The Federal Aviation Administration on Saturday grounded dozens of 737 Max 9s after the panel blew out midflight on Alaska Flight 1282, calling for inspections.
United has 79 of the Max 9 planes in its fleet and is the biggest operator of the jet model.
"Since we began preliminary inspections on Saturday, we have found instances that appear to relate to installation issues in the door plug — for example, bolts that needed additional tightening," United said in a statement. "These findings will be remedied by our Tech Ops team to safely return the aircraft to service."
Plane manufacturer Boeing said earlier Monday it issued instructions to airlines to conduct the inspections of the Max 9s in their fleets. United had begun some preliminary inspection work in the past few days.
No one was seriously injured in the accident aboard the Alaska Airlines flight, though the blown-out panel produced a force so violent that some headrests and seatbacks were ripped from the cabin and the cockpit door was flung open, according to initial details of a federal safety investigation. No passengers were seated in the two seats next to the panel.
The National Transportation Safety Board said the accident would have been worse at cruising altitude when passengers and crews are walking around the cabin.
But the accident places fresh scrutiny on Boeing, which has spent years trying to clean up a host of quality defects, while also ramping up aircraft production, including of the 737 Max. CEO Dave Calhoun has spent months trying to assure airlines, investors and financial analysts that the company is improving its supply chain and working to resolve its quality problems.
Calhoun canceled a company leadership summit this week and plans to hold an all-employee call on Tuesday.
Alaska Airlines, which is set to inspect its fleet of more than 60 737 Max 9s, didn't immediately comment on whether it, too, has found loose bolts. The FAA declined to comment on United's findings. Boeing didn't immediately respond to CNBC's request for comment.
The 737 Max is Boeing's best-selling aircraft, with more than 4,000 orders to fill. However, the more common Max 8, which is not affected by the grounding, makes up the majority of those orders.
Aviation publication The Air Current first reported that United had discovered the loose bolts.
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