Boeing's latest 737 MAX crisis deepened after United Airlines and Alaska Airlines said they had found loose parts on multiple grounded MAX 9 aircraft, raising new concerns among industry experts about how its best-selling jet family is manufactured.
US regulators grounded 171 MAX 9 planes after a panel blew off an Alaska Airlines-operated flight not long after taking off from a Portland, Oregon, airport on Friday, forcing pilots to scramble to land the plane safely.
Alaska Airlines said late on Monday that initial reports from its technicians indicated some "loose hardware" was visible on some aircraft in the relevant area when it conducted checks of its fleet.
It was waiting for final documentation from Boeing and the US Federal Aviation Administration (FAA) before formal inspections could begin.
United, the other US carrier that flies this Boeing model with the panels, said its preliminary checks found bolts that needed tightening on several panels.
The disclosures heightened concerns about the production process of the MAX 9 jets that have been grounded.
A source familiar with the matter said United has so far found close to 10 airplanes with loose bolts during its preliminary checks, up from an initial five first reported by industry publication The Air Current, and the figure may increase.
There are still ongoing discussions between Boeing, the FAA and the airlines on the precise inspection guidelines.
Boeing is expected to revise the guidelines it submitted to airlines earlier on Monday, and the FAA would have to sign off on those changes before the airlines could begin repairs, sources said.
Boeing said it was staying in close contact with MAX 9 operators and would help customers address any findings during inspections.
"We are committed to ensuring every Boeing airplane meets design specifications and the highest safety and quality standards," the planemaker said. "We regret the impact this has had on our customers and their passengers."
Several industry insiders said airlines have started to hear passengers voice concerns about the safety of the aircraft, even though the MAX 9 in question is only used by a handful of carriers.
Any prolonged concerns may increase pressure on Boeing, which has suffered from numerous production issues since the wider grounding of the 