Pressure altitude

In aviation, pressure altitude is the indicated altitude when an altimeter is set to an agreed baseline pressure setting. This setting – 1013.25 hPa, equivalent to 1013.25 millibar (or hectopascals), or 29.92 inches Hg – 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). In radio communication, the baseline pressure setting is referred to by the Q code QNE.^[1]

The relationship between static pressure and pressure altitude is defined in terms of the properties of the International Standard Atmosphere. Up to 36,090 ft this can be expressed as:

Where:

• z = pressure altitude (feet)
• P_o = static pressure (kPa)

For example:

Simplification

One simplification of the Pressure Altitude that is a bit more practical to pilots than the above formula is the following.

In the United States and Canada :

Where

• A_pressure = Pressure Altitude (z of the previous equation) in feet,
• A_ASL = Physical Altitude above Sea Level in feet,
• QNH = Current air-pressure at sea level in inHg (in air-traffic radio conversation named "Altimeter" when use inHg),

In the rest of the world:

Where

• A_pressure = Pressure Altitude (z of the previous equation) in feet,
• A_ASL = Physical Altitude above Sea Level in feet,
• QNH = Current air-pressure at sea level in millibar (in air-traffic radio conversation also named QNH),

See also

• QNH
• flight level
• density altitude
• Standard conditions for temperature and pressure
• Barometric formula

References