Pressure altitude is the height above the standard datum plane (SDP). This is when an altimeter’s Kollsman window is set to 29.92 inches of mercury or 1013.2 millibars.
How to Calculate Pressure Altitude
Pressure altitude can be calculated via:
1. Setting the barometric scale of the altimeter to 29.92 and reading the indicated altitude
2. Applying a correction factor to the indicated altitude according to the reported altimeter setting
3. A flight computer
4. Manually using the following formula: (altimeter setting – 29.92) x 1000 + current altitude
Note that this formula is more of a rule of thumb and good for a quick, easy rough estimation
When is Pressure Altitude Used?
In the United States and Canada, pressure altitude is used by all aircraft at and above 18,000 feet.
When flying at or above 18,000 feet MSL, you must set your altimeter to 29.92 inches Hg to ensure that your flight level can be properly assigned, and all aircraft at the same flight level have the same altimeter setting.
Pressure Altitude & Aircraft Performance Charts
As aircraft performance charts are typically based on pressure altitude, it’s important to know that the values you see on these charts come from when an altimeter was set to 29.92 inches of mercury in its Kollsman window.
Additionally, if you’re planning on using a performance chart, you should know the outside air temperature and pressure altitude.
This is because to get an estimate of aircraft performance, you need to get the reading from the altimeter when set to 29.92, along with the air temperature.
Pressure Altitude vs. Density Altitude
Density altitude is pressure altitude corrected for non-standard temperature. Under standard conditions, pressure altitude and density altitude identify the same level. An increase in density altitude is the result of the temperature rising above standard, and thus the density of the air decreases.
Used in calculating aircraft performance, density altitude is calculated by using pressure altitude and temperature. More specifically, to calculate density altitude, you need to find the pressure altitude and then correct it for non-standard temperature variation.
A flight computer can quickly perform this calculation by inputting the pressure altitude and outside air temperature at flight level. Density altitude can also be determined by referring to relevant charts and tables.
Related: How High Do Planes Fly?