The European Union is under increasing pressure to further ease rules on airport take-off and landing slots to cut the number of "ghost flights" airlines are running to retain them.
Carriers say the requirement for them to use 50% of their slots -- down from 80% in pre-pandemic days -- or lose them is forcing them to operate empty or half-empty flights.
A sluggish return to air travel, as travelers shrink away from the omicron COVID variant and quickly changing rules for passengers, is dragging out the practice longer than they planned.
Belgium's Brussels Airlines, for instance, says it will have to operate 3,000 under-capacity flights up to the end of March.
Its parent company Lufthansa warned last month it expected it would have to run 18,000 "pointless flights" over the European winter.
Belgium's transport minister, Georges Gilkinet, has written to the European Commission urging it to loosen the slot rules, arguing the consequences run counter to the EU's carbon-neutral ambitions.
The current reduced quotas were introduced in March last year in a nod to the hardship airlines faced as COVID washed over Europe for a second year running, shriveling passenger numbers.
In December, the commission said the 50% threshold would be raised to 64% for this year's April-to-November summer flight season.
"Despite our urgings for more flexibility at the time, the EU approved a 50%-use rule for every flight schedule/frequency held for the winter. This has clearly been unrealistic in the EU this winter against the backdrop of the current crisis," a spokesperson for the International Air Transport Association (IATA) told AFP.
He said the commission needed to show more "flexibility ... given the significant drop in passengers and impact of omicron numbers on crewing planned schedules.”
But a commission spokesperson on Wednesday said the EU executive believed "the overall reduced consumer demand... is already reflected in a much-reduced rate of 50% compared to the usual 80%-use rate rule.”
The spokesperson, Daniel Ferrie, said: "The Commission expects that operated flights follow consumer demand and offer much needed continued air connectivity to citizens."