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[[:en:Template:Mergefrom]]<!-- {{Mergefrom|Transition altitude|date=February 2007}} -->
Flight levels are not used close to the ground, for the obvious reason of the true height of an aircraft relative to an object on the ground needing to be known. For this reason, anフライト・レベルは、地表に近いところでは、地上にある物体に対する航空機の真の高度を知る必要があるために、用いることができない。この理由により、[[転移高度]]（transition altitude called the '''transition altitude''' (、TA）と呼ばれる高度が定められている。TA) is defined. When operating at or below the TA, aircraft altimeters are set to show the true altitude above sea level. The pressure setting to achieve this is called 以下の高度で飛行するとき、航空機高度計は真の高度を示すように設定される。このときに用いられる圧力は[[Q符号|QNH]] or "altimeter setting" and is available from various sources, including あるいは[[高度計規正値]]と呼ばれ、[[航空交通管制|air traffic control]] and the local や地域の[[定時飛行場実況気象通報式|METAR]]-issuing station. による航空用気象情報などの様々な情報源から入手することができる。The TA varies from country to country and, in some cases, within a country. It is clearly desirable that it is chosen to be above any obstacles within some distance of the operating area, however the size of this area varies very widely. In the [[イギリス|United Kingdom]], the standard TA outside [[:en:controlled airspace|controlled airspace]] is 3,000 ft, but within controlled airspace it varies and will be specified on the appropriate charts. For example, in the controlled airspace around [[ロンドン・ヒースロー空港|London Heathrow]], the transition altitude is 6,000 ft; for Manchester, it is 5,000 ft; for Birmingham, 4,000 ft. Throughout Germany the TA is 5,000 ft; throughout Belgium the TA is 4,500 ft. There is considerable discussion of standardisation of the TA throughout Europe, or at least a large part of it, but so far no agreement has been reached. Throughout the USA and southern Canada, the TA is 18,000 ft. In other parts of the world, other TA values apply.
The lowest usable flight level above the TA is called the '''transition level''' (TL). Because the transition altitude is fixed and the atmospheric pressure varies, the TL varies from time to time. It is therefore possible to have a valid flight level of 30 in the UK when the atmospheric pressure is above 1013.25 mbar. Note that vertical separation is not guaranteed between an aircraft flying at the transition altitude and one flying at the transitional level. For example, in the UK with a transition altitude of 3,000 ft and a QNH of 996, the Transition Level is FL35; equivalent to an altitude of less than 3,100 ft. (See [http://www.caa.co.uk/docs/33/CAP493Part1.pdf Manual of Air Traffic Services Part 1 Appendix A])
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