U.S. Transportation Secretary Sean Duffy said on Wednesday that he would order a 10% cut in flights at 40 major U.S. airports, citing air-traffic control safety concerns as a government shutdown hit a record 36th day.

The drastic plan sent airlines scrambling to make significant reductions in flights in just 36 hours, and passengers flooded airline customer-service hotlines with concerns about air travel in the coming days.

Duffy said the cuts could be reversed if Democrats agreed to reopen the government.