Common Rivet and Fabric-Covering Mistakes That Fail an Aircraft Inspection (AC 43.13-1B)

The rivet and fabric errors that most often send work back are covered by a handful of AC 43.13-1B rules. For a single row of rivets, when original values can't be copied, edge distance at least 2 rivet diameters and spacing at least 3 diameters (4-57c). Never substitute a weaker rivet without compensating (4-57b). No blind rivets in prohibited locations. Grade A fabric (TSO-C15d) on aircraft with wing loading of 9 psf or greater or a placarded Vne of 160 mph or greater (Table 2-1).

None of these are obscure. They come up because the job looked simple, the original value wasn't checked, or a figure was remembered rather than looked up. Below are the most common errors, grouped by trade, each with the paragraph that settles it. You can query every one of them in the Kopik AC 43.13 base.

Rivet mistake 1: inventing a pattern instead of copying the original

Paragraph 4-57c defines edge distance as the distance from the center of the rivet hole to the nearest edge of the sheet, and spacing as center to center of adjacent holes. Its first instruction is easy to skip: "Unless structural deficiencies are suspected, the rivet spacing and edge distance should duplicate those of the original aircraft structure." Only if structural deficiencies are suspected does the AC give minimums:

• Single row: edge distance not less than 2 times the rivet diameter, spacing not less than 3 times the rivet diameter.
• Double row and triple or multiple rows: not less than the minimums shown in Figure 4-5 (Rivet Hole Spacing and Edge Distance for Single-Lap Sheet Splices). The figure is a drawing, and its values are not available as text in the base, so this article doesn't quote any. Read them from the figure in the AC itself.

For rivet size, paragraph 4-57g gives a general rule when designing a pattern per 4-58g to 4-58n: a diameter approximately three times the thickness of the thicker sheet. And per 4-57b, if a hole becomes enlarged and you step up to the next rivet size, re-check that edge distance and spacing still meet the 4-57c minimums. Oversizing near an edge is a classic way to fail them.

Rivet mistake 2: a weaker rivet, or the wrong temper

• Weaker substitutes: rivets may not be replaced by a type with lower strength properties unless compensated by a larger size or more rivets. The AC's specific allowance: 2017 rivets of 3/16 inch or less and 2024 rivets of 5/32 inch or less may be replaced with 2117 rivets for general repairs, provided the replacements are 1/32 inch larger in diameter (4-57b).
• Driving rivets too hard: 2117 can be driven as received, but 2017 above 3/16 inch and all 2024 rivets must be kept in dry ice or refrigerated in the quenched condition until driven, or reheat-treated just before driving (4-57d).
• Misreading head markings: per 4-57a(2), 1100 ("A") has no head marking, 2117-T ("AD") a dimple, 2017-T ("D") a raised teat, 2024-T ("DD") two dashes, 5056 ("B") a raised cross. The AC's Table 4-8 (rivet identification) is not legible in the base, so don't rely on this article for anything beyond these markings.
• Rivets in tension: do not use rivets where they would be placed in tension, tending to pull the heads off (4-57g).

Rivet mistake 3: blind and POP rivets in the wrong place

Blind rivets are for access to one side only, and the AC notes their locking characteristics are typically not as good as a driven rivet's. The note in 4-57f says they shall not be used:

• in fluid-tight areas;
• in air intake areas where rivet parts may be ingested by the engine;
• on control surfaces, hinges, hinge brackets, flight control actuating systems;
• on wing attachment fittings, landing gear fittings;
• on floats or amphibian hulls below the water level, or other heavily stressed locations.

Two more lines to remember: common pull-type POP rivets produced for non-aircraft applications "are not approved for use on certificated aircraft structures or components" (4-57f(7)), and for sheet metal repairs to the airframe, blind rivets must be authorized by the airframe manufacturer or approved by an FAA representative (7-2j).

Fabric mistake 1: undergrade fabric for the wing loading

Table 2-1 ties fabric grade to wing loading and never-exceed speed. Paragraph 2-5 adds: recover or repair with fabric of equal quality and strength to the original.

So a wing loaded at 9 psf or more, or a placarded Vne of 160 mph or more, needs Grade A. Intermediate fabric on that aircraft is an inspection failure regardless of workmanship.

Fabric mistake 2: misjudging when fabric is unairworthy

• Fabric condition must be determined at every 100-hour and annual inspection (2-33a).
• Fabric is airworthy until it deteriorates below 70 percent of the strength of new fabric required for the aircraft, not of the fabric actually installed. Grade A used where only intermediate is required can go down to 46 lb per inch (2-33a).
• Unairworthy below 46 lb per inch when wing loading is under 9 psf and Vne under 160 mph; below 56 lb per inch at 9 psf and over, or Vne 160 mph and over; below 35 lb per inch for gliders at 8 psf or less and Vne 135 mph or less (2-33b, c).
• Punch testers are not FAA approved; they inform the mechanic's opinion. If readings are low or the cover looks poor, cut a 1-1/4 by 4-inch sample, ravel to 1 inch and pull-test it, or send it to a lab for ASTM D5035 (2-34).

Fabric mistake 3: paperwork and compatibility

• Recovering or repairing with a fabric or coating other than the type used by the original manufacturer is a major alteration requiring FAA approval of fabric and installation data (2-5 note).
• Replacing fabric on wings, fuselage, stabilizers or control surfaces is an airframe major repair, and so is a fabric repair larger than needed for two adjacent ribs (Part 43 Appendix A (b)(1)(xxvi), (xxvii)).
• Cotton and linen rib lacing cord, thread and finishing tapes should not be used with polyester or glass fabric (2-5 note).
• Rib lace spacing on wings should be no greater than the original manufacturer's (2-11).

And the small one that turns up in every inspection: cotter pins and safety wire are never reused (7-38). Try the base with "What's the minimum edge distance and spacing for a single row of rivets if I don't know the original factory values?" or "My fabric-covered wing has a wing loading over 9 psf, what grade of covering fabric do I need?" The source is the FAA's AC 43.13-1B.