Details of even&odd altitude

I thought that, 0-180 are odd;
180-360 are even;

And this aircraft was flying at 35.000 ft but track is 210.

Shouldn’t it fly at 34.000 or 36.000?

It depends. Was he in free route airspace? Was he in a one way airway?

It’s rare to change level once in the cruise just to satisfy the semicircular rule.

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This is another topic with more in depth information, which says:

  • When on a magnetic heading of 360º - 179º, fly thousand foot altitudes beginning at FL290 at 4000 ft intervals
  • When on a magnetic heading of 180º - 359º, fly thousand foot altitudes beginning at FL310 at 4000 ft intervals

This is because above FL290, there’s 2000ft vertical separation. Also, some countries have different cruising altitudes, such as China, Mongolia, North Korea, Turkmenistan.

EDIT: As @Stu pointed out, most of the world is RVSM, which basically means the even/odd rule; look at the photo.


In Reduced Vertical Separation Minima (RVSM) airspace which is most of the world the requirement is only for 1000’ separation between opposite direction traffic. So you could fly at, for example, 290, 310, 330, 350 or 370.

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Good morning everyone. I would rather ask how it is better to coordinate the altitudes assigned by the flight plan, for example written by symbrief, with the international rules even / odd. Example: if I fly to the west and have to travel at an altitude of 340FL, as calculated by simbrief, is it better to stay at 310FL or 350FL?



I refer to long-haul flights where, perhaps, it is important to stick as much as possible to the calculated altitudes

But it didn’t solve my question, 35.000 ft to west is not normal.

Also i got some flights and they were 30.000 ft to east and 31.000 ft to west.

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