In the density altitude formula: DA = PA + [120 x (OAT - ISA Temp.)], should one use the ISA temperature at the field elevation or the ISA temperature at the pressure altitude?
Calculating Density Altitude
Density altitude in feet = pressure altitude in feet + (120 x (OAT - ISA temperature))
Altimeter Pressure altitude is determined by setting the altimeter to 29.92 and reading the altitude indicated on the altimeter. OAT stands for outside air temperature (in degrees Celsius).
ISA stands for standard temperature (in degrees Celsius).
Keep in mind the standard temperature is 15 degrees C but only at sea level. It decreases about 2 degrees C (or 3.5 degrees F) per 1,000 feet of altitude above sea level. The standard temperature at 7,000 feet msl, therefore, is only 1 degree C (or 34 degrees F). 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 the aircraft will perform as if it were at 9,040 feet
ISA is 15 degrees C at sea level. For the calculation, decrease temperature by 2 degrees C for every 1,000 ft of altitude above sea level.