Pressure altitude

Pressure altitude is the altitude in the International Standard Atmosphere (ISA) with the same atmospheric pressure as that of the part of the atmosphere in question.

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

${\displaystyle 145366.45\left[1-\left({\frac {\text{Station pressure in millibars}}{1013.25}}\right)^{0.190284}\right].}$

In aviation, pressure altitude is the height above a standard datum plane (SDP), which is a theoretical level where the weight of the atmosphere is 29.921 inches of mercury (1,013.2 mbar; 14.696 psi) as measured by a barometer.^[2] It indicates altitude obtained when an altimeter is set to an agreed baseline pressure under certain circumstances in which the aircraft’s altimeter would be unable to give a useful altitude readout. Examples would be landing at a high altitude or near sea level under conditions of exceptionally high air pressure. Old altimeters were typically limited to displaying the altitude when set between ${\displaystyle 950~\mathrm {mb} }$ and ${\displaystyle 1030~\mathrm {mb} }$. Standard pressure, the baseline used universally, is ${\displaystyle 1013.25}$ hectopascals (${\displaystyle \mathrm {hPa} }$), which is equivalent to ${\displaystyle 1013.25~\mathrm {mb} }$ or ${\displaystyle 29.92}$ inches of mercury (${\displaystyle \mathrm {inHg} }$). This setting is equivalent to the atmospheric pressure at mean sea level (MSL) in the ISA. Pressure altitude is primarily used in aircraft-performance calculations and in high-altitude flight (i.e., above the transition altitude).

Inverse equation[]

Solving the equation for the pressure gives

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

where m are meter and hPa hecto-Pascal. This may be interpreted as the lowest terms of the Taylor expansion of

${\displaystyle p=1013.25e^{-h/8431m}hPa}$

QNE[]

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. In other words, it is the pressure altitude at the landing runway threshold.

Most aviation texts for PPL and CPL exams describe a process for finding the pressure altitude (in feet) using the following rule of thumb formula:

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

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

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

Alternatively,

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

For example, if the airfield elevation is ${\displaystyle 500~\mathrm {ft} }$ and the QNH is ${\displaystyle 993~\mathrm {mb} }$, then

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

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[]