Talk:Flight level

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Different rules in U.S.[edit]

The rules for altitudes and directions are not correct for VFR flight in the United States. I'm not sure what other cases they are incorrect for. See, for example: —Preceding unsigned comment added by JoelKatz (talkcontribs) 13:25, 28 October 2003

How odd that the US should have a system different from the rest of the world. I fear for visiting pilots! If you want to amend the article to mention this, please do (but add to, rather than replace, the existing info, since it is correct for the UK, Europe, Australia and international airspace. GRAHAMUK 23:55, 28 Oct 2003 (UTC)
Are you saying VFR flight in international airspace uses a quadrantal rule? Or doesn't use thousands plus 500? Or even uses flight levels at all below 10,000 feet rather than altitudes adjusted for local pressure variation? If so, that makes the United States dramatically different. —Preceding unsigned comment added by JoelKatz (talkcontribs) 00:59, 30 October 2003

Inconsistent examples[edit]

The page says 'above FL245' but now has examples below FL245! —Preceding unsigned comment added by JoelKatz (talkcontribs) 01:52, 9 November 2003

Cloud base[edit]

Someone keeps inserting "See Also cloud base". The cloud base has nothing whatever to do with flight levels. Absolutely none. So stop that already! GRAHAMUK 22:44, 9 Nov 2003 (UTC)

Transition altitude / level...[edit]

i have a suspicion that the article is wrong on one thing ... it claims that the lowest FL in Poland is FL 285... which is simply not true, because transition level for Warsaw Okecie airport is FL 060 ... and that is certainly lower than FL 285...

which means, that those "lowest flight levels" are used for something different - maybe quadrantial / semicircular rules??? - but surely not for the transition levels... i'm not correcting the article because i'm not 100% sure - maybe someone more knowledgeable than me could fix it? - Blueshade 12:04, 22 July 2005 (UTC)

Transition altitude / level...[edit]

Currently studying for my PPL (no instrument rating...) and here's what I have found so far. But IANAPY (I am not a pilot yet)

  1. When flying below the "transition altitude", your altimeter is set to "QNH" (the local or regional pressure at MSL) and your altitude is expressed in "feet". At some airports, your altimeter may also be set to "QFE", the local pressure at field elevation.
  2. When flying above the "transition level", your altimeter is set to the standard atmosphere of 1013.2 mb/hPa, or 29.92 inHg, and your altitude is expressed as a "flight level" (100s of feet).
  3. The transition altitude differs from country to country, and may actually be different within certain airspace as well. It can be as low as 3000 feet (eg. Netherlands), and as high as 18000 feet (e.g. US and Canada).
  4. The transition level will always be above the transition altitude, and is typically calculated hourly, based on the expected lowest QNH in a certain region. Typically, the transition level is the lowest flight level (according to the semi-circular or quadrantical rule) that is actually above the transition altitude, and thus actually usable as a flight level.
  5. The transition layer is the layer between the TA and the TL. It's sort of a no-mans-land where you should not be flying for long periods of time.
  6. When flying above a certain altitude (generally 3000 ft AGL), you have to fly according to a semi-circular or quadrantical rule. Note that this 3000 ft AGL is NOT the same as the transition altitude!
  7. Most countries (except the UK and some Eastern European countries, I have found so far) use the ICAO standard for the semi-circular flight levels

This is the first semi-circular rule. It applies to IFR flights, with some exceptions (see RSVM) (in the UK it only applies to IFR flights above FL 245)

  • 0-179 deg (magnetic track): Odd thousands below the TA (3000, 5000, 7000, ...), odd FL above the TL (30, 50, 70, ...)
  • 180-359 deg: Even thousands below the TA (4000, 6000, 8000, ...), even FL above the TL (40, 60, 80, ...)

