When a section of aircraft sheet metal skin, spar web, or structural panel is damaged beyond simple straightening, the approved repair method almost always involves either a splice plate or a doubler. Both devices restore the original load-carrying cross-section of the damaged member, but they do so in slightly different ways and are chosen based on the nature and location of the damage. A thorough understanding of how these repairs are designed, sized, and installed is one of the most heavily tested areas on the FAA Aviation Mechanic Airframe Knowledge Test and is foundational to safe structural maintenance practice.
The governing reference for these repairs is the FAA's Aviation Maintenance Technician Handbook — Airframe (FAA-H-8083-31), along with aircraft manufacturer's structural repair manuals (SRMs). Where the SRM provides specific repair data, it always takes precedence over general handbook guidance. General aviation aircraft without a specific SRM repair procedure may be repaired using the methods described in AC 43.13-1B, which is accepted as an approved data source for repairs that restore original strength.
How Splice Plates and Doublers Work
A doubler is a patch of sheet metal placed directly over (or behind) a damaged area without removing the original damaged material, provided the damage is contained and the remaining structure is still sound. The doubler overlaps the damage on all sides and is riveted through both the doubler and the original skin. In effect, it adds a second layer of material that carries the load around the weakened zone. Doublers are commonly used for small cracks, holes, or corrosion-thinned areas where the surrounding skin is still structurally acceptable.
A splice plate, by contrast, is used when a section of structural material must be cut out entirely and replaced. The splice plate bridges the gap between the two remaining ends of the original material, overlapping each end by a calculated distance and being riveted to both sides of the cutout. Splice plates are typically required for larger areas of damage, for damage that extends along a spar cap or stringer, or when the original material is so badly deformed or corroded that removal is necessary. Think of a splice plate as a structural bridge across a gap.
Load Path and Shear Transfer
The critical design principle for both types of repair is shear transfer through the rivet rows. The load that was once carried continuously through the original sheet metal must now be transferred into the splice plate or doubler through the rivets, carried across the patch, and transferred back out through more rivets. This means the rivets must be sized and numerous enough to carry the full design load of the original member. Engineers refer to the load transferred per rivet row as the shear flow, and each rivet must not exceed its allowable shear load. For general aviation repairs, AC 43.13-1B provides general guidance and example rivet spacing and row data for common alloys and thicknesses, but it does not eliminate the need for sound engineering judgment or shear-flow analysis on more complex or critical repairs; it is advisory and applies mainly to non-critical, non-pressurized structure.
Design Principles and Material Selection
Both splice plates and doublers must meet several fundamental design requirements to be airworthy:
- Material match: The repair material must be the same alloy and temper as the original — or an approved substitute of equal or greater strength — and of the same or slightly greater thickness. A common rule of thumb from AC 43.13-1B is that the repair piece should be the next standard gauge thicker than the original if the exact thickness is unavailable. Using a significantly heavier gauge adds unnecessary weight; using a lighter gauge is never acceptable.
- Overlap distance: The splice plate or doubler must extend far enough beyond the damaged area to provide the required number of rivet rows. A minimum of two rows of rivets on each side of the damage is almost always required; three or more rows may be needed for thicker skins or higher-stressed members. Each rivet row must be spaced at least the minimum edge distance from the end of the plate.
- Edge distance: The distance from the center of any rivet hole to the nearest edge of the sheet should generally be at least 2× to 2.5× the rivet diameter, per AC 43.13-1B guidance. Less than this causes edge tearout under load, a catastrophic failure mode.
- Rivet spacing (pitch): Along each row, rivets are typically spaced between 4× and 12× the rivet diameter apart. The minimum spacing of 4D prevents inter-rivet buckling and bearing failure; the maximum of 12D (or roughly 4× diameter for pressure vessels and critical skins) prevents skin buckling between rivets.
- Transverse row spacing: When multiple rows of rivets are used, rows should be staggered and spaced at least 4× to 6× the rivet diameter apart, measured perpendicular to the load direction. Staggering reduces the chance of the sheet splitting along a single line of holes.
