The International Standard Atmosphere & Density Altitude

1 — The International Standard Atmosphere (ISA)

Every performance number in your POH was calculated against a single agreed atmosphere. That agreed model is the ISA — think of it as a “factory specification” for the air.

The key values to memorise: 15 °C and 1013.25 hPa at mean sea level, a lapse rate of 2 °C per 1 000 ft, and the tropopause at 36 090 ft where the temperature settles at −56.5 °C. The shortcut for any altitude:

2 — Three Altitudes You Must Know

Indicated altitude is what your altimeter shows with QNH set. Pressure altitude (PA) is what it shows with 1013.25 hPa set — every POH chart uses PA. Density altitude (DA) is the altitude in a standard atmosphere where the air density equals your actual density: the altitude the aircraft performs as if it is at.

3 — Interactive Density Altitude Calculator

Drag the sliders and watch the three altitudes respond — feel the gap between where you are and where the aircraft thinks it is.

4 — Worked Example (Exam-Style)

Given elevation 1 500 ft · QNH 1 001 hPa · OAT 28 °C, find the density altitude:

1. PA = 1500 + (1013 − 1001) × 30 = 1 860 ft
2. ISA temp at PA = 15 − (2 × 1.86) ≈ 11 °C
3. DA = 1860 + 120 × (28 − 11) = 3 900 ft

5 — What High Density Altitude Does to Your Aeroplane

Less dense air means less oxygen, less lift, less thrust — every system that breathes air is impaired. The takeoff roll is the clearest victim: