What the ACS Tests in Metallic Structures

Metallic structures is Subject A of the Airframe section (AM.II) of the Aviation Mechanic ACS, covering sheet metal defects, layout and drilling, rivet selection and rivet layout, and rivet removal and installation. Skills include designing a repair from a structural repair manual and deciding whether damaged structure is repairable. Expect geometry, not vocabulary: 100 questions in two hours, 70 percent to pass under 14 CFR 65.17.

Rivets: Types, Part Numbers, and Where Each Is Used

Two families matter. The solid shank rivet, driven with a gun and bucking bar, is the structural default; the blind rivet installs from one side only.

Type	Specification	Where it is used
Universal head	MS20470, supersedes AN470	Standard protruding head rivet, replacing round, flat and brazier head rivets
100 degree countersunk	MS20426, supersedes AN426	Where a smooth surface is required, such as upper wing skins
Blind, friction lock	Self plugging Cherry rivet	One side access. Stem must be trimmed and is weak in vibration
Blind, mechanical lock	Bulbed Cherrylock, CherryMax	One side access with a locked stem that cannot fall out

Read the part number in one pass. In MS20470AD3-5, 20470 is the universal head, AD is 2117-T4 aluminum, 3 is the diameter in thirty-seconds of an inch, and 5 is the length in sixteenths.

Blind rivets are prohibited in fluid tight areas, engine air intake areas, control surfaces, hinges, flight control actuating systems, wing attachment fittings and landing gear fittings. For airframe metal repairs, AC 43.13-1B requires manufacturer authorization or FAA approval.

Edge Distance and Pitch: The 2D and 3D Rule

In AC 43.13-1B paragraph 4-57, edge distance runs from the rivet hole center to the nearest sheet edge, and pitch runs center to center. Single row rivets need at least 2 rivet diameters of edge distance and 3 of spacing; multiple rows follow Figure 4-5.

The handbook adds the upper bound: edge distance should be not less than 2 nor more than 4 rivet diameters, with 2.5 as the target, and countersunk rivets need 2.5D where protruding head rivets need 2D. A hole too close to the edge tears out; one too far lets the sheet lift.

Layout Math: Setback and Bend Allowance

Setback is the distance the brake jaws sit back from the mold line: the bend radius plus the metal thickness at 90 degrees, or the K factor, the tangent of half the bend angle, for any other angle. Table 4-7 lists 1.000 at 90 degrees and 0.4142 at 45 degrees.

Bend allowance is the material added around the bend: (0.01743R + 0.0078T) x N, where N is the degrees bent. A 0.16 inch radius in 0.040 inch sheet at 90 degrees gives 0.27 inch. Flats equal the mold line dimension minus setback, and total developed width is the flats plus bend allowance.

Minimum Bend Radius: Alloy, Temper, Thickness

Table 4-6 gives recommended radii for 90 degree bends. At 0.032 inch, 2024-T3 takes 2t to 4t and 7075-T6 takes 3t to 5t, while 5052-H32 forms to almost no minimum radius. Radius grows with thickness and alloy hardness, so when 7075-T6 appears the largest radius is the answer. Bend lines should lie at 90 degrees to the grain of the metal if possible.

Damage Assessment: Repair or Replace

FAA-H-8083-31B groups damage into four classes: negligible, repairable by patching, repairable by insertion, and necessitating replacement. Replacement wins when damage is too extensive, when its location makes repair impractical, or when the part is a casting, forging or hinge that is easier to change out.

The Structural Repair Manual is the primary authority, and its allowable damage limits define what counts as repairable. AC 43.13-1B is acceptable data where no manufacturer method exists; exceeding the SRM limits requires FAA approved data.

For cracks, AC 43.13-1B paragraph 4-59 says to drill holes of 3/32 or 1/8 inch at the extreme ends of the crack, then reinforce across the damaged portion. Stop drilling stops a crack running, it does not repair it. A scratch that penetrates the clad layer exposes the alloy to corrosion.

A repair is not finished when the last rivet is driven. Parts left unpainted must be clad material, faying surfaces and fastener holes are treated before reassembly, and the area is primed, sealed and inspected. Our aircraft corrosion control guide covers that sequence in detail.

The Traps That Cost Marks

Units: the diameter dash is in thirty-seconds, the length dash in sixteenths, in one part number.
Edge distance is measured from the hole center, never from the hole edge.
Countersunk rivets need 2.5D edge distance; protruding head rivets need 2D.
Setback is subtracted from the mold line dimension, bend allowance is added back.
The K factor is 1.000 only at 90 degrees; other angles need the chart.
7075-T6 has the largest minimum bend radius of the common sheet alloys.

Practice layout as arithmetic: run the 2D and 3D minimums, work a setback and a bend allowance on paper, then look the alloy up in Table 4-6. Pair this with our FAA A&P ACS subject area guide and our NDI/NDT guide for A&P mechanics.

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