In aviation, pressure altitude is the indicated altitude when an altimeter is set to 1013 hPa (29.92 inHg US and Canada). 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). First flight, December 17, 1903 Aviation or air transport refers to the activities surrounding human flight and the aircraft industry. ... Kollsman-type barometric aircraft altimeter as used in North America An altimeter is an active instrument used to measure the altitude of an object above a fixed level. ... Airspace means the portion of the atmosphere controlled by a particular country on top of its territory and territorial waters or, more generally, any specific portion of the atmosphere. ...
The relation between static pressure and pressure altitude is defined in terms of the properties of the International Standard Atmosphere (ISA). Up to 36,090 ft this relation can be expressed as: Temperature and air pressure can vary from one place to another on the Earth, and can also vary in the same place with time. ...
Where:
z = pressure altitude (feet)
Po = static pressure (hPa)
Simplification
One simplification of the Pressure Altitude that is a bit more practical to pilots than the above formula is the following:
Where
Apressure = Pressure Altitude (z of the previous equation),
AASL = Physical Altitude above Sea Level,
Measured or estimated Pressure at the Altitude of interest in inches of mercury (inHg).
The movement of the pressure sensing capsule is transferred and magnified, via a mechanical linkage or piezo-quartz component, to a dial pointer or pointers, or a digital display, which indicate the altitude reading.
The transponder obtains altitude data from a special altitude encoding altimeter or from a blind encoder; the latter being an electronic device which obtains current atmospheric pressure from the static pressure line and the reference pressure used is preset at 1013.2 hPa.
Density altitude is roughly 120 feet greater than pressurealtitude for each 1 °C that the temperature exceeds ISA for that level, and 120 feet less for each 1 °C that the outside air temperature is less than ISA.
Altitude is the elevation of an object from a known level or datum.
It is the fall in pressure that leads to a shortage of oxygen (hypoxia) in humans on ascent to high altitude.
Pressurealtitude divided by 100 feet is referred to as the flight level; so when the altimeter reads 18,000 ft on the standard pressure setting the aircraft is said to be at "Flight level 180".
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