Airworthy PrepBend allowance is the length of the curved part of the neutral line inside a bend, and for a 90° bend you find it with BA = 2π(R + ½T) ÷ 4, where R is the inside bend radius and T is the metal thickness (Aviation Maintenance Technician Handbook - Airframe, FAA-H-8083-31B, ch. 4, pp. 4-58 and 4-63 to 4-64). Setback for a 90° bend is simply R + T (p. 4-59).
Why sheet metal matters on the Airframe test
Bend allowance and rivet layout fall under Metallic Structures. The Airframe test has 100 questions, 2.0 hours and a passing score of 70, and Metallic Structures carries 5 to 15 percent of the questions, the widest range of any Airframe subject alongside Airframe Inspection (Companion Guide to the mechanic ACS, FAA-G-ACS-1, Change 2, ch. 3, pp. 12-13).
The Aviation Mechanic ACS, FAA-S-ACS-1 lists these knowledge elements, among others (p. 19):
- AM.II.A.K4: layout, forming, and drilling of sheet metal components.
- AM.II.A.K5: selection of rivets, hardware, and fasteners for a sheet metal repair.
- AM.II.A.K7: rivet layout.
For skill elements such as determining a rivet pattern for a specific repair (AM.II.A.S11, ACS p. 20), see the A&P oral and practical test guide.
Bend allowance terms you need
| Term | Meaning (handbook ch. 4, pp. 4-58 to 4-59) |
|---|---|
| Bend radius (R) | Measured from the radius center to the inside surface of the metal |
| Mold line (ML) | An extension of the flat side of the part beyond the radius |
| Mold line dimension (MLD) | The size the part would have if its corners had no radius |
| Bend tangent line (BL) | Where the metal starts and stops curving; the space between them is the bend allowance |
| Setback (SB) | How far the brake jaws are set back from the mold line to form the bend |
| Flat | The part not included in the bend: Flat = MLD - SB |
| Total developed width (TDW) | The width of material measured around the bends, edge to edge |
The neutral axis is an imaginary line with the same length after bending as before. The handbook puts it about 0.445 × T from the inside of the bend (p. 4-63), but says assuming it sits at the center is close enough for most work (p. 4-59). That is why formula 1 below uses ½T.
Setback: the formula and the K-factor
For a 90° bend, SB = R + T (handbook, p. 4-59; AC 43.13-1B, para 4-56a, p. 4-16). For any other angle, SB = K(R + T). K is 1 for a 90° bend, and without a K chart, K is the tangent of half the bend angle (handbook, p. 4-59). AC 43.13-1B uses the same method, with K = 0.41421 for a 45° bend (para 4-56b, pp. 4-18 to 4-19).
Watch the angle. The bend angle is 180° minus the angle between the flanges, so a bracket with 120° between its flanges has a 60° bend (handbook, p. 4-66). When a part has more than one bend, subtract a setback for each bend (p. 4-59).
The bend allowance formula (two versions)
Bend allowance depends on four factors: degree of bend, radius of bend, thickness of the metal and type of metal (handbook, p. 4-63).
| Formula | Use | |
|---|---|---|
| Formula 1 | BA = 2π(R + ½T) ÷ 4, with π = 3.1416 | 90° bends (pp. 4-63 to 4-64) |
| Formula 2 | BA = (0.01743R + 0.0078T) × N, where N is degrees of bend | Any bend from 1° to 180° (p. 4-64) |
On a bend allowance chart, the upper number in each cell is the allowance for a 90° bend and the lower number is the allowance per 1° (p. 4-64).
Two slips in the handbook to know about (p. 4-64): it heads formula 2 “for a 90° Bend” although its text says the formula works for any bend from 1° to 180°, and its formula 2 example (R = 0.16, T = 0.040) prints 0.27 inch, but the arithmetic gives about 0.279. Its chart gives 0.273 for the same bend.
Worked example: bend allowance for an L-angle (original)
You need an L-shaped angle from 0.032-inch 2024-T3 aluminum. The drawing calls for a 90° bend with a 0.125-inch inside radius. The outside (mold line) dimensions are 1.50 inches on one leg and 1.25 inches on the other. How wide must the flat blank be?
