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An IFR flight is planned across terrain where the maximum elevation is 5880 ft. The forecast temperature is ISA -18° C, and the Minimum Obstacle Clearance required is 2000 ft. A nearby aerodrome elevation 2075 ft reports a QNH of 1003 hPa. What is the minimum flight level that could be planned for the cruise?
• A
FL85
• B
FL80
• C
FL90
• D
FL100
Refer to figure.

Calculation of terrain clearance and lowest usable flight level:
The aircraft must fly at 7 880 ft (5 880 ft + 2 000 ft). 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 -18ºC. For the temperature correction, the air column between the airfield and the aircraft must be taken into account: 7 880 ft - 2 075 ft = 5 805 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 × (-18) × (5 805/1 000) = -418 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 8 298 ft.

Next, we need to compute the pressure correction by considering the deviation from the standard mean sea level pressure of 1013 hPa: 1013 hPa - 1003 hPa = 10 hPa. Since the barometric lapse rate near the surface is 30 ft/hPa, the pressure correction can be calculated as follows: Pressure correction = 10 hPa x 30 ft/hPa = 300 ft. This value needs to be added to the indicated altitude as per the rules below:

Pressure Altitude = 8 298 ft + 300 ft = 8 598 ft.

The Minimum Flight Level, which provides obstacle clearance, after both pressure and temperature correction, that must be selected is: FL90.

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.
 HIGHER PRESSURE; INDICATED ALTITUDE > PRESSURE ALTITUDE LOWER PRESSURE; INDICATED ALTITUDE < PRESSURE ALTITUDE WARMER THAN ISA; TRUE ALTITUDE > INDICATED ALTITUDE COLDER THAN ISA; TRUE ALTITUDE < INDICATED ALTITUDE
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.

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