Rivet Spacing and Fabric Covering Errors: What the FAA's AC 43.13-1B Actually Requires
AC 43.13-1B rules out several common rivet and fabric errors in so many words. Rivet spacing and edge distance should duplicate the original structure; only where structural deficiencies are suspected does it give single-row minimums of 2 diameters edge distance and 3 diameters spacing (4-57c). Weaker rivets may not be substituted without compensation. Wings loaded at 9 psf or more, or with a placarded Vne of 160 mph or more, need Grade A fabric. It is FAA guidance; the base doesn't cover CAA or EASA rules.
If you restore or build aircraft, the FAA's AC 43.13-1B may well be on your workshop shelf. It is precise and detailed, and it uses American units and FAA terms throughout. This guide picks out the rivet and fabric errors the circular explicitly addresses and gives the paragraph for each. The full text is searchable in the Kopik AC 43.13 base.
Before relying on it
AC 43.13-1B sets out methods acceptable to the FAA for non-pressurised areas of civil aircraft of 12,500 lb gross weight or less. It ranks behind the manufacturer's maintenance manual and does not approve a major repair. The base contains no UK CAA or EASA documents and nothing on UK permit or homebuilt schemes: whether, and how, the circular may be used on your aircraft is for your inspector, maintenance organisation or the competent authority to confirm.
Riveting: five errors the circular names
1. Laying out a new pattern when the original could be copied
Paragraph 4-57c: "Unless structural deficiencies are suspected, the rivet spacing and edge distance should duplicate those of the original aircraft structure." Edge distance is measured from the centre of the hole to the nearest edge of the sheet; spacing from centre to centre of adjacent holes. Where structural deficiencies are suspected, the minimums are:
- Single row: edge distance at least 2 x rivet diameter, spacing at least 3 x rivet diameter.
- Double, triple or multiple rows: the minimums shown in Figure 4-5. That figure is a drawing whose values aren't available as text in the base, so none are given here; consult the figure in the AC.
Worked example (our arithmetic from the 4-57c(1) rule): for a 1/8 in rivet, minimum edge distance is 2 x 1/8 = 1/4 in and minimum spacing is 3 x 1/8 = 3/8 in. For a 5/32 in rivet: 5/16 in and 15/32 in. The AC gives these in inches only, and so does this article.
2. Drilling oversize without re-checking the edge
If a hole is enlarged, deformed or damaged, paragraph 4-57b allows drilling or reaming for the next larger rivet, "however, make sure that the edge distance and spacing is not less than minimums listed in subparagraph 4-57c." A larger diameter raises the minimum edge distance with it.
3. Substituting a weaker or harder rivet
- Rivets may not be replaced by a type with lower strength unless compensated by a larger size or a greater number (4-57b).
- Permitted for general repairs: replacing 2017 rivets of 3/16 in or less and 2024 rivets of 5/32 in or less with 2117 rivets that are 1/32 in larger in diameter.
- 2117 can be driven as received; 2024 rivets (and 2017 above 3/16 in) must be kept in dry ice or refrigerated in the quenched condition until driven, or reheat-treated just before (4-57d).
- Head markings per 4-57a(2): "A" (1100) none; "AD" (2117-T) dimple; "D" (2017-T) raised teat; "DD" (2024-T) two dashes; "B" (5056) raised cross. Table 4-8 (rivet identification) isn't legible in the base, so nothing more is drawn from it here.
4. Blind rivets where they are banned
The note to 4-57f lists where blind rivets shall not be used: fluid-tight areas; air intake areas where parts could be ingested; control surfaces, hinges, hinge brackets and flight control actuating systems; wing attachment and landing gear fittings; floats or amphibian hulls below the water line; and other heavily stressed locations. Common pull-type POP rivets made for non-aircraft use are not approved on certificated aircraft structures or components (4-57f(7)), and for FAA airframe sheet metal repairs blind rivets must be authorised by the airframe manufacturer or an FAA representative (7-2j).
5. Rivets in tension and undersized rivets
Paragraph 4-57g warns against rivets placed in tension, "tending to pull the heads off", and gives a general sizing rule for joining aluminium sheets when using its repair instructions: a diameter about three times the thickness of the thicker sheet. Lap joints of thin sheet should be backed up with a stiffener section.
