Sheet-metal repair principles

More than covering a hole

Aircraft sheet-metal repairs are not simply about covering a hole or making damage look acceptable. A proper structural repair must restore the aircraft's ability to carry loads safely, resist fatigue cracking, maintain corrosion protection and preserve aerodynamic smoothness. Every repair becomes part of the aircraft's structure, which is why approved repair data, not appearance alone, determines whether a repair is acceptable.

A repair should allow the structure to carry loads as intended by the original design while minimizing new stress concentrations and maintaining long-term durability.

The goal of a structural repair

A repair that merely looks good may still be structurally unacceptable. Likewise, a repair that is substantially stronger than the original structure is not necessarily desirable. When damage occurs, the objective is to restore:

• Strength
• Stiffness
• Fatigue resistance
• Corrosion protection
• Aerodynamic shape

Why stronger is not always better

Aircraft structures are designed so loads flow smoothly through the airframe. An excessively thick or rigid repair can create a local "hard spot". A successful repair restores the original structural behavior as closely as possible. A hard spot may:

• Alter load paths
• Increase local stress concentrations
• Accelerate fatigue cracking
• Transfer loads into adjacent structure

Sheet-metal fasteners

Most aircraft sheet-metal repairs use rivets as the primary fastening method. The correct rivet type, size, spacing and installation method are critical to repair integrity. Rivets provide:

• Good fatigue performance
• Light weight
• Reliable load transfer
• Simple inspection

Rivet identification

Aircraft solid rivets are identified by both material and head markings. Different alloys have different strength and installation requirements.

• 2117-T4 (AD rivets): the most commonly used aircraft repair rivet. They can be driven in the supplied condition, need no refrigeration, and offer good strength and corrosion resistance. They are frequently used in routine structural repairs.
• 2017 (D rivets): higher strength, with more difficult installation requirements. Large-diameter 2017 rivets typically require refrigeration after heat treatment to maintain drivability.
• 2024 (DD rivets): very high strength, with strict handling controls. They age-harden at room temperature, so they must be installed within specified time limits after heat treatment, kept refrigerated ("icebox rivets"), or re-treated before use.

Rivet function

Properly installed rivets distribute loads evenly across the repair. Rivets transfer loads through:

• Shear: most aircraft rivets primarily carry shear loads.
• Clamping force: rivets also clamp sheets together, helping maintain joint integrity and reduce fretting.

Edge distance

Edge distance is the distance from the center of the rivet hole to the edge of the material. Adequate edge distance prevents the material from tearing under load.

For a typical single-row rivet installation, AC 43.13-1B recommends a minimum edge distance of 2D, where D is the rivet diameter. For example, a 1/8-inch rivet needs a minimum edge distance of 1/4 inch or greater.

Why edge distance matters

Excessive edge distance may add unnecessary weight and reduce efficiency. Insufficient edge distance may cause:

• Tearing
• Bearing failure
• Reduced joint strength
• Crack initiation

Rivet spacing (pitch)

Pitch is the distance between adjacent rivets in the same row. For many single-row repairs, the minimum pitch is 3D, where D is the rivet diameter. This spacing helps distribute loads effectively without weakening the sheet through excessive hole concentration.

Multi-row patterns

Multiple-row repairs follow approved rivet patterns specified in structural repair manuals (SRM), engineering orders and AC 43.13-1B guidance. Spacing may vary depending on load direction, material thickness and structural function.

Always use the approved pattern rather than improvising.

Rivet installation quality

Proper rivet installation is as important as rivet selection. A correctly installed rivet develops full joint strength and minimizes fatigue risk. The exact acceptable dimensions are specified by maintenance data. After driving, the shop head should be:

• Wider than the original shank (AC 43.13-1B gives about 1.5D)
• Shorter than the shank protruded before driving (about 0.5D high)
• Properly formed
• Free of cracks

Common rivet defects

Defective rivets generally require removal and replacement. Examples include:

• Clinched rivets: bent during installation.
• Cracked heads: may reduce strength and initiate fatigue.
• Undersized shop heads: insufficient clamping or strength.
• Oversized shop heads: may indicate overdriving and material damage.

