1.3 The International Standard Atmosphere

The basis for all performance calculations is the International Standard Atmosphere (ISA) which is defined as a perfect dry gas, having a mean sea level temperature of +15 degrees C, which decreases at the rate of 1.98 degrees C for every 1000 ft increase of altitude up the tropopause which is at an altitude of 36 090 ft above which the temperature is assumed to remain constant at –56.5 degrees C. The mean sea level (MSL)atmospheric pressure is assumed 1013.2 hPa (29.92 in. Hg). See Table 1.2.

1.3.1 ISA Deviation

It is essential to present performance data at temperatures other than the ISA temperature for all flight levels within the performance-spectrum envelope. If this were to be attempted for the actual or forecast temperatures, it would usually be impracticable and in some instances impossible.

To overcome the presentation difficulty and retain the coverage or range required, it is necessary to use ISA deviation. This is simply the algebraic difference between the actual (or forecast) temperature and the ISA temperature for the flight level under consideration. It is calculated by subtracting the ISA temperature from the actual (or forecast) temperature for that particular altitude. In other words:

ISA Deviation = Ambient temperature − Standard Temperature

Usually, 5 degrees C bands of temperature deviation are used for data presentation in Flight Manuals to reduce the size of the document or to prevent any graph from becoming overcrowded and unreadable.

1.3.2 Height and Altitude

Three parameters are used for vertical referencing of position in aviation. They are the airfield surface level, mean sea level (MSL) and the standard pressure level of 1013.2 hPa. It would be convenient if the performance data could be related to the aerodrome elevation because this is fixed and published in the Aeronautical Information Publication.

However, this is impractical because of the vast range that would have to be covered. Mean sea level and pressure altitude are the only permissible references for assessing altitude for the purposes of aircraft performance calculations, provided that the one selected by the manufacturers for the Flight Manual is used consistently throughout the manual. Alternatively, any combination of them may be used in a conservative manner.

1.3.3 Pressure Altitude

In Aeroplane Flight Manuals (AFMs) the word altitude refers strictly to pressure altitude, which can be defined as the vertical distance from the 1013.2 hPa pressure level. Therefore, aerodrome and obstacle elevations must be converted to pressure altitude before they can be used in performance graphs. Many large aerodromes provide the aerodrome pressure altitude as part of their hourly weather reports.

To correct an aerodrome elevation to become a pressure altitude if Table 1.3 is not available use the following formulae:

To correct an altitude for the temperature errors of the altimeter use the following formula:

1.3.4 Density Altitude

The performance data for small piston/propeller-driven aeroplanes is calculated using density altitude, which is pressure altitude corrected for non-standard temperature. It is the altitude in the standard atmosphere at which the prevailing density occurs and can be calculated by using the formula:

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