificate
• Any additional information required by regulation

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:

• Excellent driving characteristics
• Good corrosion resistance
• Adequate shear strength for general applications
• No heat treatment required prior to installation

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:

• Heat treatment requirements differ
• Driving characteristics differ
• The structural properties of the joint may be affected

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:

• White corrosion indicates oxidation of the cadmium plating
• Red rust indicates the base steel is corroding

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:

• Small cracks in non-critical areas may be repaired by welding, provided the surrounding structure is sound
• Stop-drilling is a temporary measure, not a permanent repair
• Patching is not an approved method for tube structures
• Replacement of the entire tube is not necessary for a small crack if it can be welded

When welding repairs are performed, the welder must ensure:

• The crack is completely removed before welding
• Proper weld technique is used to avoid heat-affected zone cracking
• The repair is inspected per approved procedures

---

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:

Critical Distinction: MS20470 (universal head) and MS20426 (countersunk head) are not interchangeable without engineering approval. Substituting one for the other can affect:

• Aerodynamic smoothness
• Structural strength
• Approved repair method compliance

3.1.2 Rivet Material Codes

Rivet part numbers include material designators:

• AD = 2117-T4 aluminum alloy
• D = 2024-T4 aluminum alloy
• DD = 2024-T4 (higher strength, requires heat treatment)

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:

• Manufactured head must be flush with the skin surface for countersunk rivets
• Shop head must be properly formed, not mushroomed or cracked
• No damage to the surrounding skin

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:

1. Drill through the head with a drill one size smaller than the rivet shank
2. Use a punch to drive out the remaining shank

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:

• Increase the rivet size to the next larger diameter, provided edge distance and spacing requirements are still met
• This maintains the structural integrity of the joint
• Filling with epoxy is not an approved method for structural rivet holes
• Using a standard rivet in an oversized hole compromises joint strength

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:

• Standard tolerances
• Adequate strength for general applications
• Cadmium-plated finish

NAS (National Aerospace Standard) close-tolerance bolts are required in applications subject to:

• Shear loads requiring precise fit
• Vibration-prone installations
• Critical structural attachments

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:

• Damaged shanks
• Corrosion (even minor, as it can initiate stress concentrations)
• Stripped threads
• Bent or deformed bodies
• Worn or damaged heads

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:

• Material properties
• Strength characteristics
• Corrosion resistance

3.2.4 Torque and Cotter Pin Alignment

When torqueing a castellated nut:

1. Torque to the specified value
2. Check cotter pin hole alignment
3. If the hole does not align, tighten to the next castellation, provided the maximum torque is not exceeded

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:

• Fatigue failure
• Loss of securement

Proper installation procedure:

1. Torque the castellated nut to the specified value
2. Install a new cotter pin of the correct diameter and length
3. Bend the ends properly

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:

• Wire must be tight with uniform twists
• Wire must be routed so it tends to tighten the bolts
• Slack is unacceptable because it does not provide positive locking

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:

• Creates a continuous wire pattern between adjacent bolts
• Maintains uniform tension throughout
• Prevents any single bolt from loosening independently

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):

• A single wire passes through the barrel holes
• Wire is twisted (typically 6-8 twists per inch)
• Ends are secured or bent to prevent snagging

Critical Requirements:

• The turnbuckle must not rotate in either direction
• The correct tension in the control cable must be maintained
• Cotter pins are not used for turnbuckles
• Locknuts are not a substitute for safety wire on flight control turnbuckles

3.5 Rod End Bearings

Rod end bearings with threaded shanks are locked by a checknut. The procedure is:

1. Install the rod end and torque to the specified value
2. Tighten the checknut against the rod end to prevent loosening

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:

• Provide more uniform clamping force
• Are less likely to damage the hose
• Maintain consistent pressure over a wider range

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

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:

• Wire brush
• Abrasive pads
• Mechanical blending (for pitting)

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:

• Must be within allowable limits per manufacturer's data or AC 43.13-1B
• If below limits, replacement is required

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:

• Depth of corrosion
• Location (shank, threads, head)
• Criticality of the application

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:

• Replacement of the component
• Engineering-approved repair

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:

• Location of damage (critical vs. non-critical areas)
• Extent of damage (size, depth, number of affected areas)
• Type of damage (dent, crack, corrosion, impact)
• Effect on structural integrity

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:

• Smooth (no sharp edges)
• Not involving cracks or skin separation
• Within allowable limits

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:

• Visible cracks in critical components (e.g., landing gear trunnion) require replacement or engineering-approved repair
• Stop-drilling is a temporary measure only
• Welding without engineering approval is not acceptable
• Cold bonding patches are not approved for primary structural cracks

5.3.2 Crack Repair in Welded Structures

Small cracks in welded steel tube structures may be repaired by welding if:

