Metallic Structures

Overview

This chapter covers the fundamental principles, materials, and techniques used in the construction, inspection, and repair of aircraft metallic structures. It focuses on the practical application of airframe maintenance practices as outlined in FAA Advisory Circular (AC) 43.13-1B and 14 CFR Part 43. The content addresses critical areas such as riveted joint design, damage assessment, material selection, and the proper use of tools and techniques to ensure structural integrity and airworthiness. A key theme is the necessity of using approved data and exercising sound engineering judgment when performing repairs.

Riveted Joint Design and Installation

Riveting is the primary method for joining sheet metal components in aircraft construction. Proper rivet selection, hole preparation, and installation techniques are critical for creating a joint that can withstand the required loads without failure.

Rivet Sizing and Selection

The strength of a riveted joint depends on the correct choice of rivet diameter and length.

Rivet Diameter: The general rule for determining the appropriate rivet diameter is that it should be approximately 1.5 times the thickness of the thickest sheet being joined. This ensures adequate shear strength and proper hole fill. For example, when joining a 0.040-inch skin to a 0.063-inch doubler, the thickest sheet is 0.063 inches, so the rivet diameter should be around 0.0945 inches, which corresponds to a standard 3/32-inch rivet.
Rivet Length: The length of a rivet is critical for proper shop head formation and clamping force. The rivet must be long enough to fill the hole completely and protrude sufficiently to form a shop head of the correct dimensions. Using a rivet that is too short will result in an undersized shop head, reducing the joint's strength and clamping force. The required length is calculated based on the total material thickness plus an allowance for the shop head (typically 1.5 times the rivet diameter).

Edge Distance and Spacing

Edge distance and rivet spacing are critical design parameters that prevent material failure.

Edge Distance: The minimum edge distance for structural rivets is 2 times the rivet diameter (2D). This distance is measured from the center of the rivet hole to the edge of the material. It ensures adequate material is present to prevent edge tear-out under load. For a 3/32-inch rivet (0.09375 inches in diameter), the minimum edge distance is 0.1875 inches.
Rivet Spacing: Rivet spacing is the distance between the centers of adjacent rivets in a row. The maximum allowable spacing is determined by the manufacturer's engineering data or, in the absence of such data, by the guidelines in AC 43.13-1B. When manufacturer's data specifies a maximum spacing, it takes precedence over generic guidance. Exceeding the specified spacing reduces the joint's strength and fatigue life and requires corrective action, such as adding rivets to bring the installation into compliance.

Installation Tools and Techniques

The choice of installation tool depends on the accessibility of the joint and the required quality of the installation.

Rivet Gun and Bucking Bar: This is the most common method for installing solid rivets. The rivet gun delivers a series of rapid blows to the manufactured head, while a bucking bar held against the tail of the rivet forms the shop head. The bucking bar must be held firmly and squarely against the rivet tail to ensure a uniform shop head. On curved surfaces, a curved bucking bar that matches the contour must be used to ensure the rivet is set properly without damaging the skin. A flat bar on a curved surface can cause uneven setting or skin damage.
Rivet Squeezer: A rivet squeezer is a preferred tool for vertical surfaces, confined spaces, or any application where a bucking bar is difficult to control. It provides a single, controlled squeezing action that produces consistent results and reduces the risk of damage to the surrounding skin. In confined spaces, a rivet squeezer offers superior control and eliminates the risk of bucking bar slippage.
Air Pressure Settings: The air pressure supplied to a rivet gun must be properly regulated. Excessive pressure can drive the manufactured head too deep into the skin, leaving a depression and damaging the structure. The correct pressure setting depends on the rivet size and material and should be adjusted to produce a properly formed shop head without damaging the surrounding material.
Flush Rivets: Flush rivets are used where a smooth aerodynamic surface is required. The holes are countersunk to accommodate the rivet head. If a hole is slightly oversize, a larger rivet must be used to maintain the required interference fit and structural strength. Filling an oversize hole with sealant or using a smaller rivet is not acceptable.

Blind Rivets (Cherry Rivets)

Blind rivets are used in locations where access to the back side of the joint is not possible. For structural applications, a mechanical-lock type blind rivet (such as CherryMAX or equivalent) must be used. This type of rivet locks the stem in place, preventing it from becoming loose and causing core failure. The material and strength of the blind rivet must also be appropriate for the application.

Damage Assessment and Repair

The first step in any repair is a thorough damage assessment to determine the extent of the damage and whether it is within allowable limits. The manufacturer's Structural Repair Manual (SRM) is the primary source of approved data. In the absence of specific SRM data, AC 43.13-1B provides acceptable methods and damage tolerance criteria.

Dents

Dents in aluminum structures are not always cause for replacement. The acceptability of a dent depends on its location, depth, length, and the presence of cracks.

Non-Critical Areas: Dents in non-critical areas may be within allowable damage limits as defined by the manufacturer's SRM or AC 43.13-1B. If the dent is within limits (e.g., a 2-inch long, 0.25-inch deep dent in a spar web with no cracks), the aircraft can be returned to service.
Evaluation Required: Not all dents require skin replacement. The correct action is to evaluate the damage against approved data before deciding on repair or replacement. Hammering out a dent is not an approved repair method as it can cause further damage. Heating aluminum to reform it is also not approved and can degrade material properties.

