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What is the ISA standard pressure at FL180 (5500 m)?

  • A

    250 hPa

  • B

    300 hPa

  • C

    500 hPa

  • D

    1013.25 hPa

Under ISA conditions, the surface pressure is 1013 hPa, and at 18 000 ft (5500 m), it is 500 hPa. This means that at 18 000 ft, at least half of the atmosphere's mass (and therefore half the pressure) lies below this altitude.

For the high-level charts, each FL corresponds to a specific pressure surface (pressure value):

FL50 = 850 hPa
FL100 = 700 hPa
FL140 = 600 hPa
FL180 = 500 hPa
FL240 = 400 hPa
FL300 = 300 hPa
FL340 = 250 hPa
FL390 = 200 hPa
FL450 = 150 hPa

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