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At which pressure and temperature conditions may you safely assume that the minimum usable flight level at least lies at the same height, as the minimum sector altitude?
  • A
    In a cold low pressure region
  • B
    At a temperature less than or equal to that of the ISA and where the QNH is less than 1013.25 hPa
  • C
    In a very cold area with a QNH of 1015 hPa
  • D
    At a temperature greater than or equal to that of the ISA and where the QNH is greater than or equal to 1013.25 hPa

We want the condition that gives the minimum safe flight level to be at or above the minimum save altitude.
So, the question basically asks with what temperature and pressure will you get the most favourable condition related to obstacle clearance.

We know the following about pressure and temperature:

  • When an aircraft is in a lower pressure area, the altimeter will overread. The aircrafts true altitude will be lower than indicated.
  • When an aircraft is in a higher-pressure area, the altimeter will under read. The aircrafts true altitude will be higher than indicated.
  • When an aircraft is in a colder airmass than ISA, the altimeter will overread. The aircrafts true altitude will be lower than indicated.
  • When an aircraft is in a warmer airmass than ISA, the altimeter will under read. The aircrafts true altitude will be higher than indicated.

Safest would be if the altimeter would under read.Because we want our actual flight level to have the biggest probability to be higher, or the same as our indicated altitude.

The altimeter will only under read if the temperature is warmer or the pressure is higher.At these conditions we can safely assume that the minimum usable flight level at least lies at the same height as the minimum safe altitude.

So that is why we want the temperature to be greater or equal to ISA and the QNH to be greater or equal to 1013,25.

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