Blind fasteners

Solid rivets are preferred whenever both sides of the structure are accessible. Blind fasteners may be used when access is limited and approved repair data permits their use. Blind fasteners vary significantly in strength and capability, and they are not interchangeable without approval.

Fastener compatibility

Mixing fastener types improperly may create uneven load transfer, corrosion, fatigue cracking and reduced structural life. Do not substitute any of the following unless specifically authorized by approved data:

• Different materials
• Different diameters
• Different fastener types

The logic behind a doubler repair

One of the most common sheet-metal structural repairs is the doubler repair. A doubler adds reinforcement around damaged structure and restores load-carrying capability.

Step 1: Remove the damage

Cracks, corrosion or damage must be completely removed, and the damaged area is typically trimmed into a regular shape.

Avoid sharp corners. Sharp corners concentrate stress, and stress concentrations dramatically increase fatigue-crack growth. For this reason rounded corners are used and sharp internal corners are avoided. Smooth contours distribute stress more evenly.

Step 2: Install the doubler

A doubler is placed over the repaired area. Its function is to bridge the damaged section and transfer loads into the surrounding structure. The doubler is usually the same alloy as the parent structure, compatible with surrounding materials, and specified by the SRM or approved repair data. Using the wrong material may affect strength, corrosion resistance and fatigue behavior.

A doubler is often one gauge thicker than the original material. However, the structural repair manual determines the correct thickness. Always follow approved data.

Step 3: Restore the load path

The rivet pattern is designed so loads flow out of the structure, through the rivets into the doubler, and back through the rivets into the structure on the other side. Enough fasteners must be installed beyond the damaged area to transfer loads safely into undamaged structure.

This is why repairs often extend significantly past the visible damage.

Step 4: Corrosion protection

The mating surfaces between structural members are called faying surfaces. These surfaces can trap moisture and become corrosion sites. Once assembled, these surfaces are often inaccessible, so proper preparation is critical. Depending on the repair, the following may be required before final assembly:

• Primer
• Sealant
• Corrosion inhibitors
• Surface treatments

Aerodynamic considerations

External repairs must remain as smooth as practical. Poor surface contour can create drag, turbulence and local airflow disturbances. Flush-fastener repairs are often required in high-speed aerodynamic areas. Examples include:

• Wing leading edges
• Control surfaces
• Pressurized fuselage areas

Fatigue and repairs

Every repair changes the structure. New features such as fastener holes, doublers and material transitions can become fatigue initiation points. Some major repairs become permanent inspection items within the aircraft maintenance program. For this reason, repairs may include:

• Repetitive inspections
• Nondestructive testing
• Supplemental maintenance requirements

The importance of approved data

Sheet-metal repairs are not determined by technician preference. Primary sources include structural repair manuals (SRM), engineering orders, FAA-approved repair data and approved design data. The acceptability of a repair depends on compliance with this data, not merely on workmanship. Approved repair data specifies:

• Damage limits
• Repair dimensions
• Materials
• Fastener types
• Fastener spacing
• Inspection requirements

Key takeaways

• The goal of a repair is to restore strength, stiffness, fatigue life, corrosion protection and aerodynamic smoothness.
• A stronger repair is not always a better repair.
• 2117-T4 rivets can generally be installed as received, while 2017 and 2024 rivets have additional handling requirements.
• For a typical single rivet row, minimum edge distance is 2D and minimum spacing is 3D.
• Properly installed shop heads must meet approved dimensional requirements.
• Sharp repair corners are avoided because they create stress concentrations and fatigue-crack initiation sites.
• Doublers restore load paths by transferring loads around damaged structure.
• Faying surfaces must be protected against corrosion before assembly.
• Blind fasteners may only be used where approved.
• Always follow the structural repair manual or other approved repair data.

The bottom line

Sheet-metal repair is both structural engineering and practical craftsmanship. Every repair must safely restore load-carrying capability while minimizing new stress concentrations, preserving corrosion protection and maintaining aerodynamic performance. By selecting the correct materials, rivets, edge distances, spacing and doubler design, technicians ensure that a repaired structure performs as intended not only today, but throughout thousands of future flight cycles.