From longer lines at airport security checkpoints to lengthier delays between flights, U.S. airlines are grappling with fallout from the longest government shutdown in U.S. history, one that Southwest CEO Gary Kelly says will curb early 2019 sales by as much as $15 million.

“I will sum it up in a word: It’s maddening,” Kelly told investors during a call detailing the Dallas-based company’s goals for the year and its performance in the last three months of 2018. “Everyone needs to be on notice, on guard, that this shutdown could harm the economy and it could harm air travel. We will do everything we can to find a way to work through this slop.”