getting to grips with aircraft performance 如何掌握飞机性能 1(6)

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The operational aspect : Description of operational methods, aircraft computer logics, operational procedures, pilot’s actions…

1 JAR: The Joint Airworthiness Requirements are under the European authority called the Joint
Aviation Authority (JAA).
2 FAR: The Federal Aviation Regulations are under the US authority called the Federal Aviation
Administration (FAA).

A. GENERAL
1. THE INTERNATIONAL STANDARD ATMOSPHERE (ISA)
1.1. Standard Atmosphere Modeling
The atmosphere is a gaseous envelope surrounding the earth. Its characteristics are different throughout the world. For this reason, it is necessary to adopt an average set of conditions called the International Standard Atmosphere (ISA).
1.1.1. Temperature Modeling
The following diagram (Figure A1) illustrates the temperature variations in the standard atmosphere:

The international reference is based on a sea-level temperature of 15°C at a pressure of 1013.25 hPa1. The standard density of the air at sea level is 1.225 kg/m3.
1 1013.25 hPa is equal to 29.92 in Hg, ‘hPa’ meaning hectoPascal and ‘in Hg’ inches of mercury.

Temperature decreases with altitude at a constant rate of -6.5°C/1000m or -1.98°C/1000ft up to the tropopause. The standard tropopause altitude is 11,000 m or 36,089 feet.
From the tropopause upward, the temperature remains at a constant value of -56.5°C.
Therefore, the air which is considered as a perfect gas in the ISA model presents the following characteristics:
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At Mean Sea Level (MSL):

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

For a quick determination of the standard temperature at a given altitude, the following approximate formula can be used:

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Above the tropopause (36,089 feet):

This ISA model is used as a reference to compare real atmospheric conditions and the corresponding engine/aircraft performance. The atmospheric conditions will therefore be expressed as ISA +/-.
ISA at a given flight level.
Example:
Let’s consider a flight in the following conditions:
Altitude = 33,000 feet
Actual Temperature = -41oC
The standard temperature at 33,000 feet is : ISA = 15 -2 x 33 = -51oC, whereas the actual temperature is -41oC, i.e. 10oC above the standard.
Conclusion: The flight is operated in ISA+10 conditions

1.1.2. Pressure Modeling
To calculate the standard pressure P at a given altitude, the following assumptions are made:
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Temperature is standard, versus altitude.

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Air is a perfect gas.

The altitude obtained from the measurement of the pressure is called pressure altitude (PA), and a standard (ISA) table can be set up (table A1).
PA
PRESSURE ALTITUDE (ft) (km)
40000 12
10 30000 8
20000 6
　
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