Flight Level - Semicircular/Hemispheric Rule

Semicircular/Hemispheric Rule

(Versions of this apply to IFR in the UK inside controlled airspace and generally in the rest of the world)

The semicircular rule (also known as the hemispheric rule) applies, in slightly different version, in all of the world, including in the UK inside controlled airspace.

The standard rule defines an East/West track split:

• Eastbound – Magnetic Track 000 to 179° – odd thousands (FL 250, 270, etc.)
• Westbound – Magnetic Track 180 to 359° – even thousands (FL 260, 280, etc.)

At FL 290 and above, if Reduced Vertical Separation Minima are not in use, 4,000 ft intervals are used to separate same-direction aircraft (instead of 2,000 ft intervals below FL 290), and only odd flight levels are assigned, depending on the direction of flight:

• Eastbound – Magnetic Track 000 to 179° – odd flight levels (FL 290, 330, 370, etc.)
• Westbound – Magnetic Track 180 to 359° – odd flight levels (FL 310, 350, 390, etc.)

Countries where the major airways are oriented north/south (e.g. New Zealand; Italy; Portugal) have semicircular rules that define a North/South rather than an East/West track split.

In Italy and Portugal, for example, southbound traffic uses odd flight levels; in New Zealand, southbound traffic uses even flight levels. In Europe are commonly used ICAO separation levels as per the following table: