Pressure_altitude

In aviation, pressure altitude is the indicated altitude when an altimeter is set to an agreed pressure setting. This setting, 101.325 kPa - equivalent to 1013.25 millibar, or 1013.25 hectopascals, or 29.92 inches Hg, is a baseline pressure setting equivalent to the International Standard Atmosphere (ISA) at sea level. It is primarily used in aircraft performance calculations, and in high-altitude flight (Class A airspace in the United States, which is controlled airspace at or above 18,000 feet MSL. In radio communication, it 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:

$z\; =\backslash left\; (1-\backslash left(\backslash frac\{P\_o\}\{101.325\}\backslash right)^\{0.190263\}\; \backslash right\; )\; \backslash times\; \backslash frac\{288.15\}\{0.00198122\}$

Where:

• z = pressure altitude (feet)
• $P\_o$ = static pressure (kPa)

For example:

Simplification

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

$A\_\{pressure\}~=~A\_\{ASL\}~+~(~29.92~inHg-P\_\{at~Altitude\})*1000~ft/inHg.$

Where

• $A\_\{pressure\}\; =$ Pressure Altitude ($z$ of the previous equation) in feet,
• $A\_\{ASL\}\; =$ Physical Altitude above Sea Level in feet,
• $P\_\{at~Altitude\}\; =$ Measured or estimated Pressure at the Altitude of interest in inches of mercury (inHg).

See also

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

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