Aircraft instruments
Instruments
The questions test the pitot-static and gyro systems, their errors, and how to interpret readings when a source is blocked or a setting is wrong.
Core rules
- The ASI uses pitot (total) pressure and static pressure; it indicates dynamic pressure, so it is calibrated to TAS only at sea level and standard density. It overreads at altitude for a given TAS (compressibility).
- The altimeter is a pressure gauge set to QNH, QFE or 1013.2 hPa. QNH gives elevation; QFE gives height above the aerodrome; the standard setting is used above transition altitude.
- 1 hPa ≈ 27 ft near sea level. If pressure falls and you do not reset, the altimeter overreads.
- The VSI is a rate instrument with lag; it shows zero after a level-off only when the climb or descent has stabilised.
- Attitude and heading indicators are gyroscopic, vacuum or electric drive. The heading indicator precesses and must be aligned with the magnetic compass.
- The turn coordinator shows rate of turn and coordination; the inclinometer shows slip/skid, not bank angle.
- Magnetic compass: variation is geographic, deviation is aircraft-specific; acceleration and turning errors (ANDS/UNOS) are largest on easterly/westerly headings.
Worked example
QNH at departure is 1013 hPa; at destination it is 1003 hPa and you leave the altimeter set at 1013. The aircraft is 10 hPa lower in pressure than the setting implies, so the altimeter reads about 270 ft too high. Descending to an indicated 2,000 ft puts you near 1,730 ft true altitude - a terrain-risk error.
Traps
Confusing QFE with QNH; forgetting temperature error (high temperature → altimeter underreads); treating ASI as TAS; ignoring VSI lag; and using the turn needle as a bank indicator.