• The crack is not in a critical area
• The surrounding structure is sound
• Proper welding procedures are followed

5.4 Control Cables

5.4.1 Broken Wire Criteria

Control cables must be replaced when broken wires exceed allowable limits:

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:

• Visual inspection for broken wires
• Check for corrosion
• Verify proper tension
• Inspect for wear patterns

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:

• They do not exceed 20% of the tube diameter
• They are not located in a bend
• They do not restrict fluid flow

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:

• Cracks
• Scratches (deep scratches can initiate failure)
• Corrosion
• Wear at support points
• Flattening or kinking

6.2 Flared Fittings

6.2.1 Leak Troubleshooting

A leak at a flared fitting with proper torque indicates:

• Damaged flare (cracks, nicks, deformation)
• Scratched sealing surface
• Misaligned tube

Correct action: Inspect the flare; if damaged, re-flare the tube with the correct tool.

Incorrect actions:

• Increasing torque (may damage the fitting)
• Using thread sealant (sealing relies on metal-to-metal contact of the flare)

6.2.2 Flare Inspection

The flare must be inspected for:

• Cracks
• Nicks
• Deformation
• Proper angle (37° for AN fittings)

6.3 Line Replacement

When replacing fluid lines:

• Use the same material and size as specified
• Follow approved routing to avoid chafing
• Secure with proper clamps
• Verify no interference with moving parts

---

7. Propeller Maintenance

7.1 Minor Damage Repair

Minor nicks and dents on propeller blades within allowable limits should be:

1. Filed smooth to remove stress risers
2. Polished to restore surface finish

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:

• Location (leading edge, trailing edge, face, back)
• Depth and extent of damage
• Manufacturer's allowable limits
• Effect on balance and aerodynamics

---

8. Electrical Wiring

8.1 Wire Inspection

Electrical wiring must be inspected for:

• Cracked or brittle insulation
• Chafing or wear
• Heat damage
• Corrosion
• Proper support

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:

• Deform over time
• Cut into insulation
• Loosen due to vibration

8.3 Wire Replacement

Damaged wiring must be replaced, not repaired with tape or varnish. The replacement must:

• Match the original wire specification (gauge, insulation type, temperature rating)
• Follow approved routing
• Be properly terminated and secured

---

9. Measurement and Inspection Tools

9.1 Micrometer Usage

Proper micrometer procedure ensures accurate measurements:

1. Clean the part and micrometer surfaces
2. Zero the micrometer to verify calibration
3. Use the ratchet stop to ensure consistent measuring pressure
4. Read the measurement accurately

The ratchet stop is critical because it provides:

• Consistent measuring pressure
• Prevents damage to the part
• Ensures repeatable measurements

9.2 Other Measurement Tools

• Calipers – For general measurements
• Dial indicators – For runout and deflection checks
• Go/no-go gauges – For quick tolerance verification
• Surface plates – For flatness checks

---

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:

• Piston rings
• Valves
• Other components

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:

11.2 The Relationship Between Fastener Type and Application

11.3 The Relationship Between Corrosion and Structural Integrity

Corrosion affects structural integrity by:

• Reducing material thickness
• Creating stress concentrations
• Initiating cracks
• Compromising fastener fit

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:

---

12. Summary of Critical Standards

12.1 Rivet Standards

• Rivet length formula: Material thickness + 1.5 × diameter
• Drill size for removal: One size smaller than rivet shank
• Oversized hole: Use next larger rivet size
• Loose rivets: Remove and replace

12.2 Bolt Standards

• Damaged bolts: Replace, never reuse
• Corroded bolts: Replace if plating is compromised
• Close-tolerance applications: NAS bolts required
• Cotter pin alignment: Tighten to next castellation, never loosen

12.3 Safety Wire Standards

• Bolt clusters: Figure-eight method
• Turnbuckles: Double-twist method, 6-8 twists per inch
• Wire tension: Must be tight, no slack
• Wire routing: Must tend to tighten fasteners

12.4 Corrosion Treatment Standards

• Neutralize first (for battery corrosion)
• Remove all corrosion products
• Verify remaining thickness
• Apply protective treatment
• Document per 14 CFR 43.9

12.5 Tubing Standards

• Dents: Maximum 20% of diameter, not in bends
• Flared fittings: Metal-to-metal seal, no thread sealant
• Leaks: Inspect flare, re-flare if damaged

---

13. Conclusion

The maintenance of aircraft materials, hardware, and processes requires a thorough understanding of:

• Material properties and their limitations
• Fastener selection and installation standards
• Corrosion mechanisms and treatment procedures
• Inspection criteria and allowable damage limits
• Regulatory requirements for approved data and documentation

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.