Pressure Altitude
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Given an
atmospheric pressure Atmospheric pressure, also known as air pressure or 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 1,013. ...
measurement, the pressure altitude is the imputed
altitude Altitude is a distance measurement, usually in the vertical or "up" direction, between a reference datum (geodesy), datum and a point or object. The exact definition and reference datum varies according to the context (e.g., aviation, geometr ...
that the
International Standard Atmosphere The International Standard Atmosphere (ISA) is a static atmospheric model of how the pressure, temperature, density, and viscosity of the Earth's atmosphere change over a wide range of altitudes or elevations. It has been established to provide ...
(ISA) model predicts to have the same pressure as the observed value. The
National Oceanic and Atmospheric Administration The National Oceanic and Atmospheric Administration (NOAA ) is an American scientific and regulatory agency charged with Weather forecasting, forecasting weather, monitoring oceanic and atmospheric conditions, Hydrography, charting the seas, ...
(NOAA) published the following formula for directly converting atmospheric pressure in millibars (mb) to pressure altitude in
feet The foot (: feet) is an anatomical structure found in many vertebrates. It is the terminal portion of a limb which bears weight and allows locomotion. In many animals with feet, the foot is an organ at the terminal part of the leg made up of ...
(ft): h = 145366.45 \left 1 - \left( \frac \right)^ \right In
aviation Aviation includes the activities surrounding mechanical flight and the aircraft industry. ''Aircraft'' include fixed-wing and rotary-wing types, morphable wings, wing-less lifting bodies, as well as lighter-than-air aircraft such as h ...
, 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. Ty ...
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 950 mb and 1030 mb. Standard pressure, the baseline used universally, is 1013.25  hectopascals (hPa), which is equivalent to 1013.25 mb or 29.92 
inches of mercury Inch of mercury (inHg, ″Hg, or in) is a non- SI unit of measurement for pressure. It is used for barometric pressure in weather reports, refrigeration and aviation in the United States. It is the pressure exerted by a column of mercury in ...
(inHg). This setting is equivalent to the atmospheric pressure at
mean sea level A mean is a quantity representing the "center" of a collection of numbers and is intermediate to the extreme values of the set of numbers. There are several kinds of means (or "measures of central tendency") in mathematics, especially in statist ...
(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, a flight level (FL) is an aircraft's altitude as determined by a pressure altimeter using the International Standard Atmosphere. It is expressed in hundreds of feet or metres. The altimeter setting used is the ISA sea level pressur ...
).


Inverse equation

Solving the equation for the pressure gives p = 1013.25\left(1-\frac\right)^ \text where are meter and refers to hecto- Pascal. This may be interpreted as the lowest terms of the
Taylor expansion In mathematics, the Taylor series or Taylor expansion of a function is an infinite sum of terms that are expressed in terms of the function's derivatives at a single point. For most common functions, the function and the sum of its Taylor ser ...
of p = 1013.25 \exp\left(\frac\right) \text where is the exponential function.


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 signals, operating signal initially developed for commercial radiotelegraphy, radiotelegraph communication and later adopted b ...
. The term refers to the indicated altitude at the landing runway threshold when 1013.25 ~ \mathrm or 29.92 ~ \mathrm is set in the altimeter's Kollsman window. 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 one of the following rule of thumb formulae. Internationally, pressure altitude is approximated as: : \text = \text + 30 \times (1013 - \text). For example, if the airfield elevation is 500 ~ \mathrm and the QNH is 993 ~ \mathrm , then : \begin \text & = 500 + 30 \times (1013 - 993) \\ & = 500 + 30 \times 20 \\ & = 500 + 600 \\ & = 1100. \end If the altimeter uses
inches of mercury Inch of mercury (inHg, ″Hg, or in) is a non- SI unit of measurement for pressure. It is used for barometric pressure in weather reports, refrigeration and aviation in the United States. It is the pressure exerted by a column of mercury in ...
, as common in the United States, Canada, and Japan, the following formula is used: : \text = \text + 1000 \times (29.92 - \text). For example, if the airfield elevation is 500 ~ \mathrm and the altimeter setting is 29.32 ~ \mathrm , then : \begin \text & = 500 + 1000 \times (29.92 - 29.32) \\ & = 500 + 1000 \times 0.6 \\ & = 500 + 600 \\ & = 1100. \end 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 In aviation, a flight level (FL) is an aircraft's altitude as determined by a pressure altimeter using the International Standard Atmosphere. It is expressed in hundreds of feet or metres. The altimeter setting used is the ISA sea level pressur ...
*
Cabin altitude Cabin may refer to: Buildings * Beach cabin, a small wooden hut on a beach * Log cabin, a house built from logs * Cottage, a small house * Chalet, a wooden mountain house with a sloping roof * Cabin, small free-standing structures that serve as in ...
*
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 ...
*
Standard conditions for temperature and pressure Standard temperature and pressure (STP) or standard conditions for temperature and pressure are various standard sets of conditions for experimental measurements used to allow comparisons to be made between different sets of data. The most used ...
*
Barometric formula The barometric formula is a formula used to model how the air pressure (or air density) changes with altitude. Pressure equations There are two equations for computing pressure as a function of height. The first equation is applicable to the ...


References

{{reflist Altitudes in aviation