9.2 Riveted Repair Design: Rivet Alloys, Substitution, Edge Distance & Spacing

9.2 Riveted Repair Design: Rivet Alloys, Substitution, Edge Distance & Spacing

[!IMPORTANT] The inspector's question is never "does it look neat?" It is: does this joint carry the load the original carried? A repair can be beautifully driven, perfectly flush, and completely rejectable because the mechanic changed the rivet alloy, shortened the edge distance, or substituted blind rivets where the approved data called for solid ones. Rivet questions on the IAR test are almost always substitution questions in disguise.

The governing guidance is AC 43.13-1B, Chapter 4, Section 4, paragraph 4-57 (Riveting), with the hardware identification tables in Chapter 7, Section 11. Design allowables for riveted assemblies are specified in MIL-HDBK-5.

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Rivet Identification and Relative Strength

Solid shank rivets are identified by material, head type, shank diameter, and temper condition. The alloy is the part that governs joint strength, and it is marked on the rivet head:

Two facts drive nearly every rivet question on the exam:

1. 2117 (AD) is weaker in shear than 2017 (D), which is weaker than 2024 (DD).
2. 2117 can be driven as received; 2017 and 2024 cannot. AC 43.13-1B notes specifically that 2117 rivets may be driven in the condition received, which is exactly why shops reach for them.

That combination creates a standing temptation: swap the AD rivet in for the D rivet because it is on the shelf and does not need heat treating. It is also exactly the substitution the exam asks you to reject.

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The Substitution Rule

An inspector is presented with a repair in which a row of 2017-T rivets installed by the manufacturer was replaced with the same number of 2117-T rivets of the same diameter. Approve or reject?

Reject. Same diameter and same count of a lower-strength alloy produces a joint with less shear capacity than the original — which fails the § 43.13(b) standard that the article be restored to a condition at least equal to its original or properly altered condition with regard to structural strength. The fact that 2117 is the recommended general substitute for 2017 in new design work does not license a one-for-one downgrade of an existing structure.

The industry practice that makes the substitution workable is to go up one rivet diameter when substituting AD for D — but that is a design change, and it has consequences the inspector must then check:

• A larger rivet requires a larger hole, which reduces the remaining net section of the sheet.
• A larger rivet requires greater edge distance (edge distance scales with diameter), which may not exist on the existing part.
• A larger rivet requires greater spacing, which may not fit the existing pattern.

So the honest answer to "may I substitute?" is usually: not without data that shows the substituted pattern still meets edge distance, spacing, and net-section requirements.

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Edge Distance and Spacing

AC 43.13-1B paragraph 4-57(c) defines the two terms precisely, and the definitions are testable on their own:

• Edge distance is the distance from the center of the rivet hole to the nearest edge of the sheet.
• Rivet spacing (pitch) is the distance from the center of one rivet hole to the center of the adjacent rivet hole.

Both are measured center-to-center or center-to-edge, never edge-of-hole to edge-of-sheet.

The Primary Rule: Duplicate the Original

The AC states the governing principle before it gives any numbers: unless structural deficiencies are suspected, the rivet spacing and edge distance should duplicate those of the original aircraft structure. The original design was substantiated; your job on a repair is to reproduce it, not to improve on it. An inspector who finds a repair laid out to generic minimums on a structure whose original pattern was tighter should ask why.

The Fallback Minimums

Only if structural deficiencies are suspected does the AC supply generic minimums, and for the simplest case they are:

Note carefully that the 2D/3D pair applies to single-row layouts. Multi-row patterns send you to figure 4-5 rather than to a memorized number — an answer choice that applies 2D/3D to a triple-row pattern is quietly wrong.

Why the Minimums Exist

• Too little edge distance and the rivet tears out through the edge of the sheet in shear, taking the joint with it.
• Too much edge distance and the sheet can buckle or curl between the fastener line and the edge, so bigger is not automatically safer.
• Too little spacing and the holes interact, cutting the net section of the sheet to the point where the sheet fails before the rivets do.

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Rivet Diameter Selection and Hole Fit

The IAR test asks diameter questions in the reverse direction — given a hole, name the fastener. AN bolt part numbers express shank diameter in sixteenths of an inch (solid rivets use thirty-seconds, so an AD4 rivet is 1/8 inch — do not mix the two scales):

A bolt pattern drilled to 0.3125 inch therefore takes AN5 hardware. Reading the decimal back to sixteenths is the whole trick: multiply by 16 and the AN number appears (0.3125 × 16 = 5).

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Blind Rivets Where Solid Rivets Were Specified

A common inspection scenario: an aileron major repair was accomplished using previously approved manufacturer structural repair manual data, but the mechanic substituted blind rivets for the AN427 solid rivets the data called out.

The reasoning an IA must apply:

1. The repair was accomplished to approved data, and the approved data specified solid rivets.
2. A blind rivet is not equivalent to a solid rivet of the same nominal diameter — it has different shear and bearing allowables, different hole-filling behavior, and different fatigue characteristics.
3. Therefore the installed configuration does not conform to the approved data, and the IA cannot certify conformity under § 65.95(a)(1).
4. AC 43.13-1B cannot cure this. It is acceptable data, not approved data (see Section 5.1). Citing a chapter of the AC does not convert an unapproved substitution into an approved one.

The correct disposition is to require additional approval of the substitution — through the design approval holder, a DER-approved Form 8110-3, or a field approval — before the repair can be approved for return to service.

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What the IA Checks on a Riveted Repair

[ ] Alloy: does the head marking match the alloy specified in the data?
[ ] Diameter: does it match, and if substituted, is the substitution approved?
[ ] Edge distance: >= 2D single row, or per figure 4-5 / original pattern
[ ] Spacing: >= 3D single row, or per figure 4-5 / original pattern
[ ] Pattern: does it duplicate the original unless deficiencies were suspected?
[ ] Driving: heads formed correctly, no cracked or clinched shop heads
[ ] Type: solid where solid was specified — no undocumented blind substitution
[ ] Countersinking: correct method for sheet thickness, no knife edge

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High-Yield Exam Traps

• AD for D at the same diameter is a downgrade. 2117 is weaker in shear than 2017; the joint no longer meets § 43.13(b).
• "2117 can be driven as received" is true and irrelevant to whether the substitution is legal.
• 2D edge distance, 3D spacing — single row only, and only when structural deficiencies are suspected. Otherwise duplicate the original.
• Edge distance is measured to the hole center, not to the edge of the hole.
• Blind for solid is a change to approved data, requiring further approval, not an inspector's judgment.