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An aircraft is flying over a mountain range, with a maximum elevation of 4 920 ft AMSL. What is the minimum altitude required to maintain terrain clearance by 1000 ft, assuming that the correct QNH is set on the altimeter and the outside air temperature is ISA -10ºC?
  • A
    5 680 ft
  • B
    6 160 ft
  • C
    5 920 ft
  • D

    6 120 ft

Refer to figure.
This question gives us a true altitude and is asking for the indicated altitude (the altitude our altimeter shows when the correct QNH is set).

The aircraft must fly at 5 920 ft (*obstacle elevation + terrain clearance). This altitude must be the true altitude of the aircraft. This altitude must be corrected for the temperature ISA deviation to get the indicated altitude.

The ISA deviation is ISA -10ºC. 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 × (10) × (5920/1000) = 236.8 ft.

The temperature correction is added to the true altitude as per the rules below, to give an indicated altitude of ­­­­­6156.8 ft. Closest answer 6 160 ft.


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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