Pressure, Density and Altimetry
This domain tests the altimeter chain from station pressure to the height you fly, and the errors that make an altimeter read incorrectly.
Pressure references
QFE is the pressure at aerodrome elevation. QNH is aerodrome pressure reduced to mean sea level using ISA. QNE is the standard setting 1013.25 hPa. Below the transition altitude you fly QNH; above the transition level you fly standard pressure, and the transition layer is the buffer between them. Near sea level 1 hPa is about 30 ft.
Altitudes
Indicated altitude uses QNH; pressure altitude uses 1013 hPa; true altitude is height above MSL; absolute altitude is above ground; density altitude is pressure altitude corrected for temperature, roughly plus 120 ft per 1°C above ISA.
Worked example
Aerodrome elevation 1,000 ft, QNH 1003 hPa. Pressure altitude = 1,000 + (1013 - 1003) × 30 = 1,300 ft. Temperature 40°C; ISA at 1,000 ft is 15 - (0.305 × 6.5) ≈ 13°C, so the ISA deviation is +27°C. Density altitude ≈ 1,300 + 27 × 120 ≈ 4,540 ft. Take-off run, climb rate and engine performance all degrade accordingly.
Traps
"High to low, look out below": flying toward lower pressure or colder air with the subscale unchanged leaves the aircraft lower than indicated. Do not confuse QFE with QNH. In Sindh and Balochistan summer heat, density altitude can exceed pressure altitude by several thousand feet.