When an aviation maintenance technician bends a piece of sheet metal, the material does not simply fold at a sharp angle like a piece of paper — it stretches, compresses, and redistributes along a curved arc called the bend radius. If a technician cuts a flat blank to the same total length as the sum of the outside mold-line dimensions without subtracting for the bend, the blank will come out too long, because the bending process consumes extra material. Calculating exactly how much material is used up in the bend — the bend allowance — is therefore one of the most critical mathematical skills an airframe technician must master. Get it right and your repair fits flush, carries its design load, and passes inspection. Get it wrong and you waste expensive aluminum sheet, misalign rivet holes, or introduce dangerous residual stress into the structure.
This article walks through every element of a proper sheet metal layout: understanding the geometry of a bend, computing bend allowance, locating setback and mold lines, calculating flat-pattern dimensions, and applying practical shop-floor techniques — all grounded in the FAA Aviation Maintenance Technician Handbook – Airframe (FAA-H-8083-31).
The Geometry of a Bend
Before any numbers can be computed, you must understand what physically happens inside a bend. When sheet metal is pressed over a radius, the outer surface of the bend is placed in tension and actually stretches slightly, while the inner surface is placed in compression and shortens. Somewhere between those two surfaces lies a theoretical layer that neither stretches nor compresses — the neutral axis. For practical sheet metal work, the neutral axis is assumed to lie approximately 0.445 times the material thickness from the inside surface of the bend (a commonly used approximation). This is the layer whose arc length we measure when computing how much flat material is consumed.
Several terms define the geometry precisely:
- Bend radius (R): The radius of curvature measured at the inside surface of the bend. Aircraft manufacturers specify a minimum bend radius for each alloy and temper to prevent cracking.
- Bend angle (A): The angle through which the material is bent, measured in degrees. A 90° bend is the most common, but bends of any angle are possible.
- Thickness (T): The material's nominal thickness in inches.
- Bend allowance (BA): The arc length of the neutral axis through the bend — the amount of flat stock consumed by one bend.
- Setback (SB): The distance from the mold point (the intersection of the outside flat surfaces) back to the bend tangent line. Setback is used to locate layout lines on the flat blank.
- Mold line (ML): The imaginary line formed by extending the outside flat surfaces of the finished part until they intersect. It is the reference from which setback is measured.
- Flat (F): The straight, unbent portion of a leg, measured from the bend tangent line to the edge of the part or to the next bend tangent line.
The Bend Allowance Formula
The bend allowance formula calculates the arc length of the neutral axis through the bend. Because the neutral axis lies slightly inside the center of the material, its radius is not simply R — it is R plus a fraction of the thickness:
BA = (0.01745 × A) × (R + 0.4468 × T)
Breaking this down: the constant 0.01745 converts degrees to radians (it is π ÷ 180). The bend angle A is in degrees. The quantity (R + 0.4468 × T) represents the radius of the neutral axis — the inside bend radius plus the offset to the neutral axis. Multiply those two quantities together and you have the arc length in inches that the bend consumes.
A worked example helps cement this. Suppose you are bending 0.040-inch 2024-T3 aluminum sheet to a 90° angle, using a 3/16-inch (0.1875-inch) inside bend radius:
- Neutral axis radius = 0.1875 + (0.4468 × 0.040) = 0.1875 + 0.01787 = 0.2054 inches
- BA = 0.01745 × 90 × 0.2054 = 1.5705 × 0.2054 = 0.3226 inches
This means each 90° bend consumes approximately 0.323 inches of flat stock. On a part with two such bends, you subtract roughly 0.646 inches total from the sum of the outside mold-line dimensions to arrive at the correct flat-blank length.
Setback and Locating the Bend Tangent Lines
Setback is the key measurement that tells you where to draw the bend tangent lines (the start and end of the bend zone) on your flat blank. For a 90° bend:
SB = R + T
For bends other than 90°, the formula uses a trigonometric factor: SB = tangent(A/2) × (R + T). For a 90° angle, tangent(45°) = 1.000, so the formula simplifies neatly to R + T.
Using our example (R = 0.1875, T = 0.040): SB = 0.1875 + 0.040 = 0.2275 inches. This means the bend tangent line is located 0.2275 inches inward from the mold line on each leg of the finished part. When you lay out your flat blank, you measure the outside mold-line dimension of each leg, subtract one setback for each leg of that bend, and that gives you the flat dimension of each straight leg.
