The atmosphere, temperature, pressure, density, standard atmosphere and altimetry
This domain tests whether you can convert between the atmosphere’s physical variables and the altimeter and aircraft-performance consequences. Expect ISA values, lapse rates, pressure–height relationships, QNH/QFE/QNE and temperature-error corrections.
Standard atmosphere and pressure
The ICAO ISA assumes MSL temperature 15°C, pressure 1013.25 hPa, density 1.225 kg/m³ and a tropospheric lapse rate of 6.5°C/km (about 1.98°C/1000 ft) up to 11 km. Pressure falls roughly 1 hPa per 27–30 ft near MSL. Pressure altitude is the altitude indicated when the subscale is 1013.25 hPa; density altitude is pressure altitude corrected for non-ISA temperature.
Altimetry
QNH makes the altimeter read aerodrome elevation; QFE reads height above the aerodrome reference point; QNE is the standard setting 1013.25 hPa used at or above transition altitude. The transition altitude is fixed by procedure; the transition level varies with QNH. Cold air makes true altitude lower than indicated, so minimum safe altitudes need a positive correction.
Worked example
Aerodrome elevation 1,000 ft, QNH 1003 hPa. Pressure altitude = 1,000 + (1013 − 1003) × 27 ≈ 1,270 ft. If the aircraft is at 5,000 ft indicated with mean temperature 15°C colder than ISA, true altitude is about 5,000 − (5 × 15 × 4) = 4,700 ft, using approximately 4 ft per 1,000 ft per °C. The aircraft is lower than indicated.
Traps
Do not confuse QFE with QNH. “High to low, look out below” describes the pressure error. Pressure altitude is not density altitude, and lapse rate means temperature decreasing with height, not increasing.