Rivet Spacing, Edge Distance, and Pitch Requirements

When an aviation maintenance technician drives a rivet, the work is not finished when the manufactured head is flush and the shop head is properly formed. The location of that rivet within the joint is just as important as its installation quality. Rivet spacing, edge distance, and pitch are geometric requirements that govern how forces are distributed across a riveted lap joint or splice. Get these values wrong and the surrounding sheet metal will tear, buckle, or fatigue long before its design life. Get them right and the repair carries load as efficiently as the original structure. Every AMT preparing for the FAA Airframe Knowledge Test must understand not only the numeric limits but also the engineering logic behind them.

These requirements are rooted in AC 43.13-1B, Acceptable Methods, Techniques, and Practices — Aircraft Inspection and Repair, and are reinforced by the Aviation Maintenance Technician Handbook — Airframe (FAA-H-8083-31). The values presented here reflect FAA-approved standard practice; always verify against the applicable aircraft manufacturer's structural repair manual (SRM) before performing any actual repair, because type-certificate data takes precedence.

The Three Core Geometric Terms

Before working through the numbers, it is important to be precise about definitions, because the FAA knowledge test exploits confusion between similar-sounding terms.

Edge distance (ED) — the distance from the center of a rivet hole to the nearest edge of the sheet. This is sometimes called the margin. Edge distance is measured perpendicular to the edge if the rivet row runs parallel to the edge, or simply as the shortest distance from hole center to sheet edge.
Pitch — the distance from the center of one rivet to the center of the next rivet in the same row, measured along the row. Pitch is also called rivet spacing within a row.
Transverse pitch (also called row spacing or back pitch) — the distance between rows of rivets, measured center-to-center perpendicular to the direction of loading. This is relevant when a repair uses two or more parallel rows of rivets.

Edge Distance: Why It Exists and What the Numbers Are

If a rivet is placed too close to the edge of the sheet, the thin strip of metal between the rivet hole and the edge cannot carry the shear load transferred by the rivet. Instead of the surrounding sheet deforming gradually, it tears out in a shear-out failure — a clean, sudden slot cut from the rivet hole right through the edge. This is catastrophic in a primary structure.

The acceptable minimum edge distance for aircraft sheet metal work is 2× the rivet diameter (2D), with 2.5D preferred as the optimum design value per AC 43.13-1B. So for a 3/32-inch (AN470-3) rivet, the minimum edge distance is 2 × 3/32 = 6/32 inch, or about 3/16 inch, with 2.5D (about 15/64 inch) preferred where practical. For a 1/8-inch (AN470-4) rivet, minimum ED is 2 × 1/8 = 1/4 inch, with 2.5D = 5/16 inch preferred. The maximum edge distance is generally 4× the rivet diameter (4D); beyond that, the unsupported sheet edge can flutter, buckle, or allow moisture intrusion between sheets without the joint capturing it.

When you are drilling a new hole during repair and find yourself closer than 2D to an edge, you have two acceptable solutions: relocate the rivet hole further inboard, or use the next larger rivet diameter — but only if the larger hole still clears the 2D edge distance requirement and does not encroach on adjacent rivets.

Pitch: Spacing Rivets Along the Row

Pitch controls two competing failure modes. Rivets placed too close together weaken the sheet in a different way: the holes remove so much cross-sectional area that the sheet itself splits between the rivets in a net-section tension failure. Rivets placed too far apart allow the sheets to separate, breathe under cyclic loading, and admit corrosion; the unsupported span also becomes prone to inter-rivet buckling under compression loads.

Standard practice from AC 43.13-1B specifies:

Minimum pitch: 3D — the center-to-center distance must be at least three rivet diameters. For a 1/8-inch rivet, minimum pitch = 3 × 1/8 = 3/8 inch.
Maximum pitch: 6D to 8D for most structural repairs (the specific allowable depends on sheet thickness and loading). In non-structural or lightly loaded skin repairs, 10D is sometimes permitted, but always verify with the SRM.
Typical or nominal pitch: 6D — the most commonly used value in general structural skin repairs, giving adequate strength while minimizing the number of fasteners and associated hole stress concentrations.

To estimate the number of rivets needed in a row for a given splice length, divide the splice length by the chosen pitch and round to the nearest practical whole number of spaces, then add one rivet for the end position — keeping in mind this is a general layout approach rather than a fixed AC 43.13-1B formula, and actual rivet count should be verified against the applicable structural repair manual. For example, a 3-inch splice using 6D pitch with 1/8-inch rivets: pitch = 6 × 1/8 = 3/4 inch; 3 ÷ 0.75 = 4 spaces, plus one end rivet = 5 rivets.

