Density Altitude (DA) is the pressure altitude corrected for temperature deviation from ISA — the altitude in the ICAO Standard Atmosphere where air density would equal current density.
Why important?
Aircraft performance depends on air density, not direct altitude:
- Higher DA → lower density → less engine power, less lift at the same TAS, longer take-off distance, higher TAS-stall speed.
- Mnemonic: Hot, High, Humid = High DA = degraded performance.
Rule of thumb
code
DA ≈ pressure altitude + 120 × ISA deviation
ISA deviation = OAT − (15 − 2 × Alt in 1 000 ft)
For precision use a navigation computer (CRP-5/E6B) or read directly from the AFM performance chart.
Worked example
Given: pressure altitude 3 000 ft, OAT +30 °C.
- ISA temperature at 3 000 ft = 15 − 2 × 3 = +9 °C
- ISA deviation = 30 − 9 = +21 °C
- DA ≈ 3 000 + 120 × 21 = 5 520 ft
The aircraft "thinks" it is at 5 520 ft — take-off distance, climb rate, cruise speed and TAS behave accordingly.
Effects with increasing DA
| Parameter | Effect |
|---|---|
| TAS at same IAS | Increases (Δ ≈ 2 % per 1 000 ft DA) |
| Take-off distance | Increases significantly |
| Climb rate | Decreases |
| Climb gradient | Decreases |
| Engine power | Decreases (except turbocharged) |
| Stall speed (IAS) | Roughly constant; TAS-stall increases |
| Landing distance | Increases (higher TAS at touchdown) |
Operational practice
- High-DA summer days: always check AFM charts, do not assume standard values.
- High-altitude airfields (e.g. Aspen, Innsbruck in summer) with high OAT: significantly reduced payload or extended runway requirements.