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Required penetration of a fillet weld

Aviation Mechanic General (AMG) practice question 11 of 21 on Inspection Concepts and Techniques. ACS element AM.I.K.K5 · difficulty: applied. Want to answer it in a timed quiz? Use the Aviation Mechanic General (AMG) practice mode.

Question bank updated 2026-09-22

QUESTIONA fillet weld joins base metal that is 0.120 inch thick. According to the penetration requirement for fillet welds, what depth of fusion is acceptable?

  1. A0.120 inch, the full thickness of the base metal
  2. B0.060 to 0.090 inch
  3. C0.030 to 0.060 inch
Show the answer and explanation

Correct answer: C. 0.030 to 0.060 inch

  • A — incorrect One hundred percent penetration is the requirement for a butt weld, not a fillet weld.
  • B — incorrect This range corresponds to 50 to 75 percent of the thickness; the fillet weld requirement is 25 to 50 percent.
  • C — correct Fillet weld penetration must be 25 to 50 percent of the base metal thickness; 25 percent of 0.120 inch is 0.030 inch and 50 percent is 0.060 inch.

BACKGROUND

The handbook judges completed welds by visual means. A good weld is uniform in width with even ripples well feathered into the base metal, shows no burn from overheating, has good penetration, and is free of gas pockets, porosity or inclusions. Penetration is the depth of fusion in a weld, and thorough fusion is the most important characteristic contributing to a sound weld. Penetration is affected by the thickness of the material, the size of the filler rod and how it is added. In a butt weld, penetration should be 100 percent of the base metal thickness; on a fillet weld, the requirement is 25 to 50 percent of the base metal thickness.

STUDY TIP

Keep the two penetration figures separate: 100 percent for butt welds, 25 to 50 percent for fillet welds, then multiply by the base metal thickness.

KEY POINTS

  • Penetration is the depth of fusion in a weld.
  • Butt welds require 100 percent penetration of the base metal thickness.
  • Fillet welds require 25 to 50 percent penetration of the base metal thickness.

SOURCES

  • FAA-H-8083-30B ch10 "Inspection of Welds"
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SkyDrill is not affiliated with the FAA. This is an original practice item based on FAA-H-8083-30B Aviation Maintenance Technician Handbook – General, 14 CFR part 43 and part 65. Regulations and handbooks are revised; confirm details against the current edition.