Pressure altitude is the altitude in the
International Standard Atmosphere (ISA) with the same
atmospheric pressure
Atmospheric pressure, also known as barometric pressure (after the barometer), is the pressure within the atmosphere of Earth. The standard atmosphere (symbol: atm) is a unit of pressure defined as , which is equivalent to 1013.25 millibars, ...
as that of the part of the atmosphere in question.
The
National Oceanic and Atmospheric Administration (NOAA) published the following formula for directly converting atmospheric pressure in
millibars (
) to pressure altitude in
feet (
):
:
In
aviation
Aviation includes the activities surrounding mechanical flight and the aircraft industry. ''Aircraft'' includes airplane, fixed-wing and helicopter, rotary-wing types, morphable wings, wing-less lifting bodies, as well as aerostat, lighter- ...
, pressure altitude is the height above a standard datum plane (SDP), which is a theoretical level where the weight of the atmosphere is as measured by a barometer.
[Pilot’s Handbook of Aeronautical Knowledge (FAA-H-8083-25B), 2016, Chapter 4, p 4-4] It indicates altitude obtained when an
altimeter
An altimeter or an altitude meter is an instrument used to measure the altitude of an object above a fixed level. The measurement of altitude is called altimetry, which is related to the term bathymetry, the measurement of depth under water. The m ...
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
and
. Standard pressure, the baseline used universally, is
hectopascals (
), which is equivalent to
or
inches of mercury (
). 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
In aviation and aviation meteorology, a flight level (FL) is an aircraft's altitude at standard air pressure, expressed in hundreds of feet. The air pressure is computed assuming an International Standard Atmosphere pressure of 1013.25  ...
).
Inverse equation
Solving the equation for the pressure gives
:
where are meter and hPa hecto-Pascal. This may be interpreted as the lowest terms of the Taylor expansion of
:
QNE
QNE is an
aeronautical code Q code
The Q-code is a standardised collection of three-letter codes that each start with the letter "Q". It is an operating signal initially developed for commercial radiotelegraph communication and later adopted by other radio services, especially ...
. The term refers to the indicated altitude at the landing runway threshold when
or
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:
:
For example, if the airfield elevation is
and the altimeter setting is
, then
:
Alternatively,
:
For example, if the airfield elevation is
and the QNH is
, then
:
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
The Q-code is a standardised collection of three-letter codes that each start with the letter "Q". It is an operating signal initially developed for commercial radiotelegraph communication and later adopted by other radio services, especially ...
*
Flight level
In aviation and aviation meteorology, a flight level (FL) is an aircraft's altitude at standard air pressure, expressed in hundreds of feet. The air pressure is computed assuming an International Standard Atmosphere pressure of 1013.25  ...
*
Cabin altitude
*
Density altitude
The density altitude is the altitude relative to standard atmospheric conditions at which the air density would be equal to the indicated air density at the place of observation. In other words, the density altitude is the air density given as a ...
*
Standard conditions for temperature and pressure
Standard temperature and pressure (STP) are standard sets of conditions for experimental measurements to be established to allow comparisons to be made between different sets of data. The most used standards are those of the International Union ...
*
Barometric formula
The barometric formula, sometimes called the '' exponential atmosphere'' or ''isothermal atmosphere'', is a formula used to model how the pressure (or density) of the air changes with altitude. The pressure drops approximately by 11.3 pascals per ...
References
{{reflist
Altitudes in aviation