Aircraft metallic structure repair is a critical aspect of maintaining aircraft airworthiness and safety. This process involves restoring lost strength and rigidity, ensuring the aerodynamic shape remains unchanged, and adhering to strict aviation standards. Understanding the fundamental techniques, materials, and tools used in these repairs is essential for aviation mechanics and students alike. This guide delves into the specifics of sheet metal work, riveting, and various welding processes used in the field.

Essential Requirements for Aircraft Metallic Structure Repair

Any repair made to a sheet metal aircraft structure must fulfill three primary requirements to ensure safety and structural integrity. These requirements are non-negotiable and form the foundation of all repair procedures.

Restore Lost Strength: The repaired section must be able to withstand the same loads as the original structure.
Restore Lost Rigidity: The repair must prevent unwanted flexing or movement, maintaining the component's original stiffness.
Maintain Aerodynamic Shape: The repair must not alter the external shape of the part, which could negatively impact the aircraft's aerodynamic performance.

When making a riveted repair, it's crucial that the bearing strength of the metal sheet is slightly greater than the shear strength of the rivet. This ensures that the rivet will shear before the sheet material deforms excessively, preventing critical damage to the structure.

Fasteners and Riveting Techniques in Aircraft Metallic Repair

Rivets are the most common fasteners in aircraft metallic structures. Proper selection and installation are paramount for structural integrity. Various types of fasteners and specific techniques are employed.

Rivet Identification and Selection

Most rivets used in aircraft construction have dimples on the head for identification. These marks indicate the specific aluminum alloy used in their manufacture. The general rule for determining the proper rivet diameter is three times the thickness of the thickest sheet being joined.

Universal head rivets can often replace round head rivets in aircraft structures.
Aluminum alloy rivets should be driven with as few blows as possible to prevent work-hardening, which makes them difficult to drive effectively.

Flush Riveting: Dimpling vs. Countersinking

For flush rivets, which provide a smooth aerodynamic surface, the method of recessing the rivet head depends on the material thickness:

Dimpling is used when the sheet thickness is less than the thickness of the rivet head. This process forms a conical depression in the sheet.
Countersinking is performed when the sheet thickness is greater than the rivet head's thickness. This involves machining a conical hole to accept the rivet head.
Certain aluminum alloys (like 7075-T6, 2024-T81) and magnesium alloys require hot-dimpling due to their material properties.

Rivet Installation and Edge Distance

When installing rivets, the minimum edge distance allowed is two times the diameter of the rivet. This prevents the rivet from tearing out of the sheet. For a riveted two-row splice, the recommended transverse pitch is three-fourths of the pitch of the rivets in the rows.

Temporary and Specialized Fasteners

Cleco fasteners are patented fasteners inserted into rivet holes to temporarily hold two pieces of sheet metal together until they can be riveted. They are widely used for assembly and alignment.
HI-LOK fasteners offer a significant advantage: they can be installed without requiring special rivet tools, simplifying certain assembly processes.

Sheet Metal Fabrication and Forming

Aircraft metallic structure repair often involves fabricating new sheet metal parts or reforming existing ones. Various tools and techniques are used to achieve the precise shapes required.

Cutting and Shaping Tools

Throatless shears are essential for cutting mild carbon steel up to 10-gauge and stainless steel up to 12-gauge. Their design allows them to cut irregular curves in metal sheets, making them versatile for complex shapes.
Slip roll formers are used to create simple curves with a large radius in sheet metal.
Bumping is a metal forming technique used to create compound curves in sheet metal.

Precision Bending with a Cornice Brake

A cornice brake is used for making precise bends. Several key terms relate to this process:

Sight line: This line allows the jaws of the cornice brake to be positioned so that the bend will start exactly at the bend tangent line.
Bend allowance: This is the amount of metal that is consumed or