Sheet metal repairs are among the most common structural tasks an Aviation Maintenance Technician (AMT) performs on metallic airframes. When skin, bulkheads, stringers, or other sheet metal components are damaged by corrosion, impact, or fatigue cracking, the FAA requires that any repair restore the original structural strength of the part. Designing a patch and laying out its rivet pattern correctly is not guesswork — it is a systematic engineering process governed by material selection rules, edge-distance requirements, rivet spacing formulas, and strength calculations. Understanding these principles thoroughly prepares you both for the AMT Airframe knowledge test and for producing airworthy repairs in the hangar.
The primary regulatory and technical guidance for sheet metal repairs comes from FAA Advisory Circular 43.13-1B, which provides accepted methods, techniques, and practices for aircraft inspection and repair. The Aviation Maintenance Technician Handbook — Airframe (FAA-H-8083-31) expands on these principles in detail. Together they form the backbone of every approved sheet metal repair.
Fundamentals of Patch Design
The overriding goal of any patch design is to restore at least 100% of the original structural strength of the damaged area. This means the repaired section must be able to carry the same loads — tension, compression, and shear — that the original undamaged structure could carry. The patch accomplishes this by transferring loads from the damaged skin into the patch material through the rows of rivets that fasten them together.
Material Selection
The patch material must match or exceed the strength characteristics of the original skin. In practice, this usually means using the same alloy and temper (for example, 2024-T3 clad aluminum for a 2024-T3 skin). If the original material specification is unknown, the technician must research the aircraft's structural repair manual (SRM) or obtain engineering data. The patch thickness must be selected carefully: if you use the same alloy and the same thickness, the patch must be large enough to accommodate a rivet pattern that develops full strength. Alternatively, if thinner material is used, it is generally not acceptable because a thinner patch would not restore original strength. If thicker material of the same alloy is used, the technician must verify that the added stiffness does not create stress concentrations at the patch edges.
Patch Shape and Sizing
Patches are typically cut in a rectangular or circular shape with rounded corners. Sharp corners concentrate stress and can initiate new cracks; rounding corners to a minimum radius (often ½ inch or greater as recommended in AC 43.13-1B) distributes load more evenly. The patch must extend far enough beyond the damage to accommodate the required number of rivet rows on each side. A good rule of thumb is that the patch must provide enough material to develop the shear strength of all rivets in the pattern, which requires adequate edge distance on every side.
For cracks, the damaged area must first be stop-drilled — a small hole drilled at each end of the crack to arrest further propagation — before the patch is applied. The patch must extend well beyond these stop-drill holes.
Rivet Pattern Layout
The rivet pattern is the heart of the repair design. It determines how load is transferred from the skin to the patch and back. Three critical dimensions define any rivet pattern: edge distance, rivet pitch (spacing), and row spacing.
Edge Distance
Edge distance is measured from the center of a rivet hole to the nearest edge of the sheet material — either the patch edge or the original skin edge. FAA guidance in AC 43.13-1B specifies a minimum edge distance of 2D (two times the rivet shank diameter) and a recommended edge distance of 2½D to 3D for most applications. This minimum prevents the material from tearing or cracking toward the edge under load. For example, using an AN470 rivet with a 3/32-inch shank (diameter = 0.094 inch), the minimum edge distance would be approximately 3/16 inch (2D), and the preferred distance would be closer to ¼ inch (2.5D). Always measure edge distance to the center of the hole, not the edge of the hole.
Rivet Pitch (Spacing Between Rivets in a Row)
Rivet pitch is the center-to-center distance between adjacent rivets in the same row, measured along the row. The standard acceptable range is 3D minimum to 12D maximum, with 6D to 8D being the most common practical range for structural skin repairs. A pitch that is too small risks cracking the material between rivets (a condition sometimes called rivet-to-rivet cracking). A pitch that is too large fails to develop the required joint strength because each rivet must carry too much load. For a 1/8-inch rivet, a 6D pitch equals ¾ inch.
Row Spacing
When a repair requires multiple rows of rivets (which is the norm for structural patches), the rows must be staggered and properly spaced. Standard row spacing is typically 75% of the rivet pitch, and rivets in adjacent rows are staggered by half the pitch so that they fall between the rivets in the neighboring row. Staggering reduces the likelihood that a line of holes will create a continuous weak plane through the sheet material.
Calculating Repair Strength
The strength of a riveted joint depends on two potential failure modes: rivet shear failure (the rivet shank shears off) and sheet bearing failure (the sheet material crushes around the rivet hole). The repair must be stronger than both. AC 43.13-1B provides shear strength tables for common rivet types and sizes. For example, a 2117-T3 aluminum alloy rivet (AN470AD) with a 3/32-inch shank has a single-shear strength of approximately 388 pounds. By multiplying this value by the total number of rivets on one side of the repair (one shear plane), the technician can compute total joint shear strength and compare it to the load-carrying requirement of the original undamaged skin section (skin thickness multiplied by the material's shear allowable stress multiplied by the section width).
This comparison — joint efficiency — should equal or exceed 100% for a full-strength repair. Many standard repairs documented in the SRM or AC 43.13-1B are pre-engineered to achieve this, which is why following the approved data exactly is so important.
Why It Matters
An improperly designed patch or poorly laid-out rivet pattern can result in a repair that appears sound on the surface but is structurally deficient. Under flight loads, an under-strength repair can fail progressively — rivets pull through the sheet, cracks propagate from misplaced holes, or the patch debonds at its edges. Any of these failure modes can lead to catastrophic structural failure, particularly in pressurized aircraft or high-stress primary structure areas. The FAA requires that repairs to primary structure be documented with approved data (SRM, AC 43.13-1B, or an FAA-accepted engineering order) and recorded in the aircraft maintenance records under 14 CFR Part 43.
Key Numbers and Rules
- Minimum edge distance: 2D (rivet shank diameter); preferred 2½D to 3D
- Minimum rivet pitch: 3D; maximum rivet pitch: 12D; practical range 6D–8D
- Row spacing: approximately 75% of the rivet pitch; rows staggered by half-pitch
- Patch corner radius: minimum ½ inch to prevent stress concentration
- Repair strength target: 100% of original structural strength
- Stop-drill holes: required at the ends of any crack before patching
- Regulatory basis: 14 CFR Part 43 requires approved data for structural repairs
- Common rivet for skin repairs: AN470AD (universal head) or AN426AD (countersunk/flush head) in 2117-T3 alloy
Common Test Traps
- Edge distance measured incorrectly: Edge distance is always measured from the center of the rivet hole to the sheet edge — not from the edge of the drilled hole. Confusing these two will produce under-strength edge distances that fail in bearing.
- Using thinner patch material: A patch must never be thinner than the original skin without engineering justification. Many students incorrectly assume that any same-alloy material is acceptable regardless of gauge.
- Forgetting to stop-drill cracks: Applying a patch over a crack without stop-drilling the ends first allows the crack to continue propagating under the patch, invalidating the repair from the start.
- Confusing pitch with row spacing: Pitch is the spacing within a row; row spacing is the distance between rows. These are different dimensions with different formulas, and mixing them up leads to incorrect pattern layouts on the knowledge test.
- Assuming AC 43.13-1B covers everything: While AC 43.13-1B is the go-to accepted reference, the aircraft's own Structural Repair Manual takes precedence for type-certificated aircraft. If an SRM repair procedure conflicts with AC 43.13-1B, the SRM governs.