Aircraft General Knowledge - Instruments
What this domain tests
Instruments covers the pitot-static group, gyroscopics, the magnetic compass and modern EFIS/ADC/IR architecture. Questions test error sources and what the pilot actually sees when a source fails.
Core rules
- Pitot-static: the ASI compares dynamic pressure; the altimeter senses static pressure against a set subscale; the VSI measures rate of change of static pressure. Blocked pitot makes the ASI behave like an altimeter; blocked static makes the ASI over-read in a climb.
- Altimetry: QNH gives elevation above mean sea level; QFE gives height above the reference datum. 'High to low, look out below': flying toward lower pressure without resetting means true altitude is lower than indicated.
- Gyroscopics: rigidity and precession; an air-driven artificial horizon relies on suction, so a pump failure erodes both the AH and the turn indicator.
- EFIS: ADCs combine pitot-static and temperature data; IRS/INS supplies attitude and heading and drifts slowly, requiring alignment before flight.
Worked example
Aerodrome elevation 300 ft, QNH 1013 hPa. Using about 27 ft per hPa, QFE is approximately 1013 minus (300/27) = 1013 minus 11 = 1002 hPa. Set 1002 on the subscale and the altimeter reads zero on the ground at the reference point.
Traps
Confusing indicated, calibrated, true and density altitude; the exam often gives one and asks for another. Machmeter indications depend on static pressure and temperature, not just speed. Do not forget position error and compressibility error at high Mach.
Key habit
For any failure, first ask which pressure source is affected and what the instrument will indicate while you troubleshoot.