FAA Aviation Mechanic Airframe (AMA)Airframe StructuresMedium
A technician is forming a 90-degree bend in a piece of 0.032-inch aluminum sheet using a bend radius of 0.125 inch. Using the standard bend allowance formula BA = (0.01743R + 0.0078T) x N, what is the bend allowance?
- A0.438 inch
- B0.109 inch
- C0.875 inch
- D0.219 inch
Show answer & explanationAnswer & explanation
Correct answer: D. 0.219 inch
BA = (0.01743 x 0.125 + 0.0078 x 0.032) x 90 = (0.00217875 + 0.0002496) x 90 = 0.0024284 x 90 ≈ 0.219 inch. This formula from AC 43.13-1B accounts for radius, thickness, and bend angle to determine flat-pattern length allowance.
Why the other options are wrong
- A. 0.438 inch is roughly double the correct value.
- B. 0.109 inch would result from halving the correct calculation, an arithmetic error.
- C. 0.875 inch is far too large and does not match the formula's output.
Bend Allowance Formula
The formula BA = (0.01743R + 0.0078T) x N calculates the amount of material needed for a bend in sheet metal, based on radius, thickness, and bend angle.
- R = bend radius, T = material thickness, N = bend angle in degrees
- Used to determine flat-pattern layout dimensions
- Found in FAA AC 43.13-1B, Chapter 4
Memory trick: 'Radius and Thickness, times the angle, gives the stretch.'