P0 = 1013.25 hPa

• Above MSL and below the tropopause (36,089 feet):

With P0 = 1013.25 hPa (standard pressure at sea level)

T0 = 288 .15 K (standard temperature at sea level)

α = 0.0065 ºC/m

g0 = 9.80665 m/s2

R = 287.053 J/kg/K

h = Altitude (m)

Note: For low altitudes, a reduction of 1 hPa in the pressure approximatelycorresponds to a pressure altitude increase of 28 feet.

• Above the tropopause (36,089 feet):

With P1 = 226.32 hPa (standard pressure at 11,000 m)

1.1.3. Density Modeling

To calculate the standard density ρ at a given altitude, the air is assumed to be a perfect gas. Therefore, at a given altitude, the standard density ρ (kg/m3) can be obtained as follows:

with R = universal gas constant (287.053 J/kg/K)

P in Pascal

T in Kelvin

• At Mean Sea Level (MSL):

ρ0 = 1.225 kg/m3

1.2. International Standard Atmosphere (ISA) Table

The International Standard Atmosphere parameters (temperature, pressure, density) can be provided as a function of the altitude under a tabulated form, as shown in Table A2:

Table A2: International Standard Atmosphere (ISA)

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2. ALTIMETRY PRINCIPLES

2.1. General

An altimeter (Figure A4) is a manometer, which is calibrated following standard pressure and temperature laws. The ambient atmospheric pressure is the only input parameter used by the altimeter.

Assuming the conditions are standard, the “Indicated Altitude” (IA) is the vertical distance between the following two pressure surfaces (Figure A3):

• The pressure surface at which the ambient pressure is measured (actual aircraft’s location), and

• The reference pressure surface, corresponding to the pressure selected by the pilot through the altimeter’s pressure setting knob.

IA = f(Pamb) - f(Pset)

IA = PAamb - PAset

Figure A4: Altimeter Function on PFD

PA

PA = f(P)

PA amb

PA set

IA

Figure A3: Ambient Pressure and Pressure Setting

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2.2. Definitions

Figure A5: QNH and Pressure Altitude

The pressure setting and the indicated altitude move in the same direction: Any increase in the pressure setting leads to an increase in the corresponding Indicated Altitude (IA).

The aim of altimetry is to ensure relevant margins, above ground and between aircraft. For that purpose, different operational pressure settings can be selected through the altimeter’s pressure setting knob (Figure A5):

• QFE is the pressure at the airport reference point. With the QFE setting, the altimeter indicates the altitude above the airport reference point (if the temperature is standard).

Note: The QFE selection is often provided as an option on Airbus aircraft.

• QNH is the Mean Sea Level pressure. The QNH is calculated through the measurement of the pressure at the airport reference point moved to Mean Sea Level, assuming the standard pressure law. With the QNH setting, the altimeter indicates the altitude above Mean Sea Level (if temperature is standard). Consequently, at the airport level in ISA conditions, the altimeter indicates the topographic altitude of the terrain.

• Standard corresponds to 1013 hPa. With the standard setting, the altimeter indicates the altitude above the 1013 hPa isobaric surface (if temperature is standard). The aim is to provide a vertical separation between aircraft while getting rid of the local pressure variations throughout Getting to the flight. After takeoff, crossing a given altitude referred to as Transition Altitude, the standard setting is selected.

• The Flight Level corresponds to the Indicated Altitude in feet divided by 100, provided the standard setting is selected.

• The Transition Altitude is the indicated altitude above which the standard setting must be selected by the crew.

• The Transition Level is the first available flight level above the transition altitude.

The change between the QNH setting and Standard setting occurs at the transition altitude when climbing, and at the transition level when descending (Figure A6).

QNH

Figure A6: Transition Altitude and Transition Level

The transition altitude is generally given on the Standard Instrument Departure (SID) charts, whereas the transition level is usually given by the Air Traffic Control (ATC).

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2.3. Effects of Altimeter Setting and Temperature

The true altitude of an aircraft is rarely the same as the indicated altitude, when the altimeter setting is 1013 hPa. This is mainly due to the fact that the pressure at sea level is generally different from 1013 hPa, and/or that the temperature is different from ISA.