An altimeter 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:
The pressure surface at which the ambient pressure is measured (current aircraft location), and The reference pressure surface, corresponding to the pressure selected by the pilot through the altimeter’s pressure setting knob.
Definitions
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:
> QFE is the pressure at the airport reference point. With the QFE setting, the altimeter indicates the height above the airport reference point (if the temperature is standard).
> 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, 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 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, according to the standard setting 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 latest crossing transition altitude when climbing, and at transition level when descending.
Effects of Altimeter Setting and Temperature
The true altitude (or geometrical 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.
Altimeter Setting Correction
In case of ISA temperature conditions, and a standard altimetric setting, the aircraft true altitude can be obtained from the indicated altitude provided by the local QNH if known.
True altitude = Indicated altitude + 28 × (QNHhPa – 1013)
Example: An airport with an elevation of 1000ft and a current QNH of 1005hPa, has a pressure altitude of 1000 – 28 x (1005-1013) = 1224 ft.
NOTE: 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). For instance, increasing the QNH setting by 10hPa will increase the Indicated Altitude by 10×28=280ft.
Temperature Correction
Flying at a given indicated altitude, the true altitude increases with the temperature. The relationship between true altitude and indicated altitude can be approximated as follows:
TA = IA ( T / Tisa )
With
TA = True altitude
IA = Indicated altitude
T = Actual temperature (in Kelvin)
Tisa = Standard temperature (in Kelvin)
If the temperature is lower, you fly lower.
Temperature correction is important when flying a departure or arrival procedure in very low temperature conditions.
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