What does this blue dotted line mean?

I knew it, but cannot remember what the blue dotted line mean in the FPLTOIF map:

Many thanks in advance!

Speed of sound decreases up to the blue line. It’s only really relevant in IF for fuel efficiency related reasons.


Thank you. So it means one would gain some bit of TAS above the line, at the same Mach speed? will check one day.

I had asked Chris S. about it once before. IIRC it is the boundary of the Troposphere and Stratosphere. Most commercial aviation flies above the Troposphere if possible, as the air is thinner, which allows less drag and more efficient fuel burn.

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In the troposphere, it’s less likely to develop weather systems

Below the blue line speed of sound decreases with altitude. About the blue line it doesn’t.

The blue line is provided by SimBrief and is the tropopause altitude. Why do you need to know where the tropopause is?

  • Tropopause (height) is when the temperature is considered constant (stops becoming colder as you go higher)
  • The air right below the tropopause may be home to turbulent winds
  • Pilots in real life need to know because the temperature can have an impact on fuel/thrust

Aviation Weather


Thanks to all of you.

I was close!

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