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The atmospheric density is expected to be 50% of the density at MSL in ISA at:
  • A
    18 000 ft
  • B
    22 000 ft
  • C
    35 000 ft
  • D
    40 000 ft

Air density is approximately half of the mean sea level value at 22 000 ft and approximately a quarter of the mean sea level value at 40 000 ft.
The ISA atmospheric pressure levels, flight levels and the corresponding altitudes and air densities from highest altitude to ground level:

Pressure Level (hPa) Flight Level Altitude (ft) Altitude (m) Air density (kg/m3)
50 hPa FL600 60 000 ft 18 288 m 0.003 kg/m3
150 hPa FL450 45 000 ft 13 720 m 0.238 kg/m3
200 hPa FL390 39 000 ft 11 890 m 0.317 kg/m3
250 hPa FL340 34 000 ft 10 360 m 0.395 kg/m3
300 hPa FL300 30 000 ft 9 150 m 0.459 kg/m3
400 hPa FL240 24 000 ft 7 315 m 0.569 kg/m3
500 hPa FL180 18 000 ft 5 500 m 0.698 kg/m3
600 hPa FL140 14 000 ft 4 270 m 0.796 kg/m3
700 hPa FL100 10 000 ft 3 050 m 0.904 kg/m3
850 hPa FL050 5 000 ft 1 525 m 1.055 kg/m3
1013 hPa - MSL (0 ft) MSL (0 m) 1.225 kg/m3
Note: do not confuse density and air pressure. Air pressure is approximately half of the MSL value at 18 000 ft.

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