Flight Instruments and Pressure Systems
Where the numbers come from
This domain is examined as both instrument theory and practical fault diagnosis: what each pressure instrument measures, how it fails, and what the crew actually sees. Pitot pressure is total pressure (static plus dynamic); static pressure is ambient pressure. The ASI displays the difference, the altimeter displays static pressure against a selected subscale, and the VSI displays the rate of change of static pressure through a calibrated leak.
Altimetry rules
- Subscale settings: QFE reads height above the aerodrome, QNH reads altitude above mean sea level, and 1013.2 hPa (QNE) is used above the transition altitude for flight levels.
- Pressure: 1 hPa is about 30 ft near sea level. Flying from high to low pressure at a constant indicated altitude, true altitude decreases - 'high to low, look out below'.
- Temperature: cold air is denser, so the altimeter over-reads. A working correction is 4 ft per 1,000 ft per °C of ISA deviation.
Speed and Mach
IAS is converted to CAS for position and instrument error, to EAS for compressibility, then to TAS for density. Mach is TAS divided by the local speed of sound; at constant IAS, Mach rises with altitude, and at constant Mach, IAS falls.
Worked example
Indicated altitude 5,000 ft, OAT −15 °C. ISA temperature at 5,000 ft is 15 − (2 × 5) = 5 °C, so the deviation is 20 °C cold. Correction = 4 × 5 × 20 = 400 ft. Cold air means the aircraft is lower than indicated, so true altitude is about 4,600 ft.
Traps
Confusing QFE with QNH on approach; applying the temperature correction in the wrong direction; forgetting that the altimeter under-reads when flying from low to high pressure; assuming VSI lag is negligible during a rapid level-off; and mixing up pitot blockage (ASI behaves like an altimeter) with static blockage (altimeter freezes, VSI reads zero, ASI over-reads in a descent).