Refer to figure.
The aircraft must fly at 12 290 ft (10 290 + 2 000). This altitude must be the true altitude of the aircraft. This altitude must be corrected for any temperature ISA deviation and then any difference in pressure from standard (1013 hPa), to get the pressure altitude, allowing us to work out the lowest usable flight level.

The first step is to determine the temperature correction. The ISA deviation is ISA -20ºC. The elevation of the aerodrome has to be taken into account: 12 290 ft - 5 100 ft = 7 190 ft. 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 × (-20) × (7 190/1000) = -575 ft.

In this case, the temperature correction is added to the true altitude as per the rules below (refer to the table below), to give an indicated altitude of ­­12 865 ft.

Next, we need to compute the pressure correction by considering the deviation from the standard mean sea level pressure of 1013 hPa. The local pressure equals standard, therefore pressure correction may be disregarded.

The lowest suitable flight level in order to achieve an obstacle clearance of 2 000 ft is FL130.

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.