This is the second semi-circular rule. It applies to VFR flights (but not in the UK):

  • 0-179 deg (magnetic track): Odd thousands + 500 below the TA (3500, 5500, 7500), odd FL + 5 above the TL (35, 55, 75, ...)
  • 180-359 deg: Even thousands + 500 below the TA (4500, 6500, 8500, ...), even FL + 5 above the TL (45, 65, 85, ...)

This is the quandrantal rule. It applies in the UK to VFR and IFR flights below FL 245:

  • 0-89 deg (magnetic track): Odd FL (30, 50, 70, ...)
  • 90-179 deg: Odd FL + 5 (35, 55, 75, ...)
  • 180-269 deg: Even FL (40, 60, 80, ...)
  • 270-359 deg: Even FL + 5 (45, 65, 85, ...)

Variations on this theme:

  • Eastern European countries may or may not use true track instead of magnetic track.
  • Eastern European countries may or may not use flight levels based on meters instead of feet.
  • Above a certain level, the spacing increases due to inaccuracies of altimeters at high altitutes.
  • RVSM (Reduced Vertical Separation Minima) decreases the spacing again, but requires that your airplane is RVSM-certified.

The IFR semi-circular rule for Russia as an example:

  • 0-179 deg (true track): 900m and then every 600m (900, 1500, 2100, ...)
  • 180-259 deg: 1200m and then every 600m (1200, 1800, 2400, ...) —Preceding unsigned comment added by (talkcontribs) 14:15, 21 August 2005

Good stuff... the article should stress that FL quadrantal and semi-circular rule are based on "track magnetic" (M) not "track true" (T) as the article might be read.

So your track true (T) as on your chart plus/minus (+ for W - for E) variation (for your position on this planet in relation to magnetic north) gives the "track magnetic" (M) that depicts your flight level (odd or even).

And then it comes, Graham you are are absolutely right, cloud base has nothing to do with flight levels, but what about terrain clearance (which is always the responsibility of the pilot in command).

So for me the FL is depicted by magnetic track (M) and minimum terrain clearance. The latter one differs depending on the rules you apply (VFR or IFR).

From my experience, it is not that much different in the US and the UK. In both countries you SHOULD apply the quadrantal/semi-circular rule when in VFR and above 3,000ft. In IFR you HAVE TO fly a FL above TA (and the controler tells which one, when you get your departure clearance).

Personally I always try to fly a flight level (in VMC), just in case the flight becomes IMC at any stage. So I (and the controller) don't have to worry that much about separation when I do my pop-up filing. —Preceding unsigned comment added by (talkcontribs) 12:16, 4 October 2005


Should the page mention the abbreviated way of writing flight levels? (For example, is "flight level 123" = 12300 feet?) Ojw 13:56, 6 October 2005 (UTC)

Errr... Lazy head?[edit]

What's with the lazy head comment at the end of the article? It appears on the article Flight_level but not on Flight_Level I'm new and confused :/ —Preceding unsigned comment added by (talkcontribs) 06:43, 31 October 2005

Altimeter calibration[edit]

Could we add a snippet about when/why the altimeter would be set to something other than 29.92? --Scott P. —Preceding unsigned comment added by (talkcontribs) 03:18, 24 May 2007

Altimeters should not normally be set to the standard setting of 1013.25 (29.92 is the equivalent) below the transition level which is well covered in the article. treesmill 21:42, 28 October 2007 (UTC)

"1013.25" millibars is false precision. Such numbers don't appear on any standard aviation instruments. The figure used in regulations, in the training syllabus, and displayed on altimeters is simply 1013. 1013 millibars converts more closely to 29.91 in Hg, and the error can barely be resolved on the face of a standard aviation altimeter; these figures are used by pilots in actual practice. It would be a rare laboratory-grade instrument that could resolve 0.25 millibar, much less an instrument mounted in an aircraft panel. Since vertical separation of aircraft at Flight Levels is 1000 ft, minimum, an altimeter setting error of ±0.01 in Hg is negligible and well within the margin of safety. —QuicksilverT @ 07:58, 19 November 2009 (UTC)

