United's CEO seemed to subtly threaten a move away from Boeing, saying the Max 9 grounding is 'the straw that broke the camel's back'

• Scott Kirby told CNBC he's building an alternative plan without the Boeing 737 Max 10.
• That's because the Max 9 grounding has added to frustrations about certification delays.

The CEO of United Airlines told CNBC he's looking to change the carrier's order book due to his frustrations with Boeing, and appeared to hint at a move away from the aviation giant.

"The Max 9 grounding is probably the straw that broke the camel's back for us," Scott Kirby said. "We're gonna build an alternative plan that just doesn't have the Max 10 in it."

The Boeing 737 Max 10 is the largest version of the narrowbody jet, but has been hampered by delays as it still awaits certification. United has ordered 150, per CNBC.

It is designed to compete with the Airbus A321neo, although the European manufacturer also expects its first delivery this year of the A321XLR — a single-aisle jet that can fly up to 11 hour routes.

Kirby told CNBC he believes the best case for 737 Max 10 deliveries is still five years behind schedule.

"We've grown increasingly to believe the Max 10 just gets pushed further and further to the right," he added.

"We're Boeing's biggest customer in the world. We need Boeing to succeed," Kirby told CNBC.

"I have a lot of confidence in the people: great mechanics, great engineers, great history. But they're having these consistent manufacturing challenges and they need to take action."

"I'm a lot more interested in seeing the actions on the backside of this," he added. "What are the actions to really get the manufacturing process back to the high levels of quality and consistency that historically existed?"

Kirby highlighted the 1997 merger with McDonnell Douglas as a pivotal point that "started a change in culture" at Boeing.

He told CNBC he believes "we're near the end game" on getting the Max 9 back into service — but he's more concerned about "Boeing's ability to deliver the airplanes they've committed to, and to get the Max 7 and Max 10 actually certified."

In a statement shared with Business Insider, Stan Deal, CEO of Boeing Commercial Airplanes, said: "We have let down our airline customers and are deeply sorry for the significant disruption to them, their employees and their passengers."

"We are taking action on a comprehensive plan to bring these airplanes safely back to service and to improve our quality and delivery performance," he added. "We will follow the lead of the FAA and support our customers every step of the way."