Transverse Pitch: Row-to-Row Spacing

When a repair requires two or more rows of rivets (which is common in doublers and splice plates), the rows themselves must be spaced appropriately. The standard minimum transverse pitch is 2.5D and is commonly set at 6D for standard structural repairs to match the longitudinal pitch. Rows that are too close together create overlapping stress fields around adjacent holes, compounding the net-section loss. Rows that are too far apart add unnecessary weight and material without proportional strength benefit.

Multiple rows are also often staggered — the rivet centers of one row are offset halfway between the centers of the adjacent row. Staggering improves load distribution and prevents a single straight line of holes from acting as a tear-initiating perforation across the sheet.

Calculating a Repair Layout: A Practical Walkthrough

Suppose you are repairing a crack in 0.040-inch 2024-T3 aluminum skin and you select AN470AD4 rivets (1/8-inch diameter, 2117-T4 alloy, universal head). The repair doubler will use two rows of rivets on each side of the damage.

Edge distance: 2D = 2 × 1/8 = 1/4 inch minimum from the edge of the doubler to the center of the first rivet row, with 2.5D = 5/16 inch preferred. Maximum is 4D = 1/2 inch.
Pitch along each row: Use 6D = 6 × 1/8 = 3/4 inch center-to-center between rivets in each row.
Transverse pitch between the two rows: Use 6D = 3/4 inch, or stagger with a 3/8-inch offset so each rivet in the inner row sits halfway between two rivets in the outer row.
Verify net section: With 1/8-inch holes on 3/4-inch pitch, approximately 17% of the sheet width at each cross-section is removed — well within acceptable limits for this alloy and thickness.

Why These Numbers Matter for Airworthiness

These are not arbitrary style points. Stress concentrations radiate outward from every rivet hole. Proper spacing ensures those stress fields do not overlap and amplify one another. Correct edge distance prevents shear-out, which is a low-energy, sudden failure mode — exactly the type that gives no warning before structural separation. On pressurized aircraft, inter-rivet buckling of improperly spaced skin panels under hoop stress is a known fatigue initiation mechanism responsible for several accident investigations. The FAA and airframe manufacturers studied decades of in-service experience to arrive at the 2D (minimum) / 2.5D (preferred) / 3D / 6D framework — treating it as approximate or optional is a path toward unsafe airworthiness.

Key Numbers and Rules

Minimum edge distance: 2D (2× rivet diameter), measured from hole center to sheet edge; 2.5D preferred/optimum.
Maximum edge distance: 4D.
Minimum pitch (rivet spacing in row): 3D.
Typical/nominal pitch: 6D (most commonly used in structural repairs).
Maximum pitch: 6D–8D structural; up to 10D in lightly loaded applications (verify with SRM).
Minimum transverse pitch (row spacing): 2.5D; commonly 6D in practice.
Stagger offset: one-half of the longitudinal pitch when rows are staggered.
All measurements use the nominal rivet shank diameter, not the hole diameter or head diameter.

Common Test Traps

Confusing pitch with edge distance. The FAA knowledge test frequently asks for edge distance when describing the distance to the sheet edge, and pitch when asking about spacing between rivets. Keep the definitions airtight: ED = hole center to sheet edge; pitch = hole center to hole center along the row.
Measuring from the edge of the hole, not the center. Both edge distance and pitch are always measured center-to-center (or center-to-edge). Measuring from the hole wall instead of the hole center will give a value that is approximately one rivet radius short — a real-world mistake that violates minimums.
Using rivet head diameter instead of shank diameter. The D in 2D, 3D, and 6D always refers to the shank (nominal) diameter of the rivet, not its manufactured head. AN470AD4 shank = 1/8 inch, not the 0.225-inch head diameter.
Assuming maximum pitch is always 6D. The test sometimes presents a scenario with a lightly loaded fairing or fillet strip and asks what the maximum pitch is. Know that 6D is the common structural value and that higher values (up to 10D) are permitted only in specific, non-structural contexts.
Forgetting that AC 43.13-1B is a default, not an override. The SRM or type-certificate data sheet for a specific aircraft may specify tighter or different values. AC 43.13-1B applies when no other approved data exists. On the test, problems usually assume AC 43.13-1B standards unless stated otherwise.