Density altitude and why hot days are dangerous

What density altitude is, how to estimate it quickly, and how it lengthens your takeoff run and cuts your climb.

The short answer

Density altitude is the altitude at which the aircraft feels like it is flying, given the actual air density. Hot, high and humid air is thin, so the engine makes less power, the propeller bites less and the wing needs more speed. On a hot day at a high airfield the aircraft can perform as if it were thousands of feet higher.

A quick estimate

Start with pressure altitude: field elevation plus 30 ft for every hPa that QNH is below 1013, minus 30 ft for every hPa above. Then add about 120 ft for every degree Celsius the temperature is above the standard temperature at that height (15 °C at sea level, falling 2 °C per 1,000 ft).

An example

An airfield at 2,000 ft, QNH 1013, temperature 31 °C. Standard temperature there is 11 °C, so it is 20 °C warmer. 20 × 120 = 2,400 ft, giving a density altitude of about 4,400 ft.

What to do about it

• Use the performance charts in your flight manual for the real figures, and add a safety margin
• Fly early in the morning when it is cooler
• Reduce weight: fewer passengers, less fuel if the trip allows
• Lean the mixture for best power before takeoff where the manual allows it