Direction of Cruising Levels

To reduce the danger of a collision between enroute traffic flying on the same cruising level, procedures described in the following article have been established. This also greatly reduces the workload for ATC, as not every aircraft needs to be coordinated individually between two sectors. Within the same sector, it is up to the controller to decide if the procedures will be applied or not. 

Semicircular Cruising Flight Level System (Even/Odd Rule)

Generally, according to the semicircular cruising flight level system, flights with a planned magnetic course of 360° to 179° fly on odd flight levels. Flights with planned courses from 180° to 359° fly on even flight levels. Where no other regulation or operational agreements exist, this system shall be applied for handing off traffic between different sectors. 

Note that above FL410, the conventional vertical separation minimum (CVSM) applies. Therefore, the odd/even levels will continue in 2000 ft steps. Because of that, FL430 is considered an even level, whereas FL450 is an odd level again. 

For VFR traffic flying above 3000 ft AGL, an additional 500 ft shall be added to each flight level (e.g. FL95, FL105, FL115, etc.). 

Airway Regulations

Besides the Semicircular Cruising Flight Level System, all airways in Germany have a defined cruising level range depending on the direction of flight. These airway definitions in the AIP overwrite the Semicircular Cruising Flight Level System and may only be overwritten by any other operational agreements (see below). 

The definition of each airway can be found in the AIP IFR chapter ENR 3. 

Example:

For each airway, a special column exists defining the use of cruising level per direction of flight. In our case, airway M736 defines the cruising levels in the following way:

• Traffic flying in the direction from SALLO to BIRMO shall use odd flight levels
• Traffic flying in the direction from BIRMO to SALLO shall use even flight levels. 

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This example also shows that the airway definitions sometimes deviate from the general Semicircular Cruising Flight Level System. As traffic proceeding on airway M736 from SALLO to BIRMO (actually flying slightly westbound) is required to use odd flight levels, whereas the Semicircular Cruising Flight Level System would require an even flight level. 

Other Operational Agreements

Due to the ongoing implementation of Free Route Airspace (FRA), the airway regulations are losing more and more relevance. Therefore, the use of operational agreements (e.g. LoA) can be useful to define cruising levels, especially for traffic on northbound/southbound routings. These operational agreements have the highest priority of all regulations described in this article. 

Example:

In the LOP Karlsruhe, specific en route level allocations between each sector are defined in paragraph 4.2. As you can see, sometimes it is even possible to use even and odd levels simultaneously for the same direction of flight ("even/odd" defines the use of all levels).