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when were flight levels established?

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2 Answers

  1. John D. Collins on May 17, 2012

    I have no clue when flight levels were first established, but the Civil Aeronautical Manual Part 60 (CAM 60) Air Traffic dated May 16, 1961 defines them starting at FL240 and above. In 1963, the Federal Aviation Regulations were released with the same definition.  The next year, flight levels were defined starting from FL180 and it has remained the same in the USA since.  Some other countries define lower flight levels, particularly in places where the infrastructure is not as good as in the USA or there are large areas of open water and it isn’t practical to obtain a local altimeter setting. Examples include in the Caribbean where flight levels are much lower (I don’t know the lowest used, but I have been assigned FL060). One good reason why flight levels are used at the higher altitudes has to do with the kind of operations flown in this airspace tend to be high performance where it is impractical to constantly be changing the altimeter setting every ten minutes.


    A Flight Level is defined as an altitude indication based on a level of constant atmospheric pressure referenced to a datum pressure of 29.92.  So FL240 is an indication on the altimeter of 24,000 feet with the pressure set to 29.92.  It is the same as a pressure altitude of 24,000 feet, but it isn’t a MSL altitude of 24,000 feet that would be what the altimeter would indicate if the altimeter setting was adjusted for the local equivalent sea level pressure. 


    MSL altitude and pressure altitude are not the same as true altitude as they are based on a model of the atmosphere that is rarely if ever found in the real atmosphere. Besides pressure, a standard temperature and lapse rate are assumed and this can affect the altimeter indication by thousands of feet, particularly as altitude increases.  If you want something close to true altitude, look at your GPS altitude, as it will generally have much less error than a barometric or pressure altitude.  GPS altitude will keep you out of the rocks, but you still have to use barometric altitude or flight levels as appropriate to maintain vertical separation from other traffic.

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  2. Jim Foley on May 17, 2012

    Not really inline with your question, but an addition to John’s:  According to one of my professors who has flown for airlines all over the world, the lowst is FL050 (5,000ft.)

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