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An aircraft is flying VFR in mountainous terrain with the highest obstacle in the region having an elevation of 7 220 ft. The weather deteriorates and the aircraft has to go IFR and climb to a safe level. Given the following information, what is the minimum usable IFR flight level, complying with the semi-circular rule and providing the required 2 000 ft obstacle clearance?

OAT = ISA -10ºC
QNH (provided from an airport close to sea level) = 990 hPa
Track = 175º(M)
Magnetic variation 15ºE
Drift: 8º left
Assume 30 ft/hPa

  • A
    FL90
  • B
    FL100
  • C
    FL110
  • D
    FL120

Refer to figure.
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.

Calculation of lowest usable flight level:

The aircraft must fly at a minimum true altitude of 9 220 ft (7220 + 2000). 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 -10ºC. Based on the 4% rule and its calculation mentioned in the rules above, the height correction for the temperature can be calculated as follows: Height correction for temperature = 4 × (10) × (9220/1000) = 369 ft.

The temperature correction is added to the true altitude as per the rules above, to give an indicated altitude of 9 589 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 - 990 hPa = 23 hPa. Since the barometric lapse rate is 30 ft/hPa, the pressure correction can be calculated as follows: Pressure correction = 23 x 30 ft/hPa = 690 ft. This value needs to be added to the indicated altitude as per the rules above:

Pressure Altitude = 9589 + 690 = 10 279 ft.

As flight levels are pressure altitudes, we can therefore say that the lowest usable flight level is the next usable flight level suitable for that magnetic track above 10 279 ft.

Semi-circular cruising level system - Non RVSM flight levels - IFR

Magnetic Track 360° - 179° = ODD LEVEL
Magnetic Track 180° - 359° = EVEN LEVEL

As the Magnetic Track is 175º, an odd level shall be used, so the next usable IFR flight level up from 10 279 ft is FL110

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