# Pressure altitude

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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. [cite web
last = Brandon
first = John
coauthors =
title = Altitude and altimeters
work =
publisher = Recreational Aviation Australia Inc
date = 2007-04-12
url = http://www.auf.asn.au/groundschool/umodule3.html#altitude
format =
doi =
accessdate = 2008-10-05
]

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:

$z =left \left(1-left\left(frac\left\{P_o\right\}\left\{101.325\right\} ight\right)^\left\{0.190263\right\} ight \right) imes frac\left\{288.15\right\}\left\{0.00198122\right\}$

Where:

*z = pressure altitude (feet)
*$P_o$ = static pressure (kPa)

For example:

implification

One simplification of the Pressure Altitude that is a bit more practical to pilots than the above formula is the following:

$A_\left\{pressure\right\}~=~A_\left\{ASL\right\}~+~\left(~29.92~inHg-P_\left\{at~Altitude\right\}\right)*1000~ft/inHg.$

Where
*$A_\left\{pressure\right\} =$ Pressure Altitude ($z$ of the previous equation) in feet,
*$A_\left\{ASL\right\} =$ Physical Altitude above Sea Level in feet,
*$P_\left\{at~Altitude\right\} =$ Measured or estimated Pressure at the Altitude of interest in inches of mercury (inHg).

ee also

*QNH
*flight level
*density altitude
*Standard conditions for temperature and pressure
*Barometric formula

References

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