Which tool
Where to buy| Tool | What it measures | Source |
|---|---|---|
| Slide caliper with depth rod | Outside and inside dimensions, and the depth of blind holes | H-30B p. 11-25 |
| Depth micrometer | One of four micrometer types; read the same way as an outside micrometer | H-30B p. 11-19 |
| Dial indicator | Variations in a surface, with a probe linked to a dial reading thousandths of an inch (or an LCD) | H-30B p. 11-24 |
| Depth gauges in corrosion inspection | Named among the tools used during visual inspection for corrosion, with mirrors and optical micrometers | H-30B Ch. 8 |
H-30B = FAA-H-8083-30B, AMT Handbook: General.
Measure the depth of a hole or pit
| Step | Action | What to do |
|---|---|---|
| 1 | Choose the tool | A caliper depth rod for a blind hole; a depth micrometer for a finer reading. For a pit or dent, check which tool the repair data names. |
| 2 | Zero and calibrate | Follow the manufacturer's instructions for calibration and the gauges to use; the Handbook does not give a zeroing procedure. |
| 3 | Seat the base | Rest the base or the caliper beam square on the surface; tilt gives a wrong depth. |
| 4 | Extend to the bottom | Bring the depth rod or spindle to the bottom with light pressure. |
| 5 | Read | Read the scale as for a caliper or micrometer; digital readouts show it directly. |
| 6 | Compare with the limit | Compare with the repair data limit, for example the maximum blend-out depth. |
Typical mistakes
| Mistake | What happens | Fix |
|---|---|---|
| Base not square to the surface | Reads deeper or shallower than the truth | Seat flat, hold square |
| Too much pressure on the rod | Compresses a soft surface, sprung tool | Light, steady pressure |
| Skipping calibration | Systematic error | Check against a known gauge first |
| Judging a limit from memory | Wrong decision | Take the limit from the repair data |
Care
Keep the rod and beam clean, undamaged and straight; a bent depth rod reads wrongly. Dropping a caliper or micrometer can permanently affect its accuracy (H-30B p. 11-24). Calibration is the maker's.
Questions people actually ask
Does my caliper have a depth gauge?
Some slide calipers contain a depth gauge for measuring the depth of blind holes (H-30B p. 11-25). Look for the rod at the end of the beam.
Which limit for a corrosion pit?
Not in the handbooks: the SRM or repair data gives the allowed depth. The blend-out calculator works with a depth you measure.
Limits of this guide
What the sources support, and no further.
- Not approved data. Depth limits for corrosion, dents and countersinks come from the SRM, not from this page.
- No procedure for zeroing. The FAA handbooks do not describe one for depth tools; follow the maker.
- Few figures. The sources give the tools but few numbers, so this page has few.
Where to buy
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Sources
- FAA-H-8083-30B, AMT Handbook: General (2023) — Ch. 11, pp. 11-19, 11-24 to 11-25, "Micrometer Calipers", "Dial indicator", "Slide Calipers"; Fig. 11-41; Ch. 8, visual inspection for corrosion.
Educational reference only. not approved data. Always consult the applicable SRM, AMM, or AC 43.13-1B and your certifying authority.