Ahead of the summer travel season, airlines in the United States usually compete to sell tickets and fill seats. But the airlines that operate the grounded Boeing 737 Max planes have a new problem: there are not enough seats to meet the demand. “The revenue is right in front of them. They can see it, but they can’t meet it,” said Mike Trevino. He is an airline industry expert and spokesman for Southwest Airlines Pilots Association. The grounding of Boeing’s 737 Max came after two deadly crashes in five months. Southwest Airlines is the world’s largest 737 Max operator. The company has 34 of the planes. American Airlines operates the second-most, with 24. These planes have all been removed from use until at least August. The grounding of the planes has led Southwest to cancel 160 of its 4,200 daily flights between June 8 and August 5. American Airlines will cancel 115 daily flights, or 1.5 percent of its total summer flights.