The chart — recommended radii for 90° bends
Radii are given as multiples of t, the sheet thickness. A range like 2t-4t means the AC recommends between two and four times the thickness; work to the upper figure unless you have a reason not to.
Column headings are the approximate sheet thicknesses of the original table.
| Alloy and temper | 0.016" | 0.032" | 0.064" | 0.128" | 0.182" | 0.258" |
|---|---|---|---|---|---|---|
| 2024-O 1 | 0 | 0-1t | 0-1t | 0-1t | 0-1t | 0-1t |
| 2024-T3 1,2 | 1½t-3t | 2t-4t | 3t-5t | 4t-6t | 4t-6t | 5t-7t |
| 2024-T6 1 | 2t-4t | 3t-5t | 3t-5t | 4t-6t | 5t-7t | 6t-10t |
| 5052-O | 0 | 0 | 0-1t | 0-1t | 0-1t | 0-1t |
| 5052-H32 | 0 | 0 | ½t-1t | ½t-1½t | ½t-1½t | ½t-1½t |
| 5052-H34 | 0 | 0 | ½t-1½t | 1½t-2½t | 1½t-2½t | 2t-3t |
| 5052-H36 | 0-1t | ½t-1½t | 1t-2t | 1½t-3t | 2t-4t | 2t-4t |
| 5052-H38 | ½t-1½t | 1t-2t | 1½t-3t | 2t-4t | 3t-5t | 4t-6t |
| 6061-O | 0 | 0-1t | 0-1t | 0-1t | 0-1t | 0-1t |
| 6061-T4 | 0-1t | 0-1t | ½t-1½t | 1t-2t | 1½t-3t | 2½t-4t |
| 6061-T6 | 0-1t | ½t-1½t | 1t-2t | 1½t-3t | 2t-4t | 3t-4t |
| 7075-O | 0 | 0-1t | 0-1t | ½t-1½t | 1t-2t | 1½t-3t |
| 7075-T6 1 | 2t-4t | 3t-5t | 4t-6t | 5t-7t | 5t-7t | 6t-10t |
1 Alclad sheet may be bent over slightly smaller radii than the corresponding tempers of uncoated alloy. 2 Immediately after quenching, this alloy may be formed over appreciably smaller radii. — Both footnotes are the AC's own wording. Transcribed from the published PDF; for a repair, check the figure against the source document linked below.
How to read it, and the two traps
Multiply by your actual thickness, not the column heading. The columns are the AC's approximate reference thicknesses. If your sheet is 0.040", you are between the 0.032 and 0.064 columns — take the more conservative of the two.
The radius is measured on the inside of the bend. Bend radius always means the inside radius, which is what the brake's radius die or the forming block establishes.
Trap one: the temper matters more than the alloy. 2024 in the annealed O condition bends flat; the same alloy in T3 needs 2t-4t at 0.032". Two sheets that look identical and carry the same alloy number can differ by a factor of four in what they will take.
Trap two: grain direction is not in this table. The AC's radii assume a bend across the grain. Bending along the grain — parallel to the rolling direction — cracks at radii this chart calls acceptable. If the sheet has a visible grain, lay the bend line across it.
The practical reading of the whole table: if you need a tight bend, choose a softer temper or a more formable alloy, not a smaller radius. 6061-T6 will do 1t at 0.064" where 2024-T3 wants 3t-5t.
Worked example
A rib flange in 0.032" 2024-T3, bent 90°.
- Find the 2024-T3 row and the 0.032" column: 2t-4t.
- t = 0.032", so the recommended inside radius is between 0.064" and 0.128".
- Work to the upper end — 0.128" — unless the drawing constrains you. Nothing is gained by running at the minimum.
- Now check the same job in 6061-T6: the 0.032" column gives ½t-1½t, or 0.016" to 0.048". Less than half the radius for the same thickness.
- If the part geometry demands a radius near 0.048" and the material is 2024-T3, the answer is not a tighter bend. It is a different material, an annealed blank formed and then heat treated, or a redesigned part.
0.128" inside radius. And a reminder that a bend that will not close is a material choice problem, not a brake setup problem.
Limits — when NOT to use this
Table 4-6 is titled recommended radii. It is guidance for general practice, not an allowable.
- Approved data governs. Where the SRM, AMM or a repair drawing specifies a bend radius, that figure applies.
- 90° bends only. The table is explicitly for 90° bends. Tighter included angles bend the outer fibre further for the same radius.
- Grain direction is not covered. See above. This is the most common cause of a crack at a radius the chart permits.
- Alclad differs. The AC's own footnote says clad sheet tolerates slightly smaller radii. Slightly is not quantified, so do not take it as licence for a specific number.
- Freshly quenched 2024-T3 is a special case. Footnote 2 allows appreciably smaller radii immediately after quenching — a heat-treat-and-form sequence, not something you can apply to stock sheet off the shelf.
- Nothing here covers joggles, flanges with relief holes, or compound curvature. Those are forming problems with their own rules.
- Springback is not in the table. The radius you bend to and the radius you get are different numbers, and the harder the temper the bigger the gap.
Questions people actually ask
What is the minimum bend radius for 0.032 2024-T3?
Table 4-6 recommends 2t to 4t, which is 0.064" to 0.128" for 0.032" sheet. Work to the upper end unless the drawing says otherwise.
Is bend radius measured inside or outside the bend?
Inside. It is the radius the radius die or forming block establishes on the concave face.
Why does my 2024-T3 crack at a radius the chart allows?
Almost always grain direction. Table 4-6 assumes a bend across the grain; bending parallel to the rolling direction cracks at radii the chart calls acceptable. Also check that the sheet really is T3 and not a harder temper, and that the bend is 90° rather than tighter.
Which bends more tightly, 2024-T3 or 6061-T6?
6061-T6, by a wide margin. At 0.064" it takes 1t-2t where 2024-T3 needs 3t-5t. That formability difference is usually what decides between them for a formed part.
Can I bend Alclad tighter than bare?
The AC's footnote says slightly smaller radii are possible for clad sheet, but it does not quantify "slightly". Treat it as margin, not as permission for a specific tighter figure.
Sources
- FAA AC 43.13-1B CHG 1, Chapter 4, Table 4-6 — "Recommended radii for 90-degree bends in aluminum alloys" — the whole table above, including both footnotes, is transcribed from it.
- FAA-H-8083-31, AMT Handbook — Airframe, Volume 1, Figure 4-124 — carries an equivalent minimum bend radius chart drawn from a manufacturer's structural repair manual. Consistent with Table 4-6 on the alloys both cover.
Want this as a printable?
The full table on one page with the footnotes and the grain-direction warning, sized to live on the brake.