Airways Cruise levels
 

Has always puzzled me - how are 'non-standard' hi airway cruise levels (odds/evens) decided? What makes someone declare a westbound hi airway to be an 'ODD'?

It's an ICAO thing... In RVSM westbound is EVEN... in non RVSM the levels are all odd?

Non RVSM
W/Bnd.........E/Bound
FL280.......... FL290
FL310.......... FL330
FL350.......... FL370
FL390.......... FL410
FL430.......... FL450

RVSM Airspace
West Bound East Bound
FL280.......... FL290
FL300.......... FL310
FL320.......... FL330
FL340.......... FL350
FL360.......... FL370
FL380.......... FL390
FL400.......... FL410
FL430.......... FL450

Interesting theory, but UN866, for example?, SAM Southwest-bound='ODD'? UN609 northeast-bound towards GIROM ='EVEN'.

I am braced to discover it is yet another 'French exception' to ICAO.:mad:

UN866 may be odd for the southbound component due to the confliction points over Jersey and Guernsey. The southbound aircraft would 'meet' the westbound oceanics on 'even' levels

Between the West End and Shannon there is a FLAS (Flight Level Allocation System) which changes to accomodate more aircraft coming off the ocean (early morning) or heading onto the ocean (mid morning to early afternoon)

Even and odd flight levels generally are in accordance with east-west directions as agreed by State authorities in ICAO forums.

One exception is a Flight Level Allocation System, which gives more Flight Levels in a particular direction to enhance capacity of the system. See the example given by Carbide Finger.

The other exception is in the case of a generally North-South route, which sometimes bends a little to the east, at other stretches a little to the west. It would be silly to change levels every other 40 miles or so. Therefor the responsible authorities (that is not ICAO, but the ATC authority of the State or group of States) decide which level to use for that complete route or at least a longer stretch of that route. The route from (beyond) Paris to Scandinavia is an example, the normal level change to an odd Flight Level is just south of the SPY area after flying many miles a bit eastbound. For tactical reasons controllers may co-ordinate and agree to put the level change for an individual aircraft further north or further south, whichever is most convenient to reduce workload and enhance capacity.

In non RVSM the rule cannot apply because of the 2000' separation.

The ICAO rules are clear, odd for eastbound and even for westbound. But local circumstances require exceptions, for good reasons.

In certain circumstances an over-simplistic interpretation of fligthtlevel convention would not allow best use of available airspace. Some UK air routes are particularly prone to this, especially those not too far from a North/South axis. Many routes south-ish bound into French airspace, for example, use eastbound level convention even though some of those routes may technically be westbound.
It works amazingly well, and system capacity is enhanced accordingly.