Corrosion

Corrosion is a common finding on aluminum structures. The severity of corrosion determines the required action.

Light Corrosion Pitting: Light corrosion pitting can often be blended out if the remaining thickness is within allowable limits. AC 43.13-1B typically permits blending if the thickness reduction does not exceed 10%. After blending, the area must be treated with a corrosion inhibitor and primed to prevent recurrence.
Severe Corrosion: If corrosion has reduced the cross-sectional area beyond allowable limits (e.g., a 15% reduction in a stringer), a structural repair is required. This may involve a scab patch to restore strength, provided the damage is not beyond the limits defined in the document. Replacement is not always necessary if a repair can restore the structural capability.

Cracks

Cracks in metallic structures are serious findings that require immediate attention.

Cracks in Rivet Heads: Cracks radiating from rivet heads, even if confined to the head only, indicate fatigue and failure of the rivet. The rivet has lost its structural integrity and must be removed and replaced. Stop drilling or polishing is not an approved repair for cracked rivets.
Cracks in Structural Members: Cracks in primary structural parts, such as spars, longerons, or landing gear components, are generally not repairable by field methods. This is especially true for forged aluminum parts or welded steel assemblies, where the manufacturing process and heat treatment are critical to the part's strength. The safe action is to replace the part with an approved replacement. Stop-drilling is only a temporary measure for certain cracks and is not a permanent repair for primary structure.
Cracks in Skin: When a crack is found in a skin panel, the damage must be assessed against approved data. If the SRM provides no approved repair, the mechanic must use approved data such as AC 43.13-1B to develop a repair (e.g., a riveted patch). A major repair may require FAA Form 337 and approved data per 14 CFR 43.5. The first step is a proper damage assessment to determine the extent of the damage and the proper repair method.

Material Selection and Substitution

The selection of materials for aircraft repairs is critical. The replacement material must be equivalent to or better than the original in strength, fatigue characteristics, and corrosion resistance.

Material Equivalency

Material substitutions in primary structure require approval. A material that is not equivalent cannot be used without engineering data to support the substitution. For example, a 2024-T3 angle is not equivalent to a 7075-T6 extrusion in strength and fatigue characteristics. Performing a repair with non-equivalent material without approval is a violation of 14 CFR 43.2 and could compromise structural integrity.

Material Properties

Different aluminum alloys have different properties that must be considered.

2024-T3: This is a common alloy for sheet metal applications. It offers good strength, fatigue resistance, and formability.
7075-T6: This is a high-strength alloy, but it is more prone to stress corrosion cracking than 2024-T3, particularly when stressed in the short-transverse direction (e.g., at rivet holes or edges). Using it as a doubler without proper design consideration and protection (e.g., alodine and primer) can lead to cracking.

Regulatory and Data Requirements

All maintenance and repairs must be performed in accordance with approved data.

14 CFR 43.2: Maintenance must be performed using methods and materials acceptable to the Administrator.
14 CFR 43.5: Major repairs may require FAA Form 337 and approved data.
AC 43.13-1B: This advisory circular provides acceptable methods, techniques, and practices for aircraft inspection and repair. It is a primary source of guidance for mechanics when manufacturer's data is not available.
Manufacturer's Data: When manufacturer's engineering data exists (e.g., SRM), it takes precedence over generic guidance in AC 43.13-1B.

Safety Wiring

Safety wiring is a method of securing fasteners to prevent them from loosening due to vibration.

Wire Size: The standard wire size for most turnbuckles and general safety wiring applications is 0.032-inch.
Twisting Method: The wire should be twisted at 6 to 8 twists per inch. Over-twisting (e.g., 10-12 twists per inch) weakens the wire and is not acceptable. The wire should be twisted snugly but not overly tight.

Torque Checks

Proper torque is essential for bolted connections to maintain the required clamping force.

Torque Check: If a bolt rotates during a torque check before reaching the specified torque, it indicates the bolt has stretched or the threads are worn, and the bolt has lost its preload. Simply re-torquing is not acceptable because it does not restore the clamping force.
Correct Action: The correct action is to replace the bolt and washer, then torque to the specified value. Increasing the torque or using thread-locking compound is not approved and could lead to failure.

Summary of Key Principles

Approved Data: Always use the manufacturer's SRM or AC 43.13-1B as the basis for repair decisions.
Rivet Sizing: Diameter is approximately 1.5 times the thickest sheet thickness; length must be sufficient for proper shop head formation.
Edge Distance: Minimum edge distance is 2D.
Tool Selection: Use the appropriate tool (rivet gun, squeezer, curved bucking bar) for the application to ensure quality and prevent damage.
Damage Assessment: Evaluate dents, corrosion, and cracks against allowable limits before deciding on repair or replacement.
Material Substitution: Use only equivalent or better materials with engineering approval.
Regulatory Compliance: All work must comply with 14 CFR Part 43 and be performed using methods and materials acceptable to the Administrator.

Chapter 1: Metallic Structures