Computing Density Altitude
Density altitude is another value the crew has asked you to report. Density altitude relates to the station pressure and pressure altitude you reported previously. You can use available computer applications to provide an accurate value. Density altitude can be calculated using applications available through the Geophysical Fleet Mission Program Library (GFMPL) and the National Oceanic and Atmospheric Administration (NOAA) or can be computed directly based on your measured parameters.
Example NOAA Density altitude calculator
Once you have obtained a pressure altitude value (see previous page), you can use that value to calculate the density altitude.
Here is the formula for computing density altitude
density altitude = pressure altitude + [120 x (air temperature °C - standard temperature for your elevation)]
Let's consider an air temperature of 35 °C on a rear flight deck located 50 feet above the sea surface. The pressure altitude is 520 feet. What would the corresponding density altitude be?
First, you need to determine the standard atmosphere temperature at your elevation. The standard atmosphere temperature is 15 °C at sea level. You can compute the standard temperature for your elevation using this formula:
standard temperature = - [2 x (elevation in thousands of feet) - 15]
If the rear flight deck is 50 feet above the sea surface, your standard temperature = - [2 x (0.05) - 15] = 14.9.
Then, the density altitude at 50 feet for air temperature 35 °C and pressure altitude 520 feet is determined as follows:
density altitude = 520 + [120 x (35 - 14.9)] = 2932 feet.
Based on your observations, the station pressure on your flight deck, 150 feet above the sea surface, is 29.09 in Hg (985.1 hPa). The air temperature is 31.6 °C and the pressure altitude is 980 feet. Use the formulas presented in this section to determine the density altitude.
standard atmosphere temperature at elevation = - [2 x (elevation in thousands of feet) - 15
density altitude = pressure altitude + [120 x (air temperature °C - standard temperature for your elevation)]
Hint: Your first step is to compute a standard atmosphere temperature at 150 feet, equal to 14.7 °C.
Choose the best answer.
The correct answer is c), 3008 feet.
density altitude = pressure altitude + [120 x (air temperature °C - standard temperature for your elevation)]
density altitude = 980 + [120 x (31.6 - 14.7)] = 3008 feet
Density altitude depends on air temperature and on the station pressure, which is based on elevation.