Fabric covering: choosing the right grade
Table 2-1 sets the fabric grade by wing loading and never-exceed speed, in pounds per square foot and miles per hour:
AC 43.13-1B Table 2-1 (extract): minimum tensile strength, undoped
| Fabric | New | Deteriorated | Where it applies |
|---|---|---|---|
| Grade A, TSO-C15d | 80 lb per inch | 56 lb per inch | All aircraft; required at 9 psf or greater, or placarded Vne 160 mph or greater |
| Intermediate, TSO-C14b | 65 lb per inch | 46 lb per inch | Below 9 psf and Vne below 160 mph |
| SAE AMS 3802 | 50 lb per inch | 35 lb per inch | Gliders at 8 psf or less, placarded Vne 135 mph or less |
Table 2-1 also lists aircraft linen (British 7F1), noting that it meets the minimum strength requirements of TSO-C15. Paragraph 2-31 adds how to recognise it: a slightly darker shade than cotton, irregular thread spacing and uneven thread size, with an average thread count about the same as Grade A fabric.
When is fabric worn out?
- The AC asks for the fabric condition to be determined at every 100-hour and annual inspection, the FAA's inspection terms (2-33a).
- Airworthy until below 70 percent of the strength of new fabric required for the aircraft. Example from the AC: Grade A on an aircraft needing only intermediate may deteriorate to 46 lb per inch.
- Unairworthy thresholds: 46 lb per inch (under 9 psf and Vne under 160 mph), 56 (9 psf and over, or Vne 160 mph and over), 35 (gliders at 8 psf or less and Vne 135 mph or less).
- Punch testers are not FAA approved; for a reliable figure, strip-test a 1-1/4 by 4 in sample ravelled to 1 in, or have a laboratory test it to ASTM D5035 (2-34).
- Good fabric with cracked dope may be rejuvenated; fabric that has lost its strength has to come off (2-35).
Compatibility and classification
- No cotton or linen lacing cord, thread or tapes with polyester or glass fabric (2-5 note).
- Rib lace spacing no greater than the original manufacturer's (2-11).
- In the FAA system, a different fabric or coating from the original is a major alteration (2-5), and fabric replacement on wings, fuselage, stabilisers or control surfaces is a major repair (14 CFR Part 43 Appendix A). How UK or EASA rules classify the same work is not in the base.
Ask the base directly, for example: "What's the minimum edge distance and spacing for a single row of rivets if I don't know the original factory values?" The source is the FAA's AC 43.13-1B.
Look it up rather than remember it
The Kopik base answers rivet, fabric, hardware and Part 43 questions from FAA sources only, citing the paragraph or table each time.
Frequently asked questions
What edge distance does AC 43.13-1B require for rivets?
It first asks you to duplicate the original structure. Where structural deficiencies are suspected, single-row rivets need an edge distance of at least 2 diameters and spacing of at least 3 diameters; double and multiple rows follow Figure 4-5.
Does AC 43.13-1B mention linen fabric?
Yes. Table 2-1 lists aircraft linen to British specification 7F1 as meeting the minimum strength requirements of TSO-C15.
Can AC 43.13-1B be used on a UK homebuilt?
The base contains no UK CAA, EASA or homebuilt-scheme documents, so it cannot answer that. On the US side, Part 43 itself does not apply to aircraft with an FAA experimental airworthiness certificate unless they previously held another kind of certificate.
When does aircraft fabric become unairworthy?
Below 70 percent of the new strength required for the aircraft: 46 lb per inch under 9 psf and Vne under 160 mph, 56 lb per inch at 9 psf or Vne 160 mph and over, 35 lb per inch for gliders at 8 psf or less and Vne 135 mph or less (paragraph 2-33).
Can lacing cord be cotton on a polyester covering?
No. The note to paragraph 2-5 says cotton and linen rib lacing cord, thread and finishing tapes should not be used with polyester and glass fabric covering.
Can 2017 rivets be replaced with 2117 rivets?
For general repairs, AC 43.13-1B paragraph 4-57b allows 2017 rivets of 3/16 inch diameter or less to be replaced with 2117 rivets, provided the replacements are 1/32 inch greater in diameter. The same applies to 2024 rivets of 5/32 inch or less.
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