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An aircraft is flying at FL80. The local QNH is 1000 hPa. After the second altimeter has been adjusted to the local QNH, the reading will be approximately:
(Barometric lapse rate: 27 ft/hPa)
  • A
    8 600 ft
  • B
    8 350 ft
  • C
    8 000 ft
  • D
    7 650 ft

With 1013 set on the altimeter the altimeter indicates an altitude of 8000 feet (FL80).
If we change the altimeter setting to a lower pressure value, we will have a lower reference pressure.
If we now set the local QNH of 1000 hPa the indicated altitude will be lower because the 1000 pressure value will;
according the standard atmosphere be found at a higher altitude than 1013 hPa.1000 hPa is normally found at:

1013- 1000 = 13 hPa
X 27 (drop in feet per 1 hPa) = 351 feet

Because the reference now is taken from 351 feet amsl (1000 hPa is normally found at 351 feet) according to ISA, the indicated altitude will decrease 351 feet.

8000 – 351 = 7649 feet indicated altitude.

Closest answer is 7650 feet.

Note: According to the new syllabus (EASA 2020) a barometric lapse rate of 30 ft /hPa should be used (instead of 27 ft/hPa). However, if the question text indicates a different lapse rate, that is the one you are expected to use.

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