A major US airline has said it expects to lose money in the three months between January and March due to the grounding of Boeing 737 Max 9 jets.
The US Federal Aviation Administration (FAA) grounded 171 of them after an unused door broke away mid-flight.
United Airlines has 79 of the aircraft in its fleet, more than any other carrier, followed by Alaska Airlines.
Both airlines have been forced to cancel hundreds of flights this month as inspections are carried out.
United has said it expects the planes to remain grounded until 26 January and its forecast assumes it won’t be able to fly them at all this month.
The issues at Boeing, which is now facing government investigations of its manufacturing process, have also scrambled United’s efforts to expand its fleet.
It warned investors on Tuesday that deliveries of dozens of new Boeing planes, including 737 Max 9s and a not yet approved Max 10, will be delayed.
Chief executive Scott Kirby told analysts the firm has not cancelled its orders with Boeing but was considering alternatives.
In an interview with CNBC, he called the latest grounding the “straw that broke the camel’s back”.
Boeing has been struggling to restore confidence since fatal crashes in 2018 and 2019 involving another version of its 737 Max plane that led to a global grounding for more than 18 months. Smaller manufacturing issues have cropped up repeatedly since production restarted.
Stan Deal, head of Boeing’s commercial airplanes division, said the firm had “let down” its customers and was “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 said.
The Alaska Airlines flight from Portland, Oregon to Ontario, California, had reached 16,000ft (4,876m) when it began its emergency descent, according to flight tracking data, after the unused emergency exit door blew out.
On Sunday, the FAA said another, older 737-900ER model should also be inspected as they use the same door design. United has 136 of these jets in its fleet.
The FAA did not order the older model to be grounded while the visual inspections are carried out by operators.
In the wake of the incident, Boeing said it would increase inspections and bring in an outside expert to review its processes, among other measures.