Editing Flight level (section)

==Transition altitude==
While use of a standardised pressure setting facilitates separation of aircraft from each other, it does not provide the aircraft's actual altitude above sea level. Below the Transition level (which varies worldwide), the altimeter is set to the local altimeter setting, which can be directly compared to the known [[elevation]] of the terrain. The pressure setting to achieve this varies with local atmospheric pressure. It is called [[QNH]] ("barometric pressure adjusted to sea level"), or "[[altimeter setting]]", the current local value is available from various sources, including [[air traffic control]] and the local airport weather frequency or a [[METAR]]-issuing station.

The '''transition altitude''' (TA) is the altitude above sea level at which aircraft change from the use of local pressure to the use of standard pressure. When operating at or below the TA, aircraft altimeters are usually set to show the altitude above sea level.<ref name="CAP410">{{cite web | title=CAP 410 Manual of Flight Information Services | publisher=UK Civil Aviation Authority | id=CAP410 | url=http://www.caa.co.uk/docs/33/CAP410PtA.pdf | access-date=25 February 2013 }}{{dead link|date=January 2018 |bot=InternetArchiveBot |fix-attempted=yes }}</ref> Above the TA, the aircraft altimeter pressure setting is changed to the standard pressure setting of 1013 [[pascal (unit)|hectopascals]] (equivalent to millibars) or 29.92 [[inches of mercury]], with the aircraft altitude will be stated as a flight level instead of altitude.

In the United States and Canada, the transition altitude is {{cvt|18000|ft|m}}.<ref>{{Cite web|url=https://www.law.cornell.edu/cfr/text/14/91.121|title = 14 CFR § 91.121 - Altimeter settings}}</ref> In Europe, the transition altitude varies and can be as low as {{cvt|3000|ft|-2}}. There are discussions to standardize the transition altitude within the [[Eurocontrol]] area.<ref>{{cite web | title=A Common European Transition Altitude; An ATC perspective | publisher=Eurocontrol | url=https://www.eurocontrol.int/sites/default/files/content/documents/nm/airspace/airspace-atmprocedures-cta-atc-perspective.pdf | access-date=3 April 2014 | url-status=dead | archive-url=https://web.archive.org/web/20130926112255/http://www.eurocontrol.int/sites/default/files/content/documents/nm/airspace/airspace-atmprocedures-cta-atc-perspective.pdf | archive-date=26 September 2013 | df=dmy-all }}</ref> In the United Kingdom, different airports have different transition altitudes, between 3000 and 6000 feet.<ref>{{cite web | url=https://flyinginireland.com/2016/01/iaa-transition-altitude-consultation/ | title=IAA Transition Altitude Consultation | date=17 January 2016 }}</ref>

On 25 November 2004 the [[Civil Aviation Authority of New Zealand]] raised New Zealand's transition altitude from {{convert|11000 to 13,000|ft|m}} and changed the transition level from FL130 to FL150.<ref>{{cite web | title=Part 91, Amendment 12 | publisher=[[Civil Aviation Authority of New Zealand]] | url=http://www.caa.govt.nz/rules/Amendments/Part_091_040705_Amendment_11.pdf | access-date=4 February 2009 | archive-url=https://web.archive.org/web/20160306222005/https://www.caa.govt.nz/rules/Amendments/Part_091_040705_Amendment_11.pdf | archive-date=6 March 2016 | url-status=dead }}</ref>

The '''transition level''' (TL) is the lowest flight level above the transition altitude. The table below shows the transition level according to transition altitude and QNH. When descending below the transition level, the pilot starts to refer to altitude of the aircraft by setting the altimeter to the [[QNH]] for the region or airfield.
{| class="wikitable" align="right"
|+ Table for determining transition level{{Citation needed|date=December 2016}}
|-
!rowspan=2| [[QNH]]<br>(in hectopascals) !! colspan=5 | Transition altitude (in feet)
|-
! 3,000 ft<br>(900 m) !! 4,000 ft<br>(1200 m) !! 5,000 ft<br>(1500 m) !! 6,000 ft<br>(1850 m) !! 18,000 ft<br>(5500 m)
|-
| align="center" | 1032–1050 || FL025 || FL035 || FL045 || FL055 || FL175
|-
| align="center" | 1014–1031 || FL030 || FL040 || FL050 || FL060 || FL180
|-
| align="center" | 996–1013 || FL035 || FL045 || FL055 || FL065 || FL185
|-
| align="center" | 978–995 || FL040 || FL050 || FL060 || FL070 || FL190
|-
| align="center" | 960–977 || FL045 || FL055 || FL065 || FL075 || FL195
|-
| align="center" | 943–959 || FL050 || FL060 || FL070 || FL080 || FL200
|-
|}

The '''transition layer''' is the airspace between the [[transition altitude]] and the [[transition level]].

According to these definitions the transition layer is {{convert|0–500|ft|-1}} thick. Aircraft are not normally assigned to fly at the "'transition level'" as this would provide inadequate separation from traffic flying on QNH at the transition altitude. Instead, the lowest usable "'flight level'" is the transition level plus 500&nbsp;ft.

However, in some countries, such as [[Norway]] for example,<ref>{{cite web |title=Eurocontrol specifications for harmonized Rules for Operational Air Traffic (OAT) under Instrument Flight Rules (IFR) inside controlled Airspace of the ECAC Area (EUROAT) |url=https://www.eurocontrol.int/sites/default/files/2020-01/change-6-eurocontrol-specificatitions-oat-ifr-rules-version-3-0.pdf |website=Eurocontrol |page=63 |language=en }}</ref> the transition level is determined by adding a buffer of minimum {{cvt|1000|ft|m}} (depending on QNH) to the transition altitude. Therefore, aircraft may be flying at both transition level and transition altitude, and still be vertically separated by at least {{cvt|1000|ft|m}}. In those areas the transition layer will be {{cvt|1000|-|1500|ft|m}} thick, depending on QNH.

In summary, the connection between "transition altitude" (TA), "transition layer" (TLYR), and "transition level" (TL) is

TL = TA + TLYR