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The pressure above 18,000 feet

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General Aviation

1.) It is my understanding that all pilots must set their altimeters to 29.92 (pressure altitude) above 18,000 feet.  However, as planes in Class A airspace fly into different areas of pressure, the altimeters should change (read higher or lower) depending on the pressure of that area.  Therefore, are planes usually constantly ascending or descending as they move into different areas of pressure above 18,000 feet?  

2.) Why was 18,000 feet chosen as the floor Class A airspace? 

5 Answers

  1. VFR Student on Jan 29, 2011

    Allow me to clarify my question better:
    In order to MAINTAIN THE SAME FLIGHT LEVEL, are planes usually either constantly ascending or descending due to area pressure changes above 18,000 feet? 

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  2. John D. Collins on Jan 29, 2011

    The short answer to your first question is yes.
    However, there are several good reasons why flight levels (or pressure altitude) is used above 18000 feet. Barometric altimeters are based on a standard atmosphere model of pressure and temperature, but no one told the atmosphere.  The higher you are above and the further from the location where the altimeter setting is measured, the greater is the error.  This is compounded by non standard temperatures which can affect the actual altitude by hundreds of feet or more.
    Below 18,000 feet, you are required to set/reset your altimeter to the local source every 100 NM. Above 18,000 feet is mostly the domain of high performance aircraft with turbo charged engines or Turbojet/Turbo Prop engines. A jet traveling at 600 NM/hour would have to change the altimeter setting every ten minutes, which would be an unnecessary burden. So above 18,000 feet, all aircraft set their altimeter settings to 29.92, and the indicated pressure altitude is called a flight level, so flight level 180 is 18,000 feet pressure altitude.  Even with a standard setting above FL 180, the standard baro altimeter starts to have too much error at higher altitudes above FL290, so altitudes are separated by 2,000 feet, instead of 1000 feet.  Recently, a more accurate requirement for altimetry allows for RVSM (Reduced Vertical Separation Minimum, so that some of the flight levels may be used with 1000 foot separation and only aircraft equipped with RVSM can occupy these flight levels.
    There is another issue with the flight levels that has to do with aircraft below 18000 feet use the nearby altimeter setting, which means that when the baro setting is higher than standard pressure, there is less and less separation between aircraft above and below the flight levels. FAR 91.121 resolves this by establishing the lowest available flight level based on the barometric pressure reading.
    Someone else will probably be able to answer to your second question, but I would guess it has to do with the performance requirements of aircraft that generally operate in the above 18,000 foot environment and the need to provide separation between these aircraft.

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  3. John A Lindholm on Jan 29, 2011

    Above 18,000 ft, the altimeter will read whatever altitude the aircraft is leveled at.  It does not change, reading higher or lower.  As the surface pressure changes, the aircraft will move up/down as it transits the area.  You cannot detect this in the aircraft other than an uncorrected GPS readout….  or using a chart to determine your actual altitude.
    The choice of 18,000 feet is the result of terrain.  In many other parts of the world, the use of 29.92 (1013mb) can be much lower… and often varies whether in climb or descent. 

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  4. Wesley Beard on Jan 29, 2011

    I don’t know why 18,000ft was the chosed transition leVel and transition Altitude.  If you search on instrument approach plates you will always see a transition level of FL180 and a transition Altitude of 18,000ft.  It’s easy to remember as the “V” in level indicates you change when you descend through that altitude and the “A” in Altitude means you set 29.92 when climbing through that altitude.
    Europe typically has a different transition altitude and transition level in the 5,000 to 7,000ft MSL range.  I’m pretty sure John L.’s answer is correct concerning terrain though.  In fact both John’s answers are correct!

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  5. Dithel Bless on Apr 30, 2012

    18,000 Ft was chosed because half (50%) of the atmosphere weight is below that level, therefor PITOT-STATIC Instruments are more accurate under those levels.
    Regards From Lima Peru

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