Aircraft instruments
Instrument questions test what each dial actually measures, which pressure source drives it, and how it fails.
Pitot-static
The ASI compares pitot (total) pressure with static pressure and is calibrated for sea-level density; it over-reads at high altitude and in high temperature. Blocked pitot with clear static: the ASI behaves like an altimeter. Blocked static: the ASI over-reads in climb and under-reads in descent; the altimeter freezes; the VSI reads zero. The altimeter is a barometer: 1 hPa ≈ 30 ft near sea level. VSI is a rate instrument with deliberate lag.
Gyroscopes and compass
Attitude and heading indicators rely on rigidity in space and are usually vacuum-driven; the turn coordinator uses a rate gyro and is often electric. The direct-reading magnetic compass suffers acceleration error (ANDS: accelerate north, decelerate south) and turning error, worst on north/south headings.
Worked example
QNH is 1003 hPa but the altimeter subscale is set to 1013. The instrument thinks the pressure at sea level is 10 hPa higher than it is, so on the ground at sea level it indicates 10 × 30 = 300 ft. Set the correct QNH before departure and below transition altitude; use 1013 above transition level.
Traps
Using QFE where QNH is required; forgetting that indicated altitude is not true altitude; reading the VSI as instantaneous; confusing vacuum failure of the attitude indicator with a pitot blockage; ignoring compass turning error when rolling out on north.