Question could arise need an answer[edit]

Does the transition layer extend and cover the hole FIR (controlled airspace and uncontrolled airspace?), i.e: in uncontrolled airspace is there any limit or altitude at which the pilot has to set his altimiter to standard 1013.25? —Preceding unsigned comment added by (talk) 22:41, 12 October 2007 (UTC)

The transition layer covers the whole airspace as it is simply the the region between two altitudes. However, outside of controlled airspace it is not obligatory for a pilot to set his altimeter to 1013 (in fact outside controlled airspace it's not even obligatory to carry an altimeter at all!). —Preceding unsigned comment added by (talk) 21:07, 2 May 2008 (UTC)

Highest used flight level[edit]

What is the highest used flight level? -- 14:34, 28 October 2007 (UTC)

There is no theoretical maximum although aerodynamic flight becomes impossible at something in the region of 100,000 feet, FL1000. The highest FL used by civil aircraft these days is somewhere in the 500s. A Gulfstream 550 executive jet has a maximum altitude of FL510 but there may be others with higher limits. Concorde had a maximum cruise level of FL600. Military aircraft obviously operate higher than this. treesmill 21:38, 28 October 2007 (UTC)

Lowest and highest real elevations[edit]

Non flyers want to know: e.g., FL320: assume two days with very different pressures. What is the

  • lowest
  • highest

actual, real, non-pressure related, tape measure elevation this could mean? Jidanni (talk) 02:35, 21 March 2008 (UTC)

The highest sea-level pressure ever recorded was 1085mb. On that day, an aircraft at FL320 would have been flying at 34,100ft. The lowest air pressure ever recorded was 850mb, which would mean an aircraft at FL320 was actually at 27,050ft. However, those were extreme conditions (850mb was measured in the middle of a tornado, and any aircraft flying in that would be torn apart by the storm). In practice, the range of pressures you are likely to encounter will be between 985mb (cold, dry day) and 1045mb (hot and humid) which means your aircraft could be anywhere between 31,100ft and 32,900ft. This works for any FL, e.g. FL200 would be 20,000ft ± 900ft.
Actually, these figures are still approximations because they make assumptions about the rate at which air pressure decreases as you get higher, but this effect is not as great as that caused by the variation in sea-level pressure. Hope that answers your question. (talk) 20:57, 2 May 2008 (UTC)

Ah, answers it with pleasure. Thanks. Consider adding something like that into the article if not there already.

Hmmm, just hope everybody has accurate pressure meters, and GPSs... if they had to do it all over again maybe they would have used hard tape measure elevations for the FL definition... but wait, if GPS fails one could fall back to the pressure meter which is not dependant on external satellites, so pressure related FLs are safer. OK over and out. Jidanni (talk) 02:28, 4 May 2008 (UTC)

