Pressure altitude

Pressure altitude within the atmosphere is the altitude in the International Standard Atmosphere with the same pressure as the part of the atmosphere in question.

The National Oceanic and Atmospheric Administration has published the following formula for converting directly from pressure in millibars to altitude in feet:

In aviation, pressure altitude is the indicated altitude when an altimeter is set to an agreed baseline pressure setting under certain circumstances where the aircraft's altimeter would be unable to give a useful readout of altitude. Examples would be landing at a very high altitude or near sea level in conditions of exceptionally high air pressure. Old altimeters were typically limited to show altitude when set between 950 mbar and 1030 mbar. "Standard" pressure, the baseline used universally, is 1013.25 hPa, equivalent to 1013.25 mbar or 29.92 inHg (inches of mercury). This setting is equivalent to the air pressure at mean sea level (MSL) in the International Standard Atmosphere (ISA). Pressure altitude is primarily used in aircraft performance calculations and in high-altitude flight (above the transition altitude).

QNE
The term "QNE" refers to the indicated altitude at the landing runway threshold when 1013.25 mbar or 29.92 inHg is set in the altimeter's Kohlsman 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 of finding pressure altitude using the following formula:

Pressure altitude (PA) = Hgt + 1000 x (29.92 - altimeter setting reading)

For example if the airfield elevation is 500 feet and the altimeter setting reading is 29.32 inHg then

PA = 500 + 1000 x (29.92 - 29.32) 

    = 500 + 1000 x 0.6
    = 500 + 600
    = 1100 feet

Alternatively, pressure altitude (PA) = Hgt + 30 x (1013 - QNH)

For example if the airfield elevation is 500 feet and the QNH is 993 mbar then

PA = 500 + 30 x (1013 - 993)

    = 500 + 30 x 20
    = 500 + 600
    = 1100 feet

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 the properties of the International Standard Atmosphere.

See also

References

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