Pressure altitude

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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.

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 read out 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 950mbs and 1030mbs. "Standard" pressure, the baseline used universally is 1013.25hPa, equivalent to 1013.25millibars(mbs), or 29.92" 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). The term "QNE" refers to the indicated altitude at the landing runway threshold when 1013.2mb/29.92 inHg is set in the altimeter's Kollsman window and not the setting itself, as is commonly misconceived.

Most aviation texts for PPL and CPL exams describe a process of finding pressure altitude using the following formula

Pressure Altitude(PA) = Hgt + 1000(29.92-altimeter setting)

For example if the airfield height is 500 feet and the altimeter setting is 29.32"Hg then

PA= 500 + 1000(29.92-29.32)

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

Alternatively, Pressure Altitude(PA) = Hgt + 30(1013-QNH)

For example if the airfield height is 500 feet and the QNH is 993 Mb then

PA= 500 + 30(1013-993)

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

Aircraft transponders report the pressure altitude; corrections for atmospheric pressure variations are applied on the ground.

The relationship between static pressure and pressure altitude is defined in terms of the properties of the International Standard Atmosphere.

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