Understanding aviation airframe metallic structure repair is crucial for aviation mechanics, ensuring aircraft safety and longevity. This guide distills essential knowledge on repairing metallic airframe components, focusing on restoring structural integrity and aerodynamic form. We'll explore rivet types, sheet metal forming, welding techniques, and critical repair principles.

Essential Requirements for Aviation Airframe Metallic Structure Repair

Any repair to an aircraft's sheet metal structure must meet three fundamental requirements to ensure the aircraft remains airworthy. These principles guide every repair decision, from material selection to the final execution.

Structural Integrity: Strength and Rigidity Restoration

The primary goal of any repair is to restore the lost strength and rigidity of the damaged part. This involves careful calculation and selection of materials and fasteners to match or exceed the original structural properties.

When making a riveted repair, the strength of the components is critical. The bearing strength of the metal sheet should be greater than the shear strength of the rivet, though they should be close to each other, to ensure the sheet can withstand the load transferred by the rivet without deformation.

Maintaining Aerodynamic Shape

Crucially, the repair must not change the aerodynamic shape of the part. Even minor alterations can affect airflow, potentially compromising performance and stability. Precision in forming and fitting replacement panels is paramount.

Fasteners and Riveting Techniques in Airframe Repair

Rivets are a cornerstone of metallic airframe construction and repair. Proper selection and installation are vital for a strong, durable repair.

Rivet Selection and Identification

Rivet Diameter: The general rule for determining the proper rivet diameter is three times the thickness of the thickest sheet being joined.
Minimum Edge Distance: When installing rivets, the minimum edge distance allowed is two times the diameter of the rivet.
Rivet Identification: Most rivets used in aircraft construction have dimples on the head. These identifying marks indicate the specific alloy used in the rivet's manufacture, which is crucial for material compatibility.
Replacement Rivets: A universal head rivet may be used to replace a round head rivet in an aircraft structure.
Driving Rivets: Aluminum alloy rivets should be driven with as few blows as possible. Excessive hammering will work-harden the rivets, making them difficult to drive and potentially weakening them.

Flush Riveting: Dimpling vs. Countersinking

For flush rivets, which provide a smooth aerodynamic surface, the method of creating the recess depends on the material thickness:

Dimpling: This method forms a recess by deforming the sheet metal. It's used when the sheet's thickness is less than the thickness of the rivet head.
Countersinking: This method involves removing material to create a conical recess. It should only be done when the thickness of the sheet is greater than the thickness of the rivet head.
Hot-Dimpling: Certain alloys, such as 7075-T6, 2024-T81 aluminum alloys, and magnesium alloys, require hot-dimpling due to their material properties.

Temporary and Specialty Fasteners

Cleco Fasteners: These are patented fasteners inserted into rivet holes to hold two pieces of sheet metal together until permanent rivets can be installed.
HI-LOK Fasteners: The main advantage of HI-LOK fasteners is that they can be installed without using special rivet tools, simplifying assembly and repair processes.

Riveted Splice Design

When making a two-row splice in sheet metal, the recommended transverse pitch (distance between rivet rows) to use is three-fourths of the pitch of the rivets in the rows.

Sheet Metal Forming Techniques for Airframe Repair

Repairing metallic structures often requires forming new sheet metal parts to match existing contours. Various tools and techniques are used to achieve the desired shapes.

Basic Bending and Curving

Cornice Brake: This tool is used for making straight bends. The sight line on the sheet metal allows the jaws of a cornice brake to be positioned so the bend will start at the bend tangent line.
Bend Allowance: This refers to the amount of metal that is used in making a bend in a piece of sheet metal.
Minimum Bend Radius: This is determined by the thickness of the material and its hardness.
Setback: When bending sheet metal, setback is the distance the jaws of the brake must be set back from the mold line to form the bend.
Slip Roll Former: This machine is used to create simple curves with a large radius.

