How do you calculate bend allowance?

Bend allowance is the length of material in the bend, measured along the neutral axis from one bend tangent line to the other. You get it from a bend allowance chart for the material thickness, bend radius, and angle, or from the standard formula that combines the radius, the thickness, and the number of degrees of bend. Add it to the flat portions for total developed length.

What is the minimum edge distance for a rivet?

Two rivet diameters, measured from the center of the rivet hole to the edge of the sheet, and two and a half diameters when the rivet is countersunk. Any less and the sheet can tear out at the edge. Common practice keeps edge distance between two and four diameters, with minimum rivet pitch of three diameters along the row.

What size rivet do I use for a given sheet thickness?

The rule of thumb is a shank diameter about three times the thickness of the thickest sheet in the joint, then rounded to the nearest standard size. Rivet length is the total grip length of the sheets plus roughly one and a half diameters of extra shank, which supplies the material needed to form the head.

When do you dimple instead of machine countersinking?

Machine countersinking cuts material away, so it needs sheet thicker than the depth of the countersunk rivet head. On thinner material the cut breaks through and leaves a knife edge with no bearing surface, so the sheet is dimpled instead, forming the recess by pressing the metal rather than removing it. As a rule of thumb, thin sheet around 0.040 inch and under is dimpled.

Is stop drilling a permanent crack repair?

No. Stop drilling removes the sharp tip where stress concentrates and keeps the crack from growing in the short term, but it restores no strength. The area still needs a repair sized from approved data, typically a patch or doubler, and the reason the crack started has to be addressed or the same damage returns nearby.

Metallic Structures — FAA A&P Test Questions (ACS AM.II.A)

Metallic Structures (AM.II.A) is the Airframe subject area covering sheet metal layout, forming, and riveted repair, which is where most Airframe structures questions come from. Expect bend allowance and setback, the neutral axis, rivet selection and driven head dimensions, edge distance and pitch, stop drilling, dimpling versus machine countersinking, and identifying corrosion in aluminum structure. Every AMTprep question tagged to this code carries a rationale and a handbook reference, so the numbers become recall rather than arithmetic you improvise under time pressure.

What ACS AM.II.A covers

The source is the sheet metal and structural repair material in FAA-H-8083-31, and the questions are written straight from it. Layout and forming come first. Bend allowance is the amount of material consumed in the bend itself, measured along the neutral axis between the bend tangent lines; setback locates those tangent lines back from the mold point and equals K times the sum of the radius and the thickness. The neutral axis is the line in the bend where the metal is neither stretched nor compressed, and it lies closer to the inside of the bend, about 0.445 times the material thickness from the inside surface. Minimum bend radius, grain direction, and the effect of bending across the grain rather than with it all appear. Riveting is the largest cluster. Rivet diameter is normally about three times the thickness of the thickest sheet in the joint. A correctly driven rivet has a formed head about one and a half shank diameters across and about one half a shank diameter high. Minimum edge distance is two rivet diameters, measured from the center of the hole to the edge of the sheet, and two and a half diameters for countersunk rivets, while minimum rivet pitch is three diameters. Machine countersinking needs material thicker than the depth of the rivet head; thinner sheet is dimpled instead. Damage assessment finishes the area: negligible versus repairable damage, stop drilling to blunt a crack tip and remove the stress concentration, patch and doubler work that restores original strength, and spotting corrosion in aluminum by its powdery deposit and pitted surface.

Where this sits on the test

ACS AM.II.A is tested on the FAA Airframe written test, one of 1,748 ACS-tagged questions in the Airframe bank. Every question tagged to this area carries a worked rationale and its FAA handbook reference, so you can drill the code itself rather than the whole test.

FAA handbook references

FAA-H-8083-31

Three traps candidates fall into

Bend allowance and setback get swapped. Bend allowance is the material inside the bend, measured along the neutral axis, and it is added into the flat layout length. Setback is K times radius plus thickness, and it locates the bend tangent line from the mold point. Use one where the other belongs and every flat length is wrong.
Driven head dimensions get flipped. The formed head should be roughly one and a half rivet diameters across and one half a diameter high, not the reverse. A head that is tall and narrow means the rivet was not upset enough; one that is flat and oversized means it was driven too far and the shank may have been damaged.
A stop drill gets treated as the repair. Drilling at the tip of a crack removes the stress concentration and buys time, but the hole has to capture the actual crack tip or the crack simply runs past it, and the structure still needs a proper repair from approved data before it is considered restored.

7 free sample questions from ACS AM.II.A

AM.II.ATap an answer
When laying out a sheet metal bend, what is the bend allowance?
AM.II.ATap an answer
The neutral axis (neutral line) of a piece of sheet metal that is being bent is the location where the metal
AM.II.ATap an answer
What is the primary purpose of an aircraft sheet metal stop drill at the end of a stress crack?
AM.II.ATap an answer
The diameter of a rivet shank should generally be about
AM.II.ATap an answer
After a solid rivet is properly driven, the formed (bucked) head should have a height of approximately
AM.II.ATap an answer
In a riveted lap joint, the minimum edge distance from the center of a rivet to the edge of the sheet is normally
AM.II.ATap an answer
Which type of corrosion appears as a powdery deposit and a roughened, pitted surface on aluminum?

