Pressure altitude

Given an atmospheric pressure measurement, the pressure altitude is the imputed altitude that the International Standard Atmosphere (ISA) model predicts to have the same pressure as the observed value.

The National Oceanic and Atmospheric Administration (NOAA) published the following formula^[1] for directly converting atmospheric pressure in millibars (mb) to pressure altitude in feet (ft):

In

).

Inverse equation

Solving the equation for the pressure gives

$${\displaystyle p=1013.25\left(1-{\frac {h}{44307.694{\text{ m}}}}\right)^{5.25530}{\text{ hPa}}}$$

where m are meter and hPa refers to hecto-

Taylor expansion

of

$${\displaystyle p=1013.25\exp \left({\frac {-h}{8431{\text{ m}}}}\right){\text{ hPa}}}$$

where exp is the exponential function.

QNE

This section does not cite any sources. Please help improve this section by adding citations to reliable sources. Unsourced material may be challenged and removed. (January 2024) (Learn how and when to remove this template message)

QNE is an aeronautical code Q code. The term refers to the indicated altitude at the landing runway threshold when ${\displaystyle 1013.25~\mathrm {mb} }$ or ${\displaystyle 29.92~\mathrm {inHg} }$ is set in the altimeter's Kollsman window. It is the pressure altitude at the landing runway threshold.

Most aviation texts for

exams describe a process for finding the pressure altitude (in feet) using one of the following rule of thumb formulae.

Internationally, pressure altitude is approximated as:

${\displaystyle {\text{Pressure altitude (PA)}}={\text{Elevation}}+30\times (1013-{\text{QNH}}).}$

For example, if the airfield elevation is ${\displaystyle 500~\mathrm {ft} }$ and the

is ${\displaystyle 993~\mathrm {mb} }$, then

${\displaystyle {\begin{aligned}{\text{PA}}&=500+30\times (1013-993)\\&=500+30\times 20\\&=500+600\\&=1100.\end{aligned}}}$

If the altimeter uses inches of mercury, as common in the United States, Canada, and Japan, the following formula is used:

${\displaystyle {\text{Pressure altitude (PA)}}={\text{Elevation}}+1000\times (29.92-{\text{Altimeter setting}}).}$

For example, if the airfield elevation is ${\displaystyle 500~\mathrm {ft} }$ and the altimeter setting is ${\displaystyle 29.32~\mathrm {inHg} }$, then

${\displaystyle {\begin{aligned}{\text{PA}}&=500+1000\times (29.92-29.32)\\&=500+1000\times 0.6\\&=500+600\\&=1100.\end{aligned}}}$

Aircraft Mode “C” transponders report the pressure altitude to air traffic control; corrections for atmospheric pressure variations are applied by the recipient of the data.

The relationship between static pressure and pressure altitude is defined in terms of properties of the ISA.

See also

• QNH
• Flight level
• Cabin altitude
• Density altitude
• Standard conditions for temperature and pressure
• Barometric formula

References