Atmospheric Physics and the International Standard Atmosphere
Why the ISA matters
All flight manual charts are built on the International Standard Atmosphere, so a candidate must be fluent in its constants and in how real atmospheres deviate. ISA sea level: 1013.25 hPa, 15 °C, density 1.225 kg/m³, temperature lapse 1.98 °C per 1,000 ft up to 36,090 ft (tropopause, −56.5 °C), then isothermal. Pressure lapse near sea level is about 1 hPa per 27 ft; pressure halves roughly every 18,000 ft.
Density, pressure and temperature
Lift, drag and engine power all scale with density, which falls with altitude, temperature and humidity. Density altitude is the altitude in the ISA at which the actual density occurs - the single most useful number for performance. Pressure altitude is what the altimeter shows with 1013 set; true altitude corrects for temperature and pressure error. Hot day = high density altitude = long take-off, poor climb.
Humidity
Water vapour is lighter than air, so humid air is less dense. The effect is small but real (roughly 1% density loss per 10 g/kg of humidity at low level) and is often ignored in simple charts - a classic exam trap when a humid coastal runway is compared with a dry inland one at the same temperature.
Worked example
Airport elevation 2,000 ft, QNH 1003, OAT 30 °C. Pressure altitude = 2,000 + (1013 − 1003) × 27 ≈ 2,270 ft. ISA temperature at 2,270 ft ≈ 15 − 2 × 2.27 ≈ 10.5 °C. ISA deviation = 30 − 10.5 = +19.5 °C. Density altitude ≈ pressure altitude + 120 × ISA deviation ≈ 2,270 + 2,340 ≈ 4,610 ft.
Traps
Mixing QFE and QNH; forgetting the 27 ft/hPa sign; using temperature lapse above the tropopause; assuming humidity increases performance.