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An aircraft is flying at FL095, OAT +5 deg C. The temperature deviation from ISA is…

  • A

    +9 deg C

  • B

    +11 deg C

  • C

    +7 deg C

  • D

    +13 deg C

The International Standard Atmosphere (ISA) defines a standard temperature model used as a reference for all aviation performance calculations. I

SA sea level temperature is +15 degrees Celsius, and the standard temperature lapse rate in the troposphere is 1.98 degrees Celsius per 1000 feet, which is commonly rounded to 2 degrees Celsius per 1000 feet for practical calculations. 

CALCULATION:
ISA sea level temperature = +15 deg C
Standard lapse rate = 2 deg C per 1000 ft (rounded)

FL095 = 9500 ft = 9.5 x 1000 ft
Temperature drop = 9.5 x 2 = 19 deg C
ISA temperature at FL095 = 15 - 19 = -4 deg C

ISA deviation = OAT - ISA temperature at altitude
ISA deviation = +5 - (-4) = +5 + 4 = +9 deg C

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