@ With all due respect, I must correct the significant misinformation in your post. (Better late than never.)
"In practice, the range of pressures you are likely to encounter will be between 985mb (cold, dry day) and 1045mb (hot and humid)."
As we speak, Tropical Storm Debby, over the Gulf of Mexico, has a central pressure of 991 mb. It is very, very wet, and quite warm. In fact, a pressure of 985 mb will almost always be an area of disturbed weather. (exception noted toward the end) In the tropics or subtropics, it is likely to be at least a tropical depression, if not a tropical storm. Lower-grade full hurricanes may develop at pressures at or only a little below this level. Outside the tropics, the famous "Nor'Easter" storms off the coast of New England, or the storms coming out of the Gulf of Alaska that may affect North America's West Coast (including California) are all low-pressure centers. These may be areas of cold, but there is still significant precipitation, liquid or frozen. Same in the rest of the world. (Direction of rotation is reversed in S. Hemisphere.)
Cold air is more dense than warm air; hence, cold air weighs more upon iteslf and upon the earth. Very high pressure is found in the high-centers *behind* polar or other cold fronts moving south. While the frontal zone (interaction of colder and warmer air) is likely to be high in precip, this is followed by very clear skies and ultra-low humidity. Typical: Storm passes over the Rockies, leaves a ton of snow, then the next day or two, it is totally sunny but much colder. (Humidity typically around 10%.) This is the cold, dry air behind the front, and the pressure is very high. Weather *underneath* high pressure centers tends to be clear and dry.
That same cold, heavy air, gravity-driven to seek lower levels than the high deserts of Utah (often, the high center is very near the "four corners" region), finds its escape towards sea level in the mountain passes between the high deserts and the coastal basin of Southern California. Winds whip through passes like Cajon Pass, forming Santa Ana Winds. The dense air is further compressed, which also warms it. So while it eventually finds its escape and lowered pressure toward the ocean, the pressure in mb in the deserts just to the north and east of the Los Angeles basin can be very high -- around 1030 mb, IIRC, while still being ghastly hot.
In short, very low pressure doesn't usually equal "cold and dry". With the exception of a thermal low over hot deserts, it equals disturbed weather, generally wet, and may be hot or cold, depending on latitude. (An extreme thermal low may induce monsoon conditions.) Very high pressure *never* equals "hot and humid". It may equal "cold and dry", or, as above, hot and dry. Regards, Unimaginative Username (talk) 05:23, 25 June 2012 (UTC)

True altitude[edit]

Trellis, thanks for pointing out my earlier error. Indeed QNH does not give pressure altitude. But nor does it give "true" altitude, except under ISA conditions (temperature 15ºC, density 1.225kg/m3 at sea-level, both decreasing uniformly with altitude). An altimeter cannot be calibrated for temperature or density (see Altimeter), so the altitude shown will vary from the actual (or true) altitude depending on the conditions at a given time and place. This time I have just removed the word "true", which incorrectly implies an accuracy which is not justified. (talk) 22:54, 2 May 2008 (UTC)

Good point, I agree with your change to just remove "true" and make it more accurate on the whole. Thanks for your contributions! —Krellis (Talk) 23:13, 2 May 2008 (UTC)

An opposite flight level can be defined as a non-standard flight level ???[edit]

Many ATCO's, especially from UK, refer to an opposite flight level (for example if the flight is eastbound at FL350 and they instruct a flight to climb to FL360 opposite) as a non-standard flight level. From my personal point of view an even or uneven flight level it is a standard one as long as it is as well defined in this document. I could say for example that FL420 is a non-standard flight level under RVSM between FL290 and FL410.

Concluding: an opposite flight level can be described as an non-standard flight level?

Thanks! — Preceding unsigned comment added by (talk) 20:01, 15 July 2011 (UTC)

Format of FL[edit]

  • I am quite sure they are written as FL45, FL50 etc. instead of FL045, FL050 for levels under FL100.--Abuk SABUK (talk) 22:16, 25 February 2013 (UTC)


This source says that, as from one minute past midnight, UTC, on the 17 November, 2011, Russia has been using "the standard ICAO Flight Levels in feet, that we are used to in the rest of the world – which is happening everywhere except Mongolia." Is this true? What is the situation in Ukraine? Martinevans123 (talk) 18:19, 29 July 2014 (UTC)

Half-circle rules for France[edit]

your text is wrong: France has since definition from ICAO (in French OACI) accepted and used the Standard rhuleset East/West. There are some heavy trafic routes arround big public aerports, where mandatory routing and FL is fixed. That spacing was made out of ICAO for responds to main traffic orientation, somewhere it could be N/S instead of east-west. But the rhules for half-cirqle according to ICAO are mandatory over the entire french airspace where no special routing is defined.

cosy-ch -- (talk) 09:51, 1 February 2016 (UTC)

Do you have a source? Thanks. Martinevans123 (talk) 23:32, 16 March 2016 (UTC)


Is there an officially recognised upper limit? Does this vary by region? The upper limit for RVSM is 41,000. Martinevans123 (talk) 23:26, 16 March 2016 (UTC)