Density altitude and aircraft performance
Why hot, high and humid conditions lengthen take-off runs and reduce climb performance.
Definition
Density altitude is the altitude in the standard atmosphere at which the air has the same density as the air at your location. It is pressure altitude corrected for non-standard temperature. Aircraft performance (engine power, propeller thrust, wing lift) depends on air density, so performance charts are entered with pressure altitude and temperature.
Rule of thumb: density altitude rises by roughly 120 ft for every 1 °C above ISA temperature. Humidity increases it further (moist air is less dense), which the dry-air formula does not include.
Examples (dry air, OIBK engine)
Kish OIBK, elev 101 ft, QNH 1004, OAT 37 °C → PA ≈ 355 ft, DA ≈ 2,920 ft Tehran OIII, elev 3,962 ft, QNH 1015, OAT 36 °C → PA ≈ 3,914 ft, DA ≈ 7,120 ft
Effects
Always use the aircraft flight manual performance charts with the actual conditions and applicable safety factors.
- Longer take-off and landing distances (higher true airspeed for the same indicated airspeed).
- Reduced rate and angle of climb.
- Reduced power from normally aspirated engines.
References
- FAA-H-8083-25 Pilot's Handbook of Aeronautical Knowledge, ch. 11 — Aircraft performance
- ICAO Doc 7488 — Manual of the ICAO Standard Atmosphere
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