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Spacing between hard knots in a spruce beam

Aviation Mechanic Airframe (AMA) practice question 13 of 30 on Non-Metallic Structures. ACS element AM.II.B.K5 · difficulty: applied. Want to answer it in a timed quiz? Use the Aviation Mechanic Airframe (AMA) practice mode.

Question bank updated 2026-09-22

QUESTIONA spruce beam contains two sound hard knots, each 3/8 inch in diameter, lying in the center third of the beam and spaced 14 inches apart. Why is this wood unacceptable for a structural repair?

  1. AKnots of this size are never permitted
  2. BKnots must lie outside the center third
  3. CThe knots must be 20 inches apart
Show the answer and explanation

Correct answer: C. The knots must be 20 inches apart

  • A — incorrect Sound, hard knots up to 3/8 inch in diameter are permitted when the stated conditions are met, so size alone does not reject this wood.
  • B — incorrect The rule requires that such knots be in the center third of the beam, so their location here satisfies the condition rather than violating it.
  • C — correct Sound hard knots of 3/8 inch are acceptable only when they are no closer than 20 inches to another knot or defect, and 14 inches falls short of that.

BACKGROUND

The handbook separates wood defects into those permitted and those not permitted. Sound, hard knots up to 3/8 inch in diameter are permitted only if they are not on projecting portions of I-beams, along the edges of rectangular or beveled unrouted beams, or along the edges of box beam flanges, except in portions of low stress. They must also not cause grain divergence beyond the published limit, and they must be in the center third of the beam and no closer than 20 inches to another knot or other defect. That 20-inch spacing pertains to 3/8 inch knots, and smaller knots may be proportionately closer. Knots greater than 1/4 inch must be used with caution.

STUDY TIP

When a question describes a knot, check all three conditions in turn: size, location in the beam, and distance to the nearest other knot or defect. Failing any one of them rejects the wood.

KEY POINTS

  • Sound hard knots up to 3/8 inch may be acceptable under stated conditions.
  • Knots of 3/8 inch must be at least 20 inches from another knot or defect.
  • Permitted knots must lie in the center third of the beam.

SOURCES

  • ama-airframe/ch06.txt "Defects Permitted"
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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.