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Given the following information, calculate the minimum flight level for an IFR flight that is planned outside airways:

Course: 235°M
MSA: 7800 ft
QNH: 995 hPa
Temperature: ISA

  • A
    FL85
  • B
    FL90
  • C
    FL100
  • D
    FL80
Refer to figure.

Calculation of terrain clearance and lowest usable flight level:
The aircraft must fly at 7 800 ft (MSA). 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 temperature is ISA, therefore temperature correction may be disregarded.

Next, we need to compute the pressure correction by considering the deviation from the standard mean sea level pressure of 1013 hPa: 1013 hPa - 995 hPa = 18 hPa. Since the barometric lapse rate near the surface is 30 ft/hPa, the pressure correction can be calculated as follows: Pressure correction = 18 hPa x 30 ft/hPa = 540 ft. This value needs to be added to the indicated altitude as per the rules below:

Pressure Altitude = 7 800 ft + 540 ft = 8 340 ft.

According to the ICAO IFR Semi-Circular Rule, below FL290:

  • For Easterly magnetic tracks (000° - 179°), select ODD FLs (FL110, FL130, etc.)
  • For Westerly magnetic tracks (180° - 359°), select EVEN FLs (FL100, FL120, etc.)

The aircraft flies a south-westerly magnetic track of 235º(M). Thus, the Minimum Flight Level, which provides obstacle clearance, after both pressure and temperature correction, that must be selected is: FL100


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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