Originally Posted by
mbreuer
I was not aware of that. In the US it's only used above 18,000MSL. Also the altitude at which one sets the altimeter to 29.92... so in fact 18000' |= FL180 unless the actual altimeter reading is 29.92.
And they don't call them FL's, just 'F', as in F80.
I'm sure you are aware that a FL altitude is just a pressure level above some standard reference level of 29.92 inches. While the absolute altitude will vary w/ pressure changes, the relative part doesn't and todays instruments are very very precise. Witness the Gol and Embraer collision over Brazil.