Computing the Total Flat-Blank Length
The total flat-blank length for a part with multiple bends is the sum of all the flat (straight) portions plus the bend allowance for each bend. The procedure is:
- Identify all outside mold-line dimensions from the engineering drawing.
- Compute the setback for each bend and subtract it from the adjacent mold-line dimensions to find each flat distance.
- Compute the bend allowance for each bend using the formula above.
- Add all flats and all bend allowances together for the total flat-blank length.
For a simple angle bracket with two legs — one 2.000 inches and one 1.500 inches (both measured to outside mold lines) — and one 90° bend using R = 0.1875 and T = 0.040:
- SB = 0.2275 inches (one setback applies to each leg)
- Flat of leg 1 = 2.000 − 0.2275 = 1.7725 inches
- Flat of leg 2 = 1.500 − 0.2275 = 1.2725 inches
- BA = 0.3226 inches
- Total blank length = 1.7725 + 1.2725 + 0.3226 = 3.3676 inches
Minimum Bend Radius and Material Considerations
Not every radius is permissible for every alloy. Bending tightly around a small radius work-hardens and may crack the outer fibers. The FAA Airframe Handbook references manufacturer data and standard bend radius tables that specify the minimum acceptable inside bend radius for each alloy, temper, and thickness. For example, the harder 2024-T3 alloy requires a larger minimum radius than softer 1100-O or 3003-H14 alloys. Clad aluminum (Alclad) has a slightly more ductile surface and may allow tighter radii than bare alloy of the same specification. Always consult the applicable aircraft manufacturer's structural repair manual (SRM) or the material specification table — using a radius smaller than the minimum is an airworthiness concern.
Grain direction also matters. Sheet metal has a rolling direction established during manufacture. Bending perpendicular to the grain (across the grain) permits tighter radii without cracking; bending parallel to the grain (with the grain) requires a larger minimum radius. When cutting blanks, orient them so that bends run across the grain wherever possible.
Practical Layout Techniques
After computing all dimensions, the technician transfers them to the flat blank using a combination of scribers, dividers, and straightedges. Never use a lead pencil on aluminum — graphite can cause corrosion. Use a scriber for layout lines that will fall inside the finished part boundary, and a felt-tip marker or layout dye on areas that will be trimmed away. Bend tangent lines are typically scribed lightly so they are visible during brake setup. Rivet hole locations and edge distances are laid out next, and all datum edges are filed smooth and deburred before bending to prevent crack initiation at a notch.
In the brake, the bend tangent line is aligned with the brake's nose radius. A test bend on scrap material of the same alloy, temper, and thickness verifies the springback characteristics before the actual part is formed. Most aluminum alloys spring back a few degrees after the brake pressure is released, so technicians often overbend slightly and then check with a protractor or angle block.
Key Numbers and Rules
- Neutral axis offset: approximately 0.4468 × T from the inside bend surface (some sources round to 0.445 × T — both are acceptable approximations).
- Degrees-to-radians conversion constant: 0.01745 (π ÷ 180).
- Setback for 90° bend: R + T (because tan 45° = 1).
- Setback for other angles: tan(A/2) × (R + T).
- Total blank = sum of all flats + sum of all bend allowances.
- Minimum bend radius is alloy-, temper-, and thickness-dependent — always check the SRM or manufacturer bend radius table.
- Grain direction: bends across the grain allow smaller radii than bends with the grain.
Common Test Traps
- Confusing bend allowance with setback: Setback locates the tangent line on the flat blank; bend allowance is the material consumed in the arc. They are related but not interchangeable, and mixing them up produces an incorrectly sized blank.
- Using inside mold-line dimensions instead of outside: The standard formula subtracts setback from the outside mold-line dimensions. Using inside dimensions will make your blank too short.
- Forgetting that each bend has two setbacks: When computing flats between adjacent bends, subtract a setback on both sides of each bend — one belonging to each neighboring leg.
- Applying a 90° setback formula to a non-90° bend: Remember that tan(45°) = 1, which simplifies the 90° formula, but for a 45° bend, tan(22.5°) ≈ 0.4142, which is very different — always use the full tangent formula for non-right-angle bends.
- Ignoring springback: The test may ask why a bend does not reach the target angle; the correct answer is springback, and the remedy is to overbend by the expected springback amount, which varies by alloy and temper.