Pressure altitude
From Wikipedia, the free encyclopedia
In aviation, pressure altitude is the indicated altitude when an altimeter is set to an agreed baseline pressure setting. This setting – 101,325 Pa, equivalent to 1013.25 millibar (or hectopascals), or 29.92 inches Hg – is equivalent to the air pressure at mean sea level (MSL) in the International Standard Atmosphere (ISA). Pressure altitude is primarily used in aircraft performance calculations and in high-altitude flight (above the transition altitude). In radio communication, the baseline pressure setting is referred to by the Q code QNE.[1]
The relationship between static pressure and pressure altitude is defined in terms of the properties of the International Standard Atmosphere. Up to 36,090 ft this can be expressed as:
Where:
- z = pressure altitude (feet)
- Po = static pressure (kPa)
For example:
| Pressure Altitude ft | Static Pressure kPa |
|---|---|
| 0 | 101.325 |
| 1000 | 97.715 |
| 2500 | 92.500 |
| 5000 | 84.306 |
| 10000 | 69.681 |
| 20000 | 46.563 |
| 30000 | 30.089 |
| 36090 | 22.631 |
[edit] Simplification
One simplification of the Pressure Altitude that is a bit more practical to pilots in the United States and Canada than the above formula is the following:
Where
- Apressure = Pressure Altitude (z of the previous equation) in feet,
- AASL = Physical Altitude above Sea Level in feet,
[edit] See also
- QNH
- flight level
- density altitude
- Standard conditions for temperature and pressure
- Barometric formula
[edit] References
- ^ Brandon, John (2007-04-12). "Altitude and altimeters". Recreational Aviation Australia Inc. http://www.auf.asn.au/groundschool/umodule3.html#altitude. Retrieved on 2008-10-05.
