Density Altitude is Pressure Altitude corrected for non-standard temperature. As temperature and altitude increase, air density decreases.
On a hot and humid day, the aircraft will accelerate more slowly, will need to move faster to attain the same lift and will climb more slowly. The less dense the air, the less lift, the more lackluster the climb, and the longer the distance needed for takeoff and landing. Fewer air molecules in a given volume of air also result in reduced propeller efficiency and therefore reduced net thrust. All of these factors can lead to an accident if the poor performance has not been anticipated.
Calculating Density Altitude
Density altitude in feet = pressure altitude in feet + (120 x (OAT - ISA temperature))
- Pressure altitude is determined by setting the altimeter to 1013.2 and reading the altitude indicated on the altimeter.
- OAT stands for outside air temperature (in degrees Celsius).
- ISA stands for International Standard Atmosphere standard temperature (in degrees Celsius).
The standard temperature is 15 degrees C but only at sea level. It decreases about 2 degrees C per 1,000 feet of altitude above sea level. The standard temperature at 7,000 feet msl, therefore, is only 1 degree C.
For example, the density altitude at an airport 7000 feet above sea level, with a temperature of 18 degrees Celsius and a pressure altitude of 7000 (assuming standard pressure) would be calculated as follows:
18 -1 = 17
17 x 120 = 2040
2040 + 7000=9040 feet Density Altitude
This means that the aircraft will perform as if it were at 9,040 feet.