Density Altitude
Density altitude is pressure altitude corrected for non-standard temperature. It is the altitude in the International Standard Atmosphere (ISA) at which the air density corresponds to the actual air density at the aircraft’s location. Density altitude is the altitude the aircraft “feels” for performance purposes.
Why Density Altitude Matters
Aircraft performance — thrust, lift, and drag — depends on air density. At high density altitude, the air is thin: the engine produces less power, the wings generate less lift, and the propeller produces less thrust. Takeoff distance increases, rate of climb decreases, and stall speed (IAS) remains the same but TAS is higher. Aircraft performance charts use density altitude as the primary variable.
Calculating Density Altitude
Approximately: Density altitude = Pressure altitude + (120 × (OAT − ISA temperature)). ISA temperature at sea level is 15°C, decreasing at 2°C per 1,000 ft. Example: at a 5,000 ft elevation airport on a 35°C day, ISA temperature would be 5°C. Density altitude ≈ 5,000 + (120 × (35 − 5)) = 5,000 + 3,600 = 8,600 ft. The aircraft performs as if it were at 8,600 ft.
High-Density-Altitude Operations
- High elevation airports on hot days produce the worst conditions
- Takeoff roll can double or triple compared to sea-level performance
- Rate of climb may be dramatically reduced — check the POH climb chart
- Lean the mixture properly — the engine will run rich at altitude with mixture at full rich
- Consider early morning departures when temperatures are lowest
On the TC Written Exam
Density altitude is a guaranteed exam topic. Know the definition; know it increases with high temperature, high altitude, and high humidity; know its effect on all aspects of aircraft performance.
Related Terms
See also: Pressure Altitude, Altitude, Standard Atmosphere.