FORT WORTH, TEXASEurope's skies are full of nearly empty planes to keep airlines' valuable slots at some of the world's most important airports.

The fast-spreading omicron variant of Covid-19 has forced many airlines around the world to cancel thousands of flights.

German group Lufthansa said it would have to conduct an additional 18,000 ghost flights through the winter to keep its landing and departure slots. Although the Christmas holiday brought a significant increase in passenger numbers, the demand for air travel in the rest of the winter season is expected to be low as omicron waves worldwide.

Landing and departure slots for popular routes at the big airports are extremely precious commodities, and airlines have to guarantee a high percentage of flights to keep them. It is why loss-making ghost flights have to be conducted.

Normally, airlines had to use 80 percent of their given slots to preserve their rights on those slots, but European Union has cut that to 50 percent so the airlines could keep their existing slots

The U.S. Federal Aviation Administration has applied similar minimum slot-use rules through March 26, due to the same reasons. Slots are limited at only a few US airports, including Kennedy and LaGuardia in New York and Reagan Washington National outside Washington.