Advanced Forming and Rigidity

Bumping: This technique is used to create compound curves in sheet metal.
Curving Angles: To curve the flanges of an angle, they must be stretched for a convex curve and shrunk for a concave curve.
Flanged Lightening Holes: Lightening holes in a sheet metal wing rib are flanged to give the rib rigidity, increasing its strength-to-weight ratio.
Hand-Forming Concave Curves: When hand-forming a piece of sheet metal that has a concave curve, the forming should start at the edges and work toward the center to prevent buckling and achieve a smooth curve.
Joggle: A joggle is a small offset near the edge of a piece of sheet metal that allows it to overlap with another piece of metal while maintaining a flush surface.

Drilling Stainless Steel

When drilling stainless steel, the drill should be turned slowly.
The drill should have a 140° angle included when drilling through stainless steel.

Welding Processes in Airframe Repair

Welding is another critical method for joining metallic aircraft structures, though specific materials and applications dictate its use.

Gas Tungsten Arc (GTA/TIG) Welding

Inert Gas Function: A stream of inert gas (like argon or helium) is used in GTA welding to prevent the formation of oxides in the puddle. This shielding ensures a clean, strong weld by keeping oxygen away.
TIG vs. Oxyacetylene: TIG welding is preferred over oxyacetylene welding for building and repairing welded steel tube aircraft structures because the heat is concentrated in the weld and does not cause as much distortion as gas welding.

Oxy-Acetylene Welding Considerations

Acetylene Pressure: The pressure of the gas in an acetylene cylinder must be kept low because acetylene gas becomes unstable when kept under pressure of more than about 15 psi.
Flame Types: The appearance of an oxy-acetylene torch flame indicates its type:
Oxidizing flame: Has a pointed inner cone, and the torch makes a hissing noise.
Neutral flame: Has a rounded inner cone, with no feathers around it.
Reducing flame: Has a definite feather around the inner cone.
Soft Flame: A soft flame is one made when the pressures of the gases are low enough that the flame does not make a noise and does not blow the puddle.
Welding Aluminum: A soft, neutral oxy-hydrogen flame is recommended when gas welding aluminum.

General Welding Principles

Butt Weld Penetration: When welding two pieces of steel with a butt weld, the bead should penetrate the material to achieve 100 percent penetration for maximum strength.
Tack Welding: This involves using small, welded spots to hold the material together until the final, continuous bead can be run.
Preheating Thick Metal: Thick plates of metal must be preheated before welding to control the expansion and contraction of the metal, minimizing stresses caused by the welding process.
Flux Removal: All traces of welding flux must be removed after welding aluminum or magnesium because welding flux is corrosive, and its residue can lead to corrosion of the metal.
Brazing vs. Welding: In brazing, the base metal is not melted, but is joined by a low-melting-point alloy. In welding, the base metal itself is melted and fused.
Heat-Treated Steel: A heat-treated steel part is normally not repairable by welding, as welding destroys the original heat treatment, altering its mechanical properties.
Electrical Solder: 60/40 resin-core solder is recommended for soldering electrical wires due to its properties for electronic connections.

Material Protection and Surface Treatments

Protecting metallic aircraft components from corrosion and wear is as important as the structural repair itself.

Anodizing Aluminum Alloys

Anodizing is a protective oxide film deposited on the surface of aluminum alloy by an electrolytic process. Its purpose is to enhance corrosion resistance and provide a durable surface finish.

FAQ: Common Questions on Airframe Metallic Structure Repair

What are the three essential requirements for any sheet metal aircraft repair?

The repair must restore the lost strength, restore rigidity, and it must not change the aerodynamic shape of the part.

Why is a soft flame important when gas welding?

A soft flame, produced with low gas pressures, prevents the flame from making noise and blowing the weld puddle, which is crucial for controlled and quality welds.

What determines whether a flush rivet installation requires dimpling or countersinking?

The thickness of the sheet determines the method. Dimpling is used when the sheet is thinner than the rivet head, while countersinking is for sheets thicker than the rivet head.

Why must all welding flux be thoroughly removed after welding aluminum or magnesium?

Welding flux is highly corrosive. If not completely removed, it will cause the metal to corrode over time, compromising the repair's integrity and the part's lifespan.

What is a joggle in sheet metal and why is it used?

A joggle is a small offset near the edge of a piece of sheet metal. It's used to allow one sheet to overlap with another piece of metal while maintaining a relatively flush outer surface, aiding in structural continuity and aerodynamic smoothness.

Aviation Airframe Metallic Structure Repair