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An aircraft is flying at 2 600 ft with altimeter set to 1000 hPa (QNH of an nearby airfield at 600 ft). Considering that the air mass at that airfield is on average 8ºC colder than standard, calculate the aircraft's true altitude over the field:
  • A
    2 605 ft
  • B
    2 595 ft
  • C
    2 540 ft
  • D
    2 665 ft
Refer to figure.

True altitude calculation from a Flight Level:
In this question, we are given a pressure altitude (in the form of a flight level) and are asked for the corresponding true altitude.

The first step is to compute the pressure correction by considering the deviation from the standard mean sea level pressure of 1013 hPa. In this case, the aircraft is flying an altitude with the correct QNH set, therefore, pressure correction shall be disregarded.

Next, we need to determine the temperature correction. The ISA deviation is ISA -8ºC. The airfield elevation must be taken into consideration for the temperature correction: 2 600 ft - 600 ft = 2 000 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 × (-8) × (2 000/1000) = -64 ft.

The temperature correction is subtracted from the indicated altitude as per the rules below, to give a true altitude of ­­­­2 536 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 (if not provided by the question).
  • 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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