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Minimum stitch rate for hand-sewn fabric seams

Aviation Mechanic Airframe (AMA) practice question 15 of 30 on Non-Metallic Structures. ACS element AM.II.B.K9 · difficulty: recall. Want to answer it in a timed quiz? Use the Aviation Mechanic Airframe (AMA) practice mode.

Question bank updated 2026-09-22

QUESTIONWhen aircraft covering fabric is hand sewn with a plain overthrow or baseball stitch, what is the minimum number of stitches per inch?

  1. ATwo, locked at every tenth stitch
  2. BEight, tied with a double half hitch
  3. CFour
Show the answer and explanation

Correct answer: C. Four

  • A — incorrect Hand sewing is locked with a double half hitch at a maximum of 10 stitch intervals, but the minimum rate is four stitches per inch, not two.
  • B — incorrect The double half hitch is used to lock and tie off the sewing, but the published minimum stitch rate is four per inch.
  • C — correct Hand sewing with plain overthrow or baseball stitches is done at a minimum of four stitches per inch.

BACKGROUND

Seams parallel to the line of flight are preferable, although spanwise seams are acceptable. Machine-sewn seams should be double stitched, and a single stitch may be used to close an envelope at a wingtip, trailing edge, or fuselage longeron when the seam lies over structure. Hand sewing uses plain overthrow or baseball stitches at a minimum of four stitches per inch. The hand sewing is locked at a maximum of 10 stitch intervals with a double half hitch, and the end stitch is tied off the same way. Where hand sewing or permanent tacking begins, the fabric is cut so it can be doubled under a minimum of 3/8 inch.

STUDY TIP

Keep the sewing numbers separate in your mind: four stitches per inch is the rate, 10 stitches is the locking interval, and 3/8 inch is the doubled-under allowance.

KEY POINTS

  • Hand sewing uses at least four stitches per inch.
  • Sewing is locked with a double half hitch at most every 10 stitches.
  • Fabric is doubled under a minimum of 3/8 inch before hand sewing.

SOURCES

  • ama-airframe/ch03.txt "Sewn Seams"
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MORE ON NON-METALLIC STRUCTURES

SkyDrill is not affiliated with the FAA. This is an original practice item based on FAA-H-8083-31B Aviation Maintenance Technician Handbook – Airframe, and 14 CFR part 43. Regulations and handbooks are revised; confirm details against the current edition.