Odd Flight Levels Westbound
 

A question about the dynamics of this incident, which shows something I've never experienced, but I haven't flown in all ATC jurisdictions.

Incident: Wizz A320 near Madrid on Nov 10th 2018, descent below cleared level causes near collision

Incident begins with two aircraft straight and level - westbound - at FL310 and FL290. Usually I'd expect westbound aircraft to be at "even" flight levels 280, 300, or 320, for example. Although the primary rule in Class A is "what ATC assigns," of course.

Is this just because the aircraft are closing on 110nm to destination and for ATC purposes are in a preliminary descent? Would the RVSM floor of FL290 be a factor (as low as possible while still allowing RVSM-rules flexibility, regardless of direction)?

Thanks - I was just waiting to see if other explanations surfaced, but yours is fine!

ODLs have been in use for over 40 years; I often handled traffic at 330, 350, 370 all going the same way (westbound) when I was at Lindholme controlling upper airspace traffic in '73.

Semi circular ( E-W) or quadrantal (E-W + N-S) were created for non surveillance airspace and VFR , when under positive ATC surveillance , you can use any level you want .
Also at time created , most airways were bi-directional, which is no longer the case. Capacity is also an issue today , maximizing the use of airspace is the norm .
ODLs were indeed already used widely in the 70,s a s Chevron said . above. ( I used them myself a lot at the time and in fact always did. )
Finally pattern is full : no nothing to do with 100 Nm to destination or RVSM , just maximizing the sue of the airspace .

ODLs are almost a thing of the past in busy airspace. Routes are becoming unidirectional so any level goes. And there are several FIR boundary points that utilises either East or Westbound levels for traffic on routes that are contrary to the East/West rules.