United Airlines CEO Scott Kirby expressed his frustration with Boeing over ongoing manufacturing problems that have led to the grounding of United's fleet of Max 9 aircraft. Kirby stated that the company is considering alternatives to buying a future, larger version of the Boeing 737 Max. The grounding of the Max 9s came after a door plug blew off an Alaska Airlines flight, causing an emergency landing. United Airlines, Boeing's largest customer, owns 79 Boeing Max 9s and has had to cancel hundreds of flights due to the grounding. Kirby said that he is disappointed with the manufacturing challenges at Boeing, which have caused delays in deliveries of Max jets and other Boeing aircraft in the past. He expressed confidence that the Max 9s will be safe once inspections are completed but is uncertain about Boeing's long-term future with the company. Kirby stated that the Max 10, the latest model of Boeing aircraft, is at least five years behind schedule and may be pushed further into the future. He mentioned that United is exploring alternative plans and that Airbus, Boeing's biggest competitor, is the only other manufacturer that is a viable option. Kirby's comments come after United disclosed that it expects to lose money in the first quarter of this year due to the grounding of the Max 9 jets. United's stock rose over six percent, while Boeing's dropped nearly two percent in response to the news.