Precision Measuring for the A&P Practical: Micrometers and Dial Indicators
For precision measuring on the Airframe and Powerplant (A&P) practical, be ready to show how you obtained each measurement, not only the number. On the oral and practical (O&P) test, each practical project pairs a hands-on skill element with two practical questions. This guide covers an inch micrometer, its vernier scale, and a dial indicator runout setup.
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What does the O&P practical evaluate?
The Federal Aviation Administration (FAA) Airman Certification Standards (ACS) list the hands-on skills below. A Designated Mechanic Examiner (DME) evaluates the assigned project against the performance standard in the downloaded test.
FAA Order 8900.1, Vol. 5, Ch. 5, Sec. 11, CHG 929, printed 16, PDF 16, 5-5-11-7C3)g)
Each practical project contains a skill element and two practical questions. You may use applicable reference material for both the skill and the questions.
FAA Order 8900.1, Vol. 5, Ch. 5, Sec. 11, CHG 929, printed 16, PDF 16, 5-5-11-7C3)c)
A project is pass/fail. Passing requires a successful skill demonstration on the first attempt and successful answers to both associated practical questions.
FAA Order 8900.1, Vol. 5, Ch. 5, Sec. 11, CHG 929, printed 16, PDF 16, 5-5-11-7C3)f)
The FAA Mechanic Test Generator (MTG) FAQ says projects are standalone. The two practical questions are asked at a safe and appropriate time during the project.
FAA Mechanic Test Generator FAQs, updated May 22, 2023, unnumbered page, PDF 4
The FAA sample test shows the project format: description, given equipment or information, performance standard, ACS code, references, and associated questions. It is marked for practice purposes only.
FAA sample oral and practical test, PDF 6
Our study advice: use this guide to rehearse the sequence and explain your decisions. A published skill is a study target, not a promise that your downloaded test will assign this exact task.
| ACS skill | Exact published skill |
|---|---|
| AM.I.E.S6 | Make precision measurements with an instrument that has a Vernier scale. |
| AM.I.E.S7 | Check the concentricity of a shaft. |
| AM.I.E.S14 | Check for proper calibration of a micrometer. |
| AM.I.K.S2 | Use micrometers. |
FAA-S-ACS-1, printed 7, PDF 12
FAA-S-ACS-1, printed 8, PDF 13
FAA-S-ACS-1, printed 17, PDF 22
Start with the assigned data and a safe setup
Use current maintenance and overhaul publications and procedures. The ACS safety appendix also lists proper information and tools, preparation of the equipment, skill in handling tools, and safety precautions such as wearing safety glasses, hearing protection, and any other required personal protective equipment (PPE).
FAA-S-ACS-1, printed A-3, PDF 67
Disregard for safety will result in failure. If a project requires an action that would jeopardize safety, the evaluator will ask you to simulate that portion.
FAA-S-ACS-1, printed A-3, PDF 67
Our study advice: identify the applicable manufacturer’s instructions and any required approved data before starting. The handbook summaries in this guide do not replace the procedure for the assigned aircraft, engine, component, or equipment.
How do you prepare the micrometer?
Handle the micrometer carefully. Dropping it may permanently affect its accuracy. Continually sliding work between the measuring faces may wear them; overtightening the spindle may permanently spring the frame and produce inaccurate readings.
FAA-H-8083-30B, General, printed 11-24, PDF 439
Follow the instrument manufacturer’s calibration procedures and specified gauges, such as gauge blocks, gauge pins, or ring gauges. Hold the frame so your thumb and forefinger remain free to turn the thimble.
FAA-H-8083-30B, General, printed 11-24, PDF 439
Our study advice: before reporting a dimension, identify the instrument’s range, units, scale increments, and calibration status.
The General handbook’s lathe safety precautions specifically call for the work to stop before it is measured.
FAA-H-8083-30B, General, printed 1-3, PDF 24
How do you read an inch micrometer?
On the inch micrometer described in the General handbook, the numbered barrel divisions represent tenths of an inch. Each smaller barrel division is 0.025 inch, and one full thimble revolution moves the spindle 0.025 inch.
FAA-H-8083-30B, General, printed 11-22, PDF 437 · FAA-H-8083-30B, General, printed 11-23, PDF 438
The thimble divides that 0.025 inch into 25 parts, each worth 0.001 inch. Read the visible barrel amount and add the aligned thimble amount. A micrometer with the described vernier scale divides 0.001 inch into ten parts of 0.0001 inch.
FAA-H-8083-30B, General, printed 11-23, PDF 438
For a vernier reading, add the barrel, thimble, and vernier amounts. Use the vernier line that aligns exactly with a thimble graduation.
