Pressure Instruments and Air Data
Domain 2 covers the pitot-static system and everything derived from it - airspeed, altitude, vertical speed, Mach - plus the air data computer that combines them. The exam tests the pressure relationships, the definition of each speed and altitude, the error sources, and the operational rules such as transition altitude and RVSM.
Pressure relationships
Total pressure = static pressure + dynamic pressure, with q = ½ρV². The airspeed indicator is a differential pressure gauge measuring q, but its dial is calibrated for ISA sea-level density, so it displays indicated airspeed.
The speed chain
IAS → CAS (corrected for position/static-source and instrument error) → EAS (corrected for density) → TAS. Rule of thumb: TAS rises about 2 % per 1000 ft of altitude at constant IAS. Compressibility error makes the ASI over-read at high Mach and high altitude, which is why a Machmeter is used. M = TAS ÷ a, with a = 38.94 × √(SAT in K) knots.
Altimetry
A law-divided aneroid capsule. Subscale settings: QNH (altitude above mean sea level), QFE (height above the aerodrome reference point), and 1013.25 hPa (flight levels). One hPa is about 27–30 ft near sea level. Errors: position (static port), instrument, pressure-setting, temperature and lag. In air colder than ISA the altimeter over-reads - the true altitude is lower.
Vertical speed and Mach
The VSI is a differential capsule with a calibrated leak, so it lags; the instantaneous VSI adds an accelerometer to anticipate. RVSM requires two independent altitude measurement systems, an altitude-hold autopilot, an altitude alert and TCAS.
Worked example
Cruising at FL350 with 1013 set, you enter airspace with QNH 995 but do not reset. The difference is 18 hPa × 27 ft ≈ 490 ft: the indicated altitude is about 490 ft higher than the true altitude relative to the new pressure datum. The aircraft is lower than the altimeter suggests.
Traps that cost marks
- Saying QNE is a subscale setting - it is the standard pressure 1013.25 hPa; the setting itself is "standard".
- Mixing up "high to low, look out below" (pressure error) with the cold-temperature error.
- Forgetting that static blockage freezes the altimeter and zeroes the VSI, while total-pressure blockage lets the ASI behave like an altimeter.