If the real temperature conditions were equal to ISA conditions, the thickness of this layer would be 9 000 ft, however this is not the case.

The ISA deviation is ISA -15ºC. Based on the 4% rule and its calculation mentioned in the rules below, the temperature correction can be calculated as follows:

Correction for temperature = 4 × (15) × (9000/1000) = 540 ft.

You have the option to apply the temperature correction to FL50 and FL140 individually or to the layer thickness (FL140 – FL50). However, we recommend applying the temperature correction directly to the layer thickness as it is a more straightforward approach.

In the case of a colder air mass, the deviation causes a thinner layer, so we must subtract the temperature correction of 540 ft, giving us a layer thickness of 8460 ft.

RULES. The following rules should be considered for altimetry calculations:

• All calculations are based on rounded pressure values to the nearest lower hPa.
• The value for the barometric lapse rate between MSL and 500 hPa to be used is 30 ft/hPa as an acceptable approximation of the barometric lapse rate.
• To determine the true altitude/height, the following rule of thumb, called the ‘4 %-rule’, shall be used: the altitude/height changes by 4% for each 10°C temperature deviation from ISA.
  For simplification: Height correction for the temp = 4 × (ISA DEV) × Indicated alt/1000 = ___ ft
• If no further information is given, the deviation of the outside-air temperature from ISA is considered to be the same throughout the whole layer.
• The elevation of the aerodrome has to be taken into account. The temperature correction has to be considered for the layer between the station (usually an aerodrome) and the position of the aircraft.

Definitions
Pressure Altitude: The altimeter indication with standard pressure (1013.2 hPa) set.
Indicated Altitude: The altimeter indication with local QNH set.
True altitude: The actual altitude of the aircraft above mean sea level.