Let’s start with a brain jiggle – ISA (International Standard Atmosphere) was created in order to standardise aircraft instruments, and thus provides a common reference for both pressure, and temperature. However, if the actual outside air temperature (OAT) differs from the ISA for the particular altitude or flight level that you are at, then this will affect your aircraft’s performance.
The first step is to check what the standard (ISA) temperature is for your chosen altitude. Remember that according to ISA, the temperature will decrease by 1.98°C per 1000ft from mean sea level (MSL). For practical, and exam purposes, we can safely use a 2°C decrease per 1000ft that we climb. So, as an example, if the chosen altitude is 8000ft then we calculate it like this:
8 x -2 = -16 now add +15 (which is the ISA temperature at MSL) = -1°C
If the actual OAT at 8000ft is 6°C, then we can see that there is a difference with the ISA temperature which is -1°C. In order to determine how the aircraft will perform, we need to work out what the deviation is. Higher than ISA temperatures, in particular, will negatively affect performance in various phases of flight such as take off, cruise and climb, so be sure to check your aircraft’s performance graphs, especially if the temperature is high, the altitude is high, and you are carrying a heavy load.
Using our example, to calculate the deviation from ISA:
6°C (actual OAT at 8000ft) -1°C (ISA temperature at 8000ft) = Deviation of +7°C. This means that the temperature is 7°C warmer than the given ISA temperature for that altitude.
For those writing theoretical exams, please remember that 0°C is actually a temperature, so remember to include it, particularly if you are working with ISA at 7000ft which is +1°C and 8000ft which is -1°C. 7500ft is in between at 0°C.
This deviation can significantly affect the way your aircraft performs – as the temperature is higher than ISA, you will find that it will perform as though it was at a much higher altitude. Temperature deviation needs to be taken into account when checking your take off, cruise, climb and landing performance graphs, as these graphs are normally designed with ISA standard temperature. Not factoring the temperature deviation will yield an inaccurate result! Similarly, it is imperative to check the density altitude, or you may just find yourself mushing along the runway unable to take off…