Southwest Airlines (LUV) exclusively uses Boeing 737 aircraft, but CEO Gary Kelly said its fleet lineup could change.

"That doesn't mean we will be an all 737 carrier into perpetuity," Kelly said on CNBC earlier.

Kelly said he's "not happy" about the grounding the Boeing 737 Max, which consists of less than 5% of the airline's fleet, but he's not concerned that Boeing (BA) is putting speed ahead of safety.

"Our negotiations and our relationship with Boeing is something I’ll take up with them privately," he said.

Earlier today, the airline revealed in its earnings that the grounding of its 737 Max planes, the government shutdown, and other "several unexpected events" cost the airline $200 million in the first quarter.