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After landing at an aerodrome (QNH 993 hPa) it is noticed that the altimeter is still set to 1013.2 hPa and that it reads 1200 feet. What is the elevation of the aerodrome above mean sea level ?
(Barometric lapse rate: 27 ft/hPa)
  • A
    1740 feet.
  • B
    2280 feet.
  • C
    660 feet.
  • D
    1200 feet.

The altimeter will indicate the airfield's elevation is the altimeter is set to the correct local QNH value set. In this case the correct local QNH is 993 hPa. However, the altimeter is still set to 1013.2 hPa. With this setting the altimeter indicates 1200 feet.

The difference between 1013 hPa and 993 hPa is 20 hPa (1013 - 993), so the altimeter is set at a value 20 hPa too high.

This corresponds to a difference of 540 feet (20 hPa x 27 ft/hPa). If we would set the actual pressure of 993 hPa on the altimeter, we would go from a higher to a lower pressure.

  • That is why we have to deduct 540 ft from our current indication of 1200 ft = 660 ft.
Note: According to the new syllabus (EASA 2020) a barometric lapse rate of 30 ft /hPa should be used. However, if the question text indicates a different lapse rate, that is the one you are expected to use.

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