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Range and accuracy of dial-type ultrasonic thickness units

Aviation Mechanic General (AMG) practice question 18 of 21 on Inspection Concepts and Techniques. ACS element AM.I.K.R1 · difficulty: recall. Want to answer it in a timed quiz? Use the Aviation Mechanic General (AMG) practice mode.

Question bank updated 2026-09-22

QUESTIONDirect-reading, dial-operated ultrasonic units measure material thickness over what range and with what accuracy?

  1. A0.001 inch to 12 inches, better than 5 percent accuracy
  2. B0.025 inch to 3 inches, better than 1 percent accuracy
  3. C0.080 inch to 3 inches, better than 10 percent accuracy
Show the answer and explanation

Correct answer: B. 0.025 inch to 3 inches, better than 1 percent accuracy

  • A — incorrect Neither the range nor the accuracy matches the handbook figures of 0.025 inch to 3 inches at better than 1 percent.
  • B — correct The handbook states that direct-reading dial-operated units are available that measure thickness between 0.025 inch and 3 inches with an accuracy of better than 1 percent.
  • C — incorrect The figure 0.080 inch is the skin-thickness limit for tap testing, not the lower limit of ultrasonic thickness measurement, and the accuracy is better than 1 percent.

BACKGROUND

The resonance thickness instrument can measure the thickness of metals such as steel, cast iron, brass, nickel, copper, silver, lead, aluminum and magnesium, and can locate and evaluate areas of corrosion or wear on tanks, tubing, wing skins and other structures. Direct-reading, dial-operated units are available that measure thickness between 0.025 inch and 3 inches with an accuracy of better than 1 percent. Ultrasonic inspection requires a skilled operator who is familiar with the equipment and with the inspection method appropriate to the parts being tested.

STUDY TIP

Memorize the trio 0.025 inch, 3 inches and 1 percent for dial-type ultrasonic thickness units; distractors typically swap in numbers from other methods.

KEY POINTS

  • Resonance ultrasonic units measure thickness in a wide range of metals.
  • Direct-reading dial units cover 0.025 inch to 3 inches.
  • Their accuracy is better than 1 percent.

SOURCES

  • FAA-H-8083-30B ch10 "Resonance"
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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.