Definition
The amount of additional sheet metal needed to form a bend of a given radius and angle. It is the length of material consumed in the curved portion of the bend itself, calculated from the bend radius, the metal thickness, and the number of degrees of the bend.
Plain English
When you bend a piece of metal, the curved part uses up some of the length of the material. Bend allowance is the extra bit of metal you have to add to your flat piece so that, after bending, the part comes out the right size.
Context Anchor
Used in aircraft sheet-metal layout and repair work, especially when making or replacing brackets, panels, ribs, or other formed metal parts.
Derivation
"Allowance" here means an amount added to compensate for something. The metal in a bend doesn't simply fold at a sharp corner — it curves, and that curve takes up length. The allowance is the extra material you allow for that curve.
Why Pilots Care
Pilots don't usually calculate bend allowance themselves, but understanding it helps when reviewing structural repair work or talking with an A&P mechanic about sheet metal repairs. A bend allowance error means a part won't fit correctly after forming.
Analogy
If you wrap a strip of paper around the corner of a box, part of the paper is used up going around the corner. Bend allowance is the sheet-metal version of that used-up length.
Intuition Check
Bend allowance does not mean permission to bend the metal. It means the measured length of metal that will become the curved part of the bend.
Example Sentence 1
Before cutting the aluminum sheet, the mechanic calculated the bend allowance to make sure the finished bracket would fit the airframe correctly.
Example Sentence 2
For a gentle bend in the fuselage fairing, a smaller bend allowance was required compared to a sharp 90-degree angle.