# ISA Deviation

To determine true altitude and for the assessment of performance data, it is necessary to determine the temperature deviation from the ISA at any specified altitude. To do this we first need to determine what the ISA temperature is at a specified altitude, then calculate the deviation from the ISA.Â

The ISA temperature at a particular pressure altitude is found by reducing the MSL temperature by 2Â°C for every 1000 ft above 1013 hPa datum:Â

ISA Temperature = 15 – 2Ã— altitude (in 1000 ft)

e.g. find the ISA temperature at 18 000 ft:

ISA temperature = 15 – 2 Ã— 18 = -21Â°C

Note: Remember the temperature is isothermal above 36 000 ft (11 km) in the ISA at -57Â°C.

Now to find the deviation from ISA we subtract the ISA temperature from the actual temperature:

ISA Deviation = actual temperature – ISA temperature

So if the actual temperature at 18 000 ft is -27Â°C, then the deviation is:

ISA Deviation = -27 – (-21) = -6Â°

For the temperatures below, calculate the ISA deviations:Â

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