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Penetration required in an acceptable fillet weld

Aviation Mechanic General (AMG) practice question 35 of 36 on Aircraft Materials, Hardware, and Processes. ACS element AM.I.E.K12 · difficulty: standard. Want to answer it in a timed quiz? Use the Aviation Mechanic General (AMG) practice mode.

Question bank updated 2026-09-22

QUESTIONA mechanic is judging the quality of a completed fillet weld. What depth of penetration is required for that weld to be acceptable?

  1. A25 to 50 percent of the base metal thickness
  2. B100 percent of the base metal thickness
  3. CTwice the thickness of the base metal being joined
Show the answer and explanation

Correct answer: A. 25 to 50 percent of the base metal thickness

  • A — correct On a fillet weld the penetration requirements are 25 to 50 percent of the thickness of the base metal.
  • B — incorrect Full penetration through the thickness of the base metal is the requirement for a butt weld, not for a fillet weld.
  • C — incorrect Penetration is measured as a fraction of the base metal thickness and never exceeds it; no requirement calls for twice the thickness.

BACKGROUND

Weld quality is judged in the field largely by visual means, and although appearance is not a positive indication of quality, it gives a good clue about the care used in making the weld. A properly designed joint weld is stronger than the base metal it joins. A good weld is uniform in width, its ripples are even and well feathered into the base metal, it shows no burn from overheating, and it 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 being joined, the size of the filler rod, and how the rod is added.

STUDY TIP

Keep the two penetration figures paired in memory: a butt weld needs the full thickness, a fillet weld needs only a quarter to a half of it. Expect the test to swap the two joint types.

KEY POINTS

  • Penetration is the depth of fusion, and thorough fusion is the most important characteristic of a sound weld.
  • A butt weld requires 100 percent penetration of the base metal thickness.
  • A fillet weld requires 25 to 50 percent penetration of the base metal thickness.

SOURCES

  • FAA-H-8083-30B ch10 "Inspection of Welds"
  • C:/Users/kuroh/faa-drill/sources/amg-general/ch10.txt (Inspection of Welds — penetration requirements)
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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.