Pressure altitude

Pressure altitude within the atmosphere is the altitude in the International Standard Atmosphere (ISA) with the same atmospheric pressure as that of the part of the atmosphere in question. Pressure altitude within the atmosphere is the altitude in the International Standard Atmosphere (ISA) with the same atmospheric pressure as that of the part of the atmosphere in question. The National Oceanic and Atmospheric Administration (NOAA) has published the following formula for directly converting atmospheric pressure in millibars ( m b {displaystyle mathrm {mb} } ) to pressure altitude in feet ( f t {displaystyle mathrm {ft} } ): In aviation, the pressure altitude is the indicated altitude obtained when an altimeter is set to an agreed baseline pressure under certain circumstances in which the aircraft’s altimeter would be unable to give a useful readout of the altitude. Examples would be landing at a very 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   m b {displaystyle 950~mathrm {mb} } and 1030   m b {displaystyle 1030~mathrm {mb} } . Standard pressure, the baseline used universally, is 1013.25 {displaystyle 1013.25} hectopascals ( h P a {displaystyle mathrm {hPa} } ), which is equivalent to 1013.25   m b {displaystyle 1013.25~mathrm {mb} } or 29.92 {displaystyle 29.92} inches of mercury ( i n H g {displaystyle mathrm {inHg} } ). 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). Most aviation texts for PPL and CPL exams describe a process for finding the pressure altitude (in feet) using the following formula: For example, if the airfield elevation is 500   f t {displaystyle 500~mathrm {ft} } and the altimeter setting is 29.32   i n H g {displaystyle 29.32~mathrm {inHg} } , then