Boeing’s disastrous start to 2024 is spilling over to airlines and their passengers as production delays at the U.S. planemaker exacerbate an already nascent shortage of single-aisle jets that form the backbone of commercial air travel.

United Airlines, Southwest Airlines and Ryanair are among the companies scrambling to respond to reduced deliveries from Boeing as the planemaker focuses on fixing quality lapses exposed by the Jan. 5 accident on an Alaska Airlines flight, during which a door plug on the 737 Max 9 blew off shortly after takeoff.

With the busy summer travel season in view, carriers say they’re trimming schedules and looking for alternatives to 737s they’ve already ordered, while also contending with issues afflicting narrow-body aircraft from Airbus. Even Boeing seems uncertain when the planes will be ready as an army of U.S. inspectors sift through its factories, meaning the company can’t make any firm predictions when things might return to normal.