If you’re planning to fly this summer, bring plenty of patience. Airlines are already struggling to keep up with the rising number of passengers. Travelers are posting pictures of crowded airports online, and they’re recounting horror stories about long delays, some of them because airlines are understaffed. The airline industry is quickly bouncing back after fighting for survival last year. They convinced tens of thousands of employees to quit last year, and now some of them don’t have enough pilots or flight attendants. The problems seem worst at Southwest and American, but others have struggled too.