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Semi-Circular rule explain

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Semi-Circular rule explain

Old 26th Apr 2019, 09:38
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Join Date: Apr 2019
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Question Semi-Circular rule explain

Hello!
I would like to ask about the semi-circular rule for flight levels as I thought I understand it pretty well, but then looked at the flights like for ex.: EGSS-LPPT or LPPT-LIME. Compared to the rule, these flights have their cruising altitude as a total opposite. (EGSS-LPPT) as it is flying southwest with a heading in the range of 180-359 deg. it should be cruising in an even flight level. Obviously they usually cruise in FL370. Now comes the question...why is that? Is there some kind of restriction?
Thank you very much for all clarifications and happy landings!
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Old 26th Apr 2019, 10:42
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First, there are exceptions to many rules in the aviation business.

When it comes to cruising levels ICAO says in Annex 2 'An IFR flight operating in cruising flight in controlled airspace shall be flown at a cruising level...selected from: a) the tables of cruising levels in Appendix 3; or b) a modified table of cruising levels, when so prescribed in accordance with Appendix 3 for flight above FL 410; except that the correlation of levels to track prescribed therein shall not apply whenever otherwise indicated in air traffic control clearances or specified by the appropriate ATS authority in Aeronautical Information Publications'. Appendix 3 essentially converts the semi-circular rule to a set of appropriate levels in both feet and metres and for RVSM and non-RVSM airspace.

So, particularly in areas where there are busy routes, or just a lot of traffic generally, one-way systems are often set up which utilise what are sometimes called ODLs - opposite direction levels. Sometimes these routing schemes are established for short periods only, to cater for lots of flights heading to a particular event at the same time, and then it's all reversed at the end of the event.
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