At my next stage check, I know I’m going to have to explain pressure altitude and density altitude. And when I become a CFI I’m going to have to teach other people what they are, so I need to learn it. And understand it. And most importantly, be able to explain it. Thank goodness for Youtube videos, I can get some free education that will supplement everything I’m paying to learn at Pinnacle.
Pressure altitude is simple – it’s what you get when you dial 29.92 in your altimeter.
Density altitude is pressure altitude corrected for temperature and humidity. Or, pressure altitude corrected for nonstandard temperature. Another way of thinking about density altitude is this: Density altitude is the altitude the airplane feels like it’s at.
At some point my examiner is going to ask me to calculate density altitude. I know there are two pieces of information I need to figure this out. Pressure altitude and temperature. I can get the temperature right off the thermometer. I can find the pressure altitude by setting the altimeter to 29.92 and reading the altitude off the altimeter. Another method of calculating pressure altitude: take standard pressure, subtract the current pressure, multiply it by 1,000 and add the field elevation.
If you don’t have a flight computer, or an E6B, you can find density altitude using this formula:
Density Altitude = PA + (120 x (OAT – ISA Temp)
PA = 120 x (Outside Air Temperature – Standard Temperature (always 15))
In the wintertime, when it’s cool, and especially near sea level, density altitude isn’t really a big deal. In the summer time, when it heats up and air is humid, density altitude can be a big deal. Not so much here in Carlsbad, but in other areas, particularly high elevation areas where it gets hot, density altitude is very important. Remember, a high density altitude is NOT a good thing.
A surprisingly accurate rule of thumb (usually any error will be less than 200-300 feet) for determining the density altitude is easy to remember. For each 10-degrees Fahrenheit above standard temperature at any particular elevation, add 600 feet to the field elevation. (And, conversely for each 10-degrees F below standard temperature, subtract 600 feet from the field elevation.)
Example: It is 79 degrees Fahrenheit at Carlsbad, so that’s 20 degrees about standard temperature (standard temperature is 15C/59F) Add 1200 feet (600×2) to the field elevation of 330. Density altitude calculated by the rule of thumb method would be 1550. Using the 29.92 barometric pressure calculated on a flight computer, density altitude would be 1,673. So, pretty close.