alt Hacker News"Maybe, or maybe FL180 is a nice clean line for class A airspace. No need to bother transcontinental flights for a local issue."
Way more plauible
"Maybe, or maybe FL180 is a nice clean line for class A airspace. No need to bother transcontinental flights for a local issue."
Way more plauible
FL180 is the floor of Class A airspace, "the flight levels", where airliners etc. operate.
Relevant chapter from FAA "Pilot's Handbook of Aeronautical Knowledge": https://www.faa.gov/sites/faa.gov/files/17_phak_ch15.pdf
In the "Flight Levels", altitudes are referred to not in feet above sea level but as "FLxxx" where xxx is a nominal altitude in 100s of feet.
Altimetry is done using barometric pressure. Since this varies with weather, airplanes at lower altitudes set their altimeters to the local barometric pressure for a reasonably accurate reading. In the flight levels, where planes are typically covering ground quickly and there is very little chance of your path conflicting with the surface of the Earth, every plane sets to an agreed-upon reference of 29.92 inches of mercury as the altimeter setting.