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Guarding against frequency drift in resonance testing

Aviation Mechanic General (AMG) practice question 16 of 21 on Inspection Concepts and Techniques. ACS element AM.I.K.R2 · difficulty: recall. Want to answer it in a timed quiz? Use the Aviation Mechanic General (AMG) practice mode.

Question bank updated 2026-09-22

QUESTIONIn resonance-type ultrasonic thickness testing, how does the operator guard against possible drift of the instrument's frequency?

  1. ABy making checks with standard test blocks
  2. BBy applying a heavier layer of couplant before each reading
  3. CBy comparing readings with a reference standard of a different material
Show the answer and explanation

Correct answer: A. By making checks with standard test blocks

  • A — correct The handbook states that the frequency corresponding to a dial setting must be accurately known and that checks are made with standard test blocks to guard against possible drift of frequency.
  • B — incorrect Couplant is needed so ultrasonic energy can pass from the transducer to the part; it has nothing to do with verifying the instrument's frequency.
  • C — incorrect Reference standards are made to match the part being inspected; a standard of a different material would not serve as a frequency check.

BACKGROUND

In resonance thickness testing, the point where the frequency matches the resonance point of the material is the thickness-determining factor. For the reading to be valid, the frequency of the ultrasonic wave corresponding to a particular dial setting must be accurately known. To guard against possible drift of frequency, checks are made with standard test blocks. The instruments normally operate in four or five frequency bands, and calibrated transparent scales placed in front of the display allow thickness to be read directly.

STUDY TIP

Whenever a question concerns verifying a precision NDI instrument, look for the answer that involves a known standard: test blocks for resonance thickness gauges, reference standards for pulse-echo and eddy current.

KEY POINTS

  • Resonance thickness readings depend on the dial frequency being accurately known.
  • Standard test blocks are used to check for frequency drift.
  • Calibration of precision inspection equipment must be current before use.

SOURCES

  • FAA-H-8083-30B ch10 "Resonance"
  • FAA-H-8083-30B ch10 "General Techniques"
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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.