Why do airliners over south-west Europe cruise at flight levels with inversed numbers?

Question

Over North America and Europe, usually when airliners go east (heading 0-179) they cruise at a flight level with an odd number (e.g. FL370) and when going west (180-359) they cruise at an even number (e.g. FL380), right?

When looking on flightradar24, all planes follow this pattern except for planes flying over south-west Europe (Iberian peninsula, Cote d'Azur, Mallorca and so, as well as Morocco): there, planes flying to the east cruise at even numbers and to the west at odd numbers. Why do planes flying over or near the Iberian peninsula have the flight levels inversed?

Answer 1

The above persons guessed correctly. Since a majority of flights in the area fly in north / south directions, rather than east / west, France, Spain, Italy and Switzerland generally apply the NorthEVEN and SouthODD rule. In Germany, some airways also follow this rule on their course between the Swiss and Danish airspace. This is easily seen in enroute IFR charts.

You can also check it, for instance, in the Spanish AIP, ENR 1.7.3, here:

_{Spanish AIP, ENR 1.7.3 , page 3.}

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In the French AIP section ENR 1.3.5, you can see how they interestingly use the east / west rule for IFR flights below FL200, and the north / south rule for IFR flights at or above FL200.)

_{French AIP ENR 1.3.5.}