Pressure altitude

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http://en.wikipedia.org/wiki/Pressure_altitude

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.

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:

$z =\left (1-\left(\frac{P_o}{101.325}\right)^{0.19025515825} \right ) \times \frac{288.15}{0.001990740740}$

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

## 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:

$A_{pressure}~=~A_{ASL}~+~(~29.92~inHg-QNH~)*1000~ft/inHg.$

Where

• Apressure = Pressure Altitude (z of the previous equation) in feet,
• AASL = Physical Altitude above Sea Level in feet,

Published - July 2009