Step 1: Check the radius. AC 43.13-1B, Table 4-6 recommends a radius of 2t to 4t for 0.032-inch 2024-T3, which is 0.064 to 0.128 inch (p. 4-17). A 0.125-inch radius falls inside that range. On a real job, use the manufacturer’s minimum bend radius chart (handbook, p. 4-59).
Step 2: Setback. 90° bend, so K = 1. SB = R + T = 0.125 + 0.032 = 0.157 inch
Step 3: Flats. Each leg is its mold line dimension minus one setback (one bend). Flat 1 = 1.500 - 0.157 = 1.343 inches Flat 2 = 1.250 - 0.157 = 1.093 inches
Step 4: Bend allowance (formula 1). R + ½T = 0.125 + 0.016 = 0.141 2π = 2 × 3.1416 = 6.2832 6.2832 × 0.141 = 0.88593 0.88593 ÷ 4 = 0.2215 inch
Step 5: Total developed width. TDW = flats + (bend allowance × number of bends) (handbook, p. 4-66). TDW = 1.343 + 1.093 + 0.2215 = 2.6575, about 2.66 inches
Check 1: a second route. Total mold lines minus two setbacks plus the bend allowance: 2.75 - 0.314 + 0.2215 = 2.6575. Same answer.
Check 2: the handbook’s sanity test. TDW must be smaller than the total mold line dimensions, because the metal follows the radius; if it is larger, the math is wrong (p. 4-66). 2.66 is less than 2.75, so it passes.
Check 3: formula 2. (0.01743 × 0.125 + 0.0078 × 0.032) × 90 = (0.00217875 + 0.0002496) × 90 = 0.2186 inch. That is within about 0.003 inch of formula 1.
Step 6: Sight line. Draw the sight line inside the bend allowance area, one bend radius (0.125 inch) from the bend tangent line that goes under the brake nose bar, not in the middle of the bend allowance, a common mistake (p. 4-66).
Rivet edge distance, pitch and transverse pitch
First rule: on most repairs, use the same rivet spacing and edge distance the manufacturer used around the damage, and check the structural repair manual (handbook, p. 4-33; AC 43.13-1B, para 4-57c, p. 4-24). The figures below are general minimums in rivet diameters (D).
| Rule | Handbook FAA-H-8083-31B (pp. 4-33 to 4-34) |
|---|---|
| Rivet edge distance, range | Not less than 2D or more than 4D; about 2½D recommended |
| Minimum edge distance, universal (protruding) head | 2D |
| Minimum edge distance, countersunk | 2½D |
| Preferred edge distance | Minimum plus 1/16 inch, or 2D using the next rivet size up |
| Rivet pitch (same row), smallest allowable | 3D |
| Rivet pitch, usual range | 4D to 6D, sometimes up to 10D |
| Minimum pitch by layout | 1 and 3 rows: 3D; 2 rows: 4D (countersunk pitch is larger) |
| Transverse pitch (between rows) | Usually 75 percent of rivet pitch |
| Transverse pitch, smallest allowable | 2½D |
Edge distance runs from the rivet (or hole) center to the sheet edge (handbook, p. 4-33; AC 43.13-1B, para 4-57c). Too close and the sheet may crack or pull away from the rivets; too far and the edge tends to turn up (handbook, p. 4-34).
Where the numbers differ:
- Inside the handbook. The rivet spacing paragraph on p. 4-33 gives a higher minimum, 3½D for protruding head and 4D for flush head rivets, while the rivet pitch paragraph on p. 4-34 gives 3D. Figure 4-80 shows countersunk minimums of 3½D (1 and 3 rows) and 4½D (2 rows).
- AC 43.13-1B. For single-row rivets: edge distance at least 2D and spacing at least 3D, matching the handbook pitch rule (para 4-57c(1), p. 4-24). Figure 4-5 shows double rows with 2D edge distance, 4D spacing and 4D between rows, and triple rows 3D apart with 2D edge distance (p. 4-25).
Example: spacing for a 1/8-inch rivet
Handbook sizing rule: skin thickness × 3, then the next larger rivet size (p. 4-32). For the 0.032 sheet above, 3 × 0.032 = 0.096 inch; 3/32 is only 0.094, so use 1/8 inch (0.125).
| Dimension (D = 0.125 inch) | Rule | Result |
|---|---|---|
| Minimum edge distance, universal head | 2D | 0.250 inch (1/4) |
| Preferred edge distance, universal head | 2D + 1/16 | 0.3125 inch (5/16) |
| Minimum edge distance, countersunk | 2½D | 0.3125 inch (5/16) |
| Minimum pitch, single row | 3D | 0.375 inch (3/8) |
| Usual pitch | 4D to 6D | 0.50 to 0.75 inch |
| Minimum transverse pitch | 2½D | 0.3125 inch (5/16) |
Rivet codes: reading AN470 and MS20470
After the alloy code in a rivet part number come two numbers split by a dash: the first is the diameter in thirty-seconds of an inch and the second is the length in sixteenths of an inch (handbook, p. 4-31). AC 43.13-1B breaks down MS20470AD3-5 as a universal head rivet of 2117-T aluminum, 3/32 inch in diameter and 5/16 inch long (para 4-57a(3), p. 4-24).
| Code | Meaning (AC 43.13-1B, para 4-57a(4), p. 4-24) |
|---|---|
| MS20470 (supersedes AN470) | Universal head, the standard protruding-head rivet |
| MS20426 (supersedes AN426) | 100° countersunk head, used where a smooth finish is needed |
| Alloy code | Alloy | Head marking | Notes (handbook Figure 4-77, p. 4-33) |
|---|---|---|---|
| A | 1100 or 3003 aluminum | None | Nonstructural uses only |
| AD | 2117 aluminum | Dimple | Most used repair rivet; installed as received (p. 4-31) |
| B | 5056 aluminum | Raised cross | |
| D | 2017 aluminum | Raised dot | |
| DD | 2024 aluminum | Two raised bars | “Ice box” rivet, driven in the “W” condition |
| E (KE) | 7050 aluminum | Raised ring | Replacement for the DD rivet |
AC 43.13-1B describes D as a raised teat and DD as two dashes (para 4-57a(2), p. 4-20).
Decoding an original example, MS20470AD4-6: universal head, 2117 aluminum, 4/32 (1/8) inch diameter, 6/16 (3/8) inch long. For length, total length = grip length (the combined thickness of the sheets) plus the shank needed to form the shop head; a properly formed shop head is 1½ times the shank diameter (handbook, p. 4-32).
Common mistakes
- Using the angle between the flanges as the bend angle. Bend angle = 180° minus that angle (handbook, p. 4-66).
- Forgetting the second setback. A flat with a bend on each side loses two setbacks (p. 4-63).
- Mixing up edge distance and pitch. Edge distance is center to edge; pitch is center to center in the same row; transverse pitch is between rows (pp. 4-33 to 4-34).
A short study plan
- Read handbook chapter 4, pp. 4-31 to 4-35 (rivets and spacing) and pp. 4-58 to 4-66 (bend terms through the flat pattern).
- Read AC 43.13-1B, paragraph 4-56 and paragraph 4-57a to c (pp. 4-16 to 4-25).
- Redo the worked example with a 0.040 sheet and a 0.16 radius, then check yourself against the handbook’s U-channel example, which uses those figures (pp. 4-61 to 4-66). Expect a small gap: the handbook uses 0.27 inch per bend (TDW 3.74 inches), while formula 1 gives about 0.283 (TDW about 3.77).
- Practice on the Metallic Structures topic page, then take the free Airframe mock test. The A&P Airframe written test guide covers the other subjects.
Common questions
What is the bend allowance formula?
For a 90° bend, BA = 2π(R + ½T) ÷ 4. For any angle from 1° to 180°, BA = (0.01743R + 0.0078T) × N, where N is the degrees of bend (handbook, pp. 4-63 to 4-64).
What is setback in sheet metal?
The distance the brake jaws are set back from the mold line to form a bend: R + T for 90°, K(R + T) for other angles (handbook, p. 4-59).
What is the minimum rivet edge distance?
2D for universal head and 2½D for countersunk rivets, with about 2½D recommended (handbook, pp. 4-33 to 4-34). AC 43.13-1B gives 2D for single rows (para 4-57c(1)).
What is the difference between rivet pitch and transverse pitch?
Rivet pitch is center to center in the same row, at least 3D. Transverse pitch is between rows, usually 75 percent of the pitch and at least 2½D (handbook, p. 4-34).
What do the numbers in an AN470 or MS20470 rivet code mean?
470 is the universal head. After the alloy letters, the first number is the diameter in 32nds of an inch and the second is the length in 16ths (handbook, p. 4-31).