True altitude calculation from a Flight Level:
In this question, we are given a pressure altitude (in the form of a flight level) and are asked for the terrain clearance, therefore we need to determine the corresponding true altitude.

The first step is to compute the pressure correction by considering the deviation from the standard mean sea level pressure of 1013 hPa: 1033 hPa - 1013 hPa = 20 hPa. The question provides a barometric lapse rate near the surface of 27 ft/hPa, the pressure correction can be calculated as follows: Pressure correction = 20 hPa x 27 ft/hPa = ­­­540 ft. This value needs to be added to the pressure altitude as per the rules below:

Indicated altitude = 20 000 ft + 540 ft = 20 540 ft.

To determine the temperature correction, it is necessary to have knowledge of the ISA air temperature for the given flight level/altitude (FL200). In the International Standard Atmosphere (ISA), the air temperature at sea level is 15ºC, and the temperature lapse rate is -2ºC/1 000 ft. The formula to calculate the ISA temperature is therefore: ISA temperature = 15ºC - (20 000/1 000) x 2, giving an ISA temperature of ­­­-25 ºC. The actual outside air temperature is ­­­-40 ºC, which means we have an ISA deviation of ISA -15ºC.

Next, we need to determine the temperature correction. The ISA deviation is ISA -15ºC. The elevation of the aerodrome has to be taken into account. Based on the 4% rule and its calculation mentioned in the rules below, the height correction for the temperature can be calculated as follows: Height correction for temperature = 4 × (-15) × (20 540/1 000) = -1 232 ft.

The temperature correction is subtracted from the indicated altitude as per the rules below, to give a true altitude of 20 540 ft - 1 232 ft = ­­­­­19 308 ft.

• 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 (if not provided by the question).
• 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.