The European Union defended its airport slots rule following complaints by some airlines including Deutsche Lufthansa AG that the requirement forces it to fly empty planes.

Lufthansa last week launched an onslaught against the EU’s use-it-or-lose-it rules that stipulate flights must take place or takeoff and landing slots will be forfeited. The carrier, whose units include Swiss, Brussels Airlines and Austrian Airlines, has claimed the regulations will force it to operate 18,000 flights without passengers over the winter season, causing unnecessary carbon-dioxide emissions.

Competitors like Ryanair Holdings Plc have accused Lufthansa of distorting the environmental issue, saying the German airline should sell cheaper tickets to make sure the planes aren’t empty.

The EU last year waived the normal requirement to use 80% of assigned slots due to the worldwide slump in global travel amid the coronavirus pandemic. The minimum was raised to 50% for this winter season and will be hiked again to 64% for summer.

The requirements are based on Eurocontrol bookings and cancellation numbers and are “very reasonable,” European Commission spokesman Stefan De Keersmaecker told reporters on Thursday. It “cannot be argued that the EU rules oblige the airline to fly,” he said, adding that airlines can apply for an exception if they cannot operate the route.

According to Eurocontrol, air traffic so far this winter has been in the range of 73% to 78% of 2019 levels and is forecast to be at 88% for 2022, the commission said.