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Kirby's comments came one day after United disclosed that it in the first three months of this year because of the grounding of its Boeing 737 Max 9 jets.

United has 79 of those planes, which federal regulators grounded more than two weeks ago after a panel blew out of an Alaska Airlines Max 9 in midflight, leaving a gaping hole in the plane. Investigators are probing whether bolts that help hold the panel in place were missing or broke off.

Kirby said on CNBC that he believes that the Max 9s could be cleared to fly again soon, “but I'm disappointed that the manufacturing challenges do keep happening at Boeing.”

At times over the past few years, manufacturing flaws have held up deliveries of Max jets and a larger Boeing plane, the 787. Last year, United received 24 fewer Boeing aircraft than it expected.

United has a standing order for Max 10 jets, a larger version of the Max line. However, that model and a smaller one, the Max 7, are years behind schedule for being certified by the Federal Aviation Administration. The grounding of the Max 9 jets is likely to further complicate Boeing's drive to get the new models approved.

Kirby said the Max 10 is at least five years behind schedule and could be pushed further into the future.

“I think this is the straw — the Max 9 grounding — is probably the straw that broke the camel's back for us,” he told CNBC. “We're going to at least build a plan that doesn't have the Max 10 in it.”

Kirby wasn't specific about what planes the airline could acquire instead of the Max 10, but he noted that there is only one other global manufacturer of such large planes — Boeing's European rival Airbus.

Doing without the Max 10 probably means United won't grow as fast as it had hoped, Kirby added.