At aneroid-instrument facilities, the margin for error on the comparison of two altimeter setting indicators is +/- ___ in. Hg.

• A. 0.02
• B. 0.03
• C. 0.04
• D. 0.05

Answer: (D)

The key idea is that calibration labs set a strict tolerance for how closely two altimeter setting indicators must agree. In aneroid-instrument facilities, the difference between two indicators must be within plus or minus 0.05 inches of mercury. This small margin ensures both indicators read essentially the same setting, which keeps altitude readings reliable during tests and prevents a noticeable drift in indications.

That 0.05 inHg tolerance translates to about a 50-foot potential difference in altitude under standard conditions, which is tight enough to catch meaningful discrepancies yet practical for routine calibration. If the difference were allowed to be larger, readings could diverge enough to mislead pilots or maintenance crews; if it were smaller, calibrations would be more costly and time-consuming for little added benefit.