When laying out a sheet metal bend, what is the bend allowance?

ACS code: AM.II.A

Correct answer: The length of material in the curved section of the bend

Rationale: Bend allowance is the length of material actually consumed in the curved portion of a bend. It must be added to the flat dimensions when computing the total developed (flat-pattern) length of a part, because the metal in the radius is neither a straight flange nor part of the setback. Setback is a separate value (the distance from the bend tangent line to the mold-point intersection), and the bend radius is the inside curvature, so those describe different quantities.

The neutral axis (neutral line) of a piece of sheet metal that is being bent is the location where the metal

ACS code: AM.II.A

Correct answer: is neither compressed nor stretched

Rationale: During bending, metal on the inside of the radius is compressed and metal on the outside is stretched. Between these zones lies the neutral axis, where the material is neither compressed nor stretched and its length does not change. Bend-allowance formulas are based on the length of this neutral line, which for practical purposes lies near 0.445 of the material thickness from the inside surface.

What is the primary purpose of an aircraft sheet metal stop drill at the end of a stress crack?

ACS code: AM.II.A

Correct answer: To prevent the crack from spreading farther

Rationale: A crack acts as a stress concentrator with a very sharp tip. Drilling a small hole precisely at the end (tip) of the crack blunts that sharp point and spreads the load over the larger radius of the hole, which arrests the crack's progress. It is a temporary measure to stop further growth until a proper repair is made; it does not relieve panel-wide stress or serve as a rivet pilot.

The diameter of a rivet shank should generally be about

ACS code: AM.II.A

Correct answer: three times the thickness of the thickest sheet being joined

Rationale: A common rule for selecting rivet diameter is to use a rivet whose shank diameter is approximately three times the thickness of the thickest sheet in the joint. A rivet that is too small will not carry the shear load; one that is too large can cause excessive bearing stress and crack the sheet. This three-times rule produces a rivet sized to develop adequate strength in the joint.

After a solid rivet is properly driven, the formed (bucked) head should have a height of approximately

ACS code: AM.II.A

Correct answer: one-half the shank diameter

Rationale: Standards for an acceptable driven rivet specify that the formed head should be about one-half the shank diameter in height and about one and one-half times the shank diameter across its width (diameter). These proportions ensure the rivet has been upset enough to fill the hole and clamp the sheets without being over- or under-driven. A formed head that is too thin or too small indicates insufficient driving.

In a riveted lap joint, the minimum edge distance from the center of a rivet to the edge of the sheet is normally

ACS code: AM.II.A

Correct answer: two times the rivet diameter

Rationale: Edge distance is measured from the center of the rivet hole to the nearest edge of the sheet. The minimum acceptable edge distance is generally two times the rivet diameter (2D) for universal-head rivets. Too little edge distance allows the sheet to tear out under load, while excessive edge distance lets the joint buckle. Rivet spacing (pitch) is a separate dimension typically ranging from about 3D to 12D.

Which type of corrosion appears as a powdery deposit and a roughened, pitted surface on aluminum?

ACS code: AM.II.A

Correct answer: Surface (uniform) corrosion

Rationale: Surface, or uniform, corrosion on aluminum shows up as a general roughening, etching, or pitting of the surface, often accompanied by a white or gray powdery oxide deposit. It is the most common form on aluminum structures. Stress corrosion cracking is intergranular cracking under sustained tensile load, and fretting corrosion occurs at the interface of close-fitting parts subject to slight relative motion, so each presents differently.

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Frequently asked questions

How do you calculate bend allowance?: Bend allowance is the length of material in the bend, measured along the neutral axis from one bend tangent line to the other. You get it from a bend allowance chart for the material thickness, bend radius, and angle, or from the standard formula that combines the radius, the thickness, and the number of degrees of bend. Add it to the flat portions for total developed length.
What is the minimum edge distance for a rivet?: Two rivet diameters, measured from the center of the rivet hole to the edge of the sheet, and two and a half diameters when the rivet is countersunk. Any less and the sheet can tear out at the edge. Common practice keeps edge distance between two and four diameters, with minimum rivet pitch of three diameters along the row.
What size rivet do I use for a given sheet thickness?: The rule of thumb is a shank diameter about three times the thickness of the thickest sheet in the joint, then rounded to the nearest standard size. Rivet length is the total grip length of the sheets plus roughly one and a half diameters of extra shank, which supplies the material needed to form the head.
When do you dimple instead of machine countersinking?: Machine countersinking cuts material away, so it needs sheet thicker than the depth of the countersunk rivet head. On thinner material the cut breaks through and leaves a knife edge with no bearing surface, so the sheet is dimpled instead, forming the recess by pressing the metal rather than removing it. As a rule of thumb, thin sheet around 0.040 inch and under is dimpled.
Is stop drilling a permanent crack repair?: No. Stop drilling removes the sharp tip where stress concentrates and keeps the crack from growing in the short term, but it restores no strength. The area still needs a repair sized from approved data, typically a patch or doubler, and the reason the crack started has to be addressed or the same damage returns nearby.

All 40 ACS areas

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