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This question already has an answer here:

in my radio book i came across this communications.

Control: "Pacific 800 report passing FL 150"

Aircraft: "Pacific 800 Wilco"... "Pacific 800 passing FL150".

next.

Control: "Fiji 58 maintain 2500 feet" Aircraft: "Maintaining 2500 feet Fiji 58"

What is the formal explanation for FL and altitude, cause i am confused both are the same thing.

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marked as duplicate by Greg Hewgill, Ralph J, Pondlife, TomMcW, Ron Beyer Jun 20 '17 at 2:47

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  • $\begingroup$ Flight level is the indicated altitude when the altimeter is set to 29.92. This is done above 18,000 feet. $\endgroup$ – Riccati Jun 19 '17 at 22:46
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    $\begingroup$ @Riccati In the U.S. it is done above 18,000'. Other places have a different transition altitude, and in these places it can be entirely normal to have FL150 or FL080 or etc. $\endgroup$ – Ralph J Jun 19 '17 at 23:55
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This depends on where you are in the world but under the FAA here in the US the FAA defines it as

Flight level means a level of constant atmospheric pressure related to a reference datum of 29.92 inches of mercury. Each is stated in three digits that represent hundreds of feet. For example, flight level 250 represents a barometric altimeter indication of 25,000 feet; flight level 255, an indication of 25,500 feet.

In other words its the altitude if you set your altimeter to 29.92. In the US all aircraft in class A airspace (18,000 and up generally) set their altimeter to 29.92 thus flight levels are used at that altitude. When an aircraft is below Fl 180 the local altimeter setting is used.

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  • $\begingroup$ "FL180" isn't quite the same as 18,000' -- with a high altimeter setting, FL180 can be much more than 500' above 17,500' MSL. And with an altimeter setting below 29.92, FL180 won't exist (in the US) as a usable flight level. I recognize that these distinctions probably boarder on pedantic, but it's good to distinguish how "17,000' MSL" is 17,000 feet above mean sea level, while "FL180" is 18,000' above a reference datum plane that usually is different from mean sea level. (When the local altimeter setting is 29.92, then the two planes match up, but not otherwise.) $\endgroup$ – Ralph J Jun 20 '17 at 0:02
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Remember all altitudes are not created equal.

  • Pressure altitude is altitude above a set reference datum
  • Density altitude is pressure altitude corrected for nonstandard temperature.
  • Absolute Altitude is the actual altitude above the ground at your present location above the planet
  • Indicated Altitude is your altitude above Mean Sea Level (MSL) corrected for pressure variations at that location.

Flight levels are a pressure altitude reading, that is a height above a standard datum (MSL at an ambient pressure of 29.92" Hg.). This is used for a standard height reference at high altitudes (>18,000 ft MSL) so crews operating there can make use of a universal reference datum. An airplane cruising at FL390 may or may not actually be 39,000 ft above MSL depending on ambient conditions but maintains a standard height from a universal datum for traffic separation in this airspace.

Lower altitudes will make use of indicated altimeter readings to indicate height above MSL. This is favored because of the aircraft's proximity to terrain, providing appropriate reading for obstacle clearance, etc.

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