getting to grips with aircraft performance 如何掌握飞机性能 1(9)

IA = f(Pamb) - f(Pset)
 IA = PAamb - PAset

2.2. Definitions

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.
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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.

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

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

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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).

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).

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.
2.3.1. 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 the local QNH is known.

2.3.2. Temperature Correction
Flying at a given indicated altitude, the true altitude increases with the temperature (Figure A7). The relationship between true altitude and indicated altitude can be approximated as follows:
　
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