FAA-H-8083-30B, General, printed 11-23, PDF 438
Illustration, calculated for this guide: suppose the last numbered barrel mark is 0.300 inch, two additional 0.025-inch barrel divisions are visible, the thimble contributes 0.016 inch, and vernier line 7 aligns. The reading is 0.300 + (2 × 0.025) + 0.016 + (7 × 0.0001) = 0.3667 inch. This is an arithmetic illustration, not an aircraft limit or a reading taken from an actual part.
How do you show dial indicator runout?
The handbook’s example places a shaft between machined V-blocks and clamps the dial indicator to a stand. Position the probe so it lightly contacts the surface. Rotate the outer dial to put the needle at zero, then rotate the part and observe the needle’s fluctuation.
FAA-H-8083-30B, General, printed 11-24–11-25, PDF 439–440
The total fluctuation is the runout. The indicator must be securely fastened so movement of the instrument itself does not introduce measurement error.
FAA-H-8083-30B, General, printed 11-25, PDF 440
For the handbook’s crankshaft inspection example, use the support locations specified in the engine overhaul manual and compare total indicator reading with the manufacturer’s table of limits. A shaft beyond those limits must not be reused; a bent crankshaft should not be straightened.
FAA-H-8083-32B, Powerplant, Chapter 10, printed 10-9, PDF 9
Which mistakes should you catch before reporting the result?
Our study checklist: do not confuse 0.025-inch barrel divisions with 0.001-inch thimble divisions; do not add a vernier digit until you identify the aligned line. Read the units aloud and show the addition. For runout, report the total fluctuation and the applicable limit separately.
Do not try to improve a crankshaft reading by straightening a bent shaft. The handbook says that attempt ruptures the nitrided journal surface and causes eventual crankshaft failure.
FAA-H-8083-32B, Powerplant, Chapter 10, printed 10-9, PDF 9
Frequently asked questions
Must every micrometer scale be visible at the same time?
No. The General handbook says all three scales on a micrometer are not fully visible without turning the micrometer. The handbook figures are drawn as though the barrel and thimble were laid out flat, so all three scales show at once; the instrument in your hand does not look like that.
FAA-H-8083-30B, General, printed 11-23, PDF 438
Can a dial indicator check something besides shaft runout?
The handbook also describes checking a rotating component such as a brake disc for warp. In some cases the disc can remain installed, with the indicator base clamped to a stationary part of the structure.
FAA-H-8083-30B, General, printed 11-25, PDF 440
Will the examiner choose my measuring tools during the O&P practical?
Not directly. The examiner must ensure you can find the tools, equipment, and materials that may be needed, but should not provide the specific tools and data for the task directly. The examiner evaluates whether you can select the appropriate tools and data needed for the assigned task.
FAA Order 8900.1, Vol. 5, Ch. 5, Sec. 11, CHG 929, printed 16, PDF 16, 5-5-11-7C3)h)
Rehearse the spoken answer with DMEReady
DMEReady is an AI voice examiner for the A&P oral in your phone or computer browser. You answer out loud, and it asks follow-up questions the way a DME would. You can also rehearse an oral built from the ACS codes on your Airman Knowledge Test Report (AKTR). It rehearses the oral; it does not train hands-on practical skills. After each session, the report shows what you covered, what you missed, what a complete answer includes, and where to read in the FAA handbooks. You can save the report as a PDF study sheet and share a result image that never shows your answers or name. The first drill is free, with no sign-up. DMEReady is an AI examiner, not a human DME. It is independent and is not affiliated with or endorsed by the FAA; it does not predict or guarantee results.
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Related guides
- Aircraft Hardware and Materials Oral Exam Questions and Answers
- Inspection Concepts and Techniques Oral Exam Questions and Answers
- Torque on the A&P Practical: Wrench Use, Extensions and Values
- A&P Practical Projects: What the DME Evaluates on the O&P
- FAA A&P Practical Test Changes 2026: What's New
Sources
Primary sources checked October 20, 2026.
- FAA Order 8900.1, Vol. 5, Ch. 5, Sec. 11, CHG 929
- FAA Mechanic Test Generator FAQs, updated May 22, 2023
- FAA sample oral and practical test
- FAA-S-ACS-1
- FAA-H-8083-30B, General
- FAA-H-8083-32B, Powerplant, Chapter 10
About this guide: drafted with AI from the FAA sources listed on this page and checked claim by claim against them. Not reviewed by an A&P mechanic or DME. How we make our guides
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