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Aviation Maintenance Technician Handbook–Airframe

FAA-H-8083-31B Version 2023
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Chapter 4

Aircraft Metal Structural Repair

will be discussed. [Figure 4-123] When using bend allowance calculations, the following steps for finding the total developed length can be computed with formulas, charts, or computer-aided design (CAD) and computer-aided manufacturing (CAM) software packages. This channel is made of 0.040-inch 2024-T3 aluminum alloy.

Step 1: Determine the Correct Bend Radius

Minimum bend radius charts are found in manufacturers’ maintenance manuals. A radius that is too sharp cracks the material during the bending process. Typically, the drawing indicates the radius to use, but it is a good practice to double check. For this layout example, use the minimum radius chart in Figure 4-124 to choose the correct bend radius for the alloy, temper, and the metal thickness. For 0.040, 2024- T3 the minimum allowable radius is 0.16-inch or 5⁄32-inch.

Step 2: Find the Setback

The setback can be calculated with a formula or can be found in a setback chart available in aircraft maintenance manuals or Source, Maintenance, and Recoverability books (SMRs). [Figure 4-125]

Figure 4-124. Minimum bend radius (from the Raytheon Aircraft Structural Inspection and Repair Manual).
Figure 4-124. Minimum bend radius (from the Raytheon Aircraft Structural Inspection and Repair Manual).
Figure 4-125. Setback chart.
Figure 4-125. Setback chart.
Formula: Figure 4-125 formula reference: Setback-chart relationship: enter with T + R, move to the bend angle BA, and read setback SB. The illustrated example uses T = 0.063 inch, R = 0.12 inch, T + R = 0.183 inch, and BA = 135°, giving SB = 0.453 inch. Source text: T SB = DIstance from mold line to bend line 1. Enter chart at bottom on appropriate scale using sum T + R ; Bend line BA = Line to bend line 2. Read up to bend angle ; BA = Bend angle 3. Determine setback from corresponding scale on left ; BA R R T = = B Th e i n c d k n ra e d ss ius Example: ; ( ; T (0.063) + R (0.12) = 0.183 ; Outside Set- Bend BA = 135° ; mold line back line ; Setback = 0.453 ; (SB) ; Bend Angle (BA) ; 170° 160° 150° 140° 135° 130° 120° ; 110° ; 100° ; 90° ; BSecnasDkcabeS ; ABegnAdneB ; 80° ; 70° ; 60° ; 50° ; 45° ; 40° ; 30° ; 20° ; 10° ; 0.50 1.0 1.5 2.0 2.5 3.0 3.5 ; 0.10 0.183 0.20 0.30 0.40 0.50 0.60 0.70 ; 0.02 0.04 0.06 0.08 0.10 0.12 0.14 ; Thickness (T) + Radius (R) ; Flat Pattern Setback Graph

Using a Formula to Calculate the Setback

Formula: SB = setback ; K = K-factor (K is 1 for 90° bends) ; R = inside radius of the bend ; T = material thickness

Since all of the angles in this example are 90° angles, the setback is calculated as follows:

Formula: SB = K(R+T) = 0.2 inches ; Note: K = 1 for a 90° bend. For other than a 90° bend, use ; a K-factor chart.

Using a Setback Chart to Find the Setback The setback chart is a quick way to find the setback and is useful for open and closed bends, because there is no need to calculate or find the K-factor. Several software packages and online calculators are available to calculate the setback. These programs are often used with CAD/CAM programs. [Figure 4-125]

  • Enter chart at the bottom on the appropriate scale with the sum of the radius and material thickness.
  • Read up to the bend angle.
  • Find the setback from corresponding scale on the left.
Formula: Example: ; • Material thickness is 0.063-inch. ; • Bend angle is 135°. ; • R + T = 0.183-inch.

Find 0.183 at the bottom of the graph. It is found in the middle scale.

  • Read up to a bend angle of 135°.
  • Locate the setback at the left hand side of the graph in the middle scale (0.435-inch). [Figure 4-125]

Step 3: Find the Length of the Flat Line Dimension

The flat line dimension can be found using the formula:

Formula: Flat = MLD – SB ; MLD = mold line dimension ; SB = setback