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Quadrantal rule
This rule applies to IFR flights in the UK both in and outside of controlled airspace except that such aircraft may be flown at a level
other than required by this rule if flying in conformity with instructions given by an air traffic control unit, or if complying with notified
en-route holding patterns or holding procedures notified in relation to an aerodrome. The rule affects only those aircraft operating
under IFR when in level flight above 3,000 ft above mean sea level, or above the appropriate transition altitude, whichever is the higher,
and when below FL195 (19,500 ft above the 1013.2 hPa datum in the UK, or with the altimeter set according to the system published by the competent authority in relation to the area over which the aircraft is flying if such aircraft is not flying over the UK.)
The rule is non-binding upon flights operating under VFR.
Minimum vertical separation between two flights abiding by the UK Quadrantal Rule is 500 ft (note these are in geopotential foot units).
The level to be flown is determined by the magnetic track of the aircraft, as follows:[5]
- Magnetic Track 000 to, and including, 089° - odd thousands of feet (FL70, 90, 110 etc)
- Magnetic Track 090 to, and including, 179° - odd thousands plus 500 ft (FL75, 95, 115 etc)
- Magnetic Track 180 to, and including, 269° - even thousands of feet (FL80, 100, 120 etc)
- Magnetic Track 270 to, and including, 359° - even thousands plus 500 ft (FL85, 105, 125 etc)
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, for example, southbound traffic uses odd flight levels. In Europe are commonly
used ICAO separation levels as per the following table