- Rivet diameter selection: A common guideline is that rivet diameter should be approximately 3× the thickness of the thinnest sheet being joined, rounded to the nearest standard size. The most common structural rivet diameters in general aviation are 3/32", 1/8", 5/32", and 3/16".
The Cutout Shape and Damage Removal
Before a splice plate can be installed, the damaged area must be trimmed to a clean, smooth-edged shape. Rectangular cutouts with sharp corners create stress concentrations — points where the local stress in the skin rises dramatically above the average. For this reason, all corners of a cutout must be given a generous radius, typically a minimum of 1/2 inch for most general aviation skin repairs (check the SRM or AC 43.13-1B for specific minimums). The smooth radius distributes stress evenly around the cutout perimeter instead of concentrating it at a corner, where a fatigue crack could rapidly propagate.
When trimming the damaged skin, the technician should also drill a stop-drill hole at the tip of any existing crack before cutting. The stop-drill — a small hole drilled at the very end of the crack — blunts the crack tip and arrests further propagation while the repair is prepared. This is a critical step that is frequently tested on the AMT exam.
Why Splice Plate and Doubler Design Matters
Aircraft structure is designed to specific load factors — 3.8g being a commonly cited limit load factor for normal category airplanes under certain Part 23 provisions, though the actual factor varies with category and weight — and every structural element carries a share of those loads in a carefully planned load path. An improperly designed repair interrupts that load path. Too few rivets and the repair fails in shear. Too small an overlap and the plate peels away under bending. Wrong material and the repair either corrodes galvanically or yields before the design limit load is reached. In the worst case, a deficient structural repair fails in flight, potentially leading to loss of control or structural breakup. This is why AC 43.13-1B and aircraft SRMs are so carefully followed — every dimension in those documents has an engineering basis.
Beyond ultimate strength, fatigue is a major consideration. Every rivet hole is a stress concentration, and in a pressurized aircraft or a wing skin that flexes thousands of times per flight cycle, crack initiation at rivet holes is the primary fatigue failure mechanism. Correct edge distance, proper rivet installation (no loose or overdriven rivets), and adequate overlap length all work together to maximize the fatigue life of the repaired structure.
Key Numbers and Rules
- Minimum edge distance: 2× to 2.5× rivet diameter from hole center to sheet edge.
- Minimum rivet pitch (spacing along row): 4× rivet diameter.
- Maximum rivet pitch for non-pressurized skins: 12× rivet diameter.
- Minimum transverse row spacing: 4× to 6× rivet diameter.
- Recommended rivet diameter: approximately 3× the thickness of the thickest joined sheet.
- Minimum rows per side of damage: 2 rows (3 or more for thicker/higher-stressed members).
- Minimum corner radius for cutouts: typically 1/2 inch (verify per SRM or AC 43.13-1B).
- Repair material thickness: same as original or one standard gauge heavier if exact match unavailable — never lighter.
- Stop-drill crack tips before trimming the cutout.
Memory Aid
For rivet layout, remember the phrase "2 Edge, 4 Pitch, 4-6 Row" — Edge distance is about 2D-2.5D, Pitch (along-row spacing) is about 4D minimum, and transverse Row spacing is about 4D-6D. The numbers increase as you move from edge to row, making them relatively easy to keep in order during an exam.
Common Test Traps
- Confusing doubler vs. splice plate: A doubler goes over existing (unsectioned) skin; a splice plate bridges a gap where material has been removed. The FAA exam distinguishes these clearly.
- Forgetting edge distance on the patch itself: The 2D edge distance applies to the patch plate's edges, not just the original skin. Students often calculate rivet rows correctly but place them too close to the edge of the new doubler.
- Using lighter-gauge material to "save weight": This is never acceptable. Repair material must match or exceed the original thickness. The exam may offer this as a tempting distractor.
- Sharp cutout corners: A question may ask what is wrong with a repair sketch that shows a rectangular cutout with 90-degree corners. The correct answer is that sharp corners create stress concentrations; rounded corners with a minimum radius are required.
- Skipping the stop-drill: Questions about crack repairs often include a distractor answer that proceeds straight to trimming without stop-drilling. Always stop-drill the crack tip first.