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An aircraft flies at flight level 40. Elevation of the aerodrome: 990 ft; QNH 976 hPa.The tower clears the pilot to fly at 3000 ft QNH. Which of the following statements is correct?
(Barometric lapse rate: 27 ft/hPa)
  • A
    The aircraft has to descend about 1000 ft.
  • B
    The aircraft has to descend about 2000 ft.
  • C
    Only a small change of altitude is necessary.
  • D
    The aircraft has to climb about 1000 ft.

The aircraft flies at flight level 40, which means that the altimeter’s subscale is a standard value set of 1013 hPa.
Elevation of the aerodrome is 990 ft and the local QNH is 976 hPa. The tower clears to pilot to descend to 3000 ft.

The altimeter subscale will change from a setting of 1013 hPa to a setting of 976 hPa (local QNH).
We need to know the change in altitude which results from this change in altimeter setting. Consdering a barometric lapse rate of 27 ft/hPa:

  • 1013 hPa – 976 hPa = 37 hPa
  • 37 hPa x 27 ft = 999 feet.

4000 ft - 999 ft = 3 001 ft => the pilot needs to descend only 1 ft in order to maintain 3000 ft on QNH 976 hPa.

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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