¶ Introduction
The semi-circular rule is the basic rule for flight level or altitude selection in function of the aircraft heading in order to ensure basic vertical separation.
This semi-circular defines the available flight levels in the conventional airspace and also in the reduced vertical separation airspace (RVSM) when applicable between FL290 and FL410.
RVSM definition = reduced vertical separation minimum
¶ Definition of odd and even flight levels
For answering to the need of flight level separation between the same types of flight, flight levels have been separated in two categories: the even and the odd flight level
- Even flight level: the last number before the final number 0 shall be even: FL 40, FL 60, FL 120...
- Odd flight level: the last number before the final number 0 shall be odd: FL 50, FL 70, FL 130
¶ Default worldwide semi-circular rule
¶ For IFR flights
The default worldwide semi-circular rule is the East/West orientation of the flight level parity:
- Your aircraft has track between 0° and 179°, your flight level or altitude must be odd.
- Your aircraft has track between 180° and 359°, your flight level or altitude must be even
By following the semi-circular rule, an IFR aircraft will limit possible conflicts between other aircraft coming in opposite direction by providing 1000ft separation between opposite west/east tracks.
¶ For VFR flights
The VFR rules of flight level and/or altitude selection is the same as IFR ones with adding 500ft to all levels.
The default worldwide semi-circular rule is the East/West orientation of the flight level parity:
- Your aircraft has track between 0° and 179°, your flight level or altitude must be odd.
- Your aircraft has track between 180° and 359°, your flight level or altitude must be even
By following the semi-circular rule, a VFR aircraft will limit possible conflicts between other aircraft coming in opposite direction by providing 1000ft separation between opposite west/east tracks.
¶ Specific semi-circular rule
¶ North/South rule
In some countries due to the IFR route or special regulations set by the local administration, the semicircular rule can be the North/South orientation of the flight level parity:
- Your aircraft has track between 90° and 269°, your flight level or altitude must be odd
- Your aircraft has track between 270° and 359° & between 0° and 89°, your flight level or altitude must be even.
¶ List of available flight levels and altitude (in feet)
¶ In airspace below FL290 (in feet)
¶ In RVSM airspace between FL290 and FL410 (in feet)
Reduced Vertical Separation Minima (RVSM) reduces the vertical separation above FL 290 and below FL410 to 1,000 ft.
This allows aircraft to safely fly more optimum routes, gain fuel savings and increase airspace capacity by adding new flight levels.
In areas where feet are used for altitude and where, in accordance with regional air navigation agreements, a vertical separation minimum of 1000 feet is applied between FL290 and FL410 inclusive.
¶ In non-RVSM airspace between FL290 and FL410 (in feet)
If your airspace is non-RVSM airspace, a vertical separation minimum of 2000 feet is applied between FL290 and FL410 inclusive.
¶ Airspace above FL410 (in feet)
¶ List of available flight levels and altitude (in meters)
¶ In metric airspace below S890 (in feet)
¶ In-RVSM metric airspace
In areas where metres (meter) are used for altitude and where, in accordance with regional air navigation agreements, a vertical separation minimum of 300 m is applied between 8 900 m and 12 500 m inclusive
¶ In a non-RVSM metric airspace
¶ Airspace above S1490 (in meter)
- ICAO documentation Annex 2 - Rules of the Air - 10th Edition July 2005
- VID 150259 - Creation
- VID 150259 - Wiki integration
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