Flight level assignment

[push&stop]

what critieria do controllers use in determining flight level to aircraft?

[Phoenix_X]

I'm not a controller but a pilot, however, I would think (hope ;)) that the answer is:
The flightlevel requested by the aircraft, if it is available. If not, an available flightlevel subsequently requested by the aircraft.

[TopBunk]

Added to which there are sometimes agreements between adjacent sectors that limit the max level available between airport pairs, so as to keep them in lower airspace to avoid the additional controller workload to climb and descend aircraft.

I am thinking of LHR-MAN at FL180/190, LHR-CDG at FL230/240, LHR-Switzerland and LYS via France at FL270, etc. Higher levels may be available on the day subject to the controller and workload, but in reality if the block time is less than 1 hour, it does not affect fuel burn by more than about 5-10% if you cruise at say FL180 vs FL340. Several times I have had a slot delay LHR-GLA at FL340 due ATC sector restrictions, which disappeared when files at FL180. On plugging in the lower level to the FMGC/FMC the increased fuel burn was maybe 100kgs - no need to refuel. Sometimes I wonder why we climb so high - you get nicer views lower down!

[rej]

Basically what the pilot wants and what is available

oh yes and of course noise abatement - aircraft make a big bang when they hit each other!