Fabrication, Sewing, and Construction Techniques
What this domain tests
This domain tests stitch and seam quality: stitch type, stitch density, thread and needle selection, seam construction, bartacks, and the difference between a structural repair and a cosmetic patch. Expect practical judgment questions.
Core rules and concepts
- Stitch type. Lock stitching is common for structural seams; zigzag stitching is used where stretch or load distribution is needed. Double-needle stitching can improve seam strength and consistency. Use the stitch type specified in the manufacturer’s data.
- Stitch density. Stitch length is usually stated as stitches per inch (SPI). A typical parachute specification may call for roughly 7–11 SPI, but the approved data controls. Too few stitches per inch reduces seam strength; too many can perforate and weaken fabric.
- Seam construction. Flat-fell, French, and lap seams distribute load differently. Structural seams should be sewn with the specified seam allowance and stitch pattern; raw edges must be finished as required. Bartacks and box-X stitches reinforce high-load points such as line attachments and hardware tabs.
- Thread, needle, and tension. Thread size must match the specification and the needle must suit the fabric. Balanced tension produces a lock in the material, not loose loops on one side. Test on scrap material before sewing the article.
Worked example
A rigger measures a 12-inch structural seam and counts 72 stitches. That is 72 ÷ 12 = 6 SPI. If the approved data requires 8–10 SPI, the seam is too coarse: each stitch spans more fabric, and the seam may not develop full strength. The rigger should not “add a few stitches” over the finished seam. The correct action is to remove the seam, reset the machine, sew a test sample at 9 SPI, verify stitch balance, and re-sew the seam to the approved pattern.
Traps that cost marks
- Judging stitch quality by appearance only, without counting SPI or checking the back side.
- Using backstitching or knots where the specification calls for a bartack or a turned seam.
- Selecting cotton or unapproved thread because it feeds smoothly.
- Sewing structural seams without a test sample, then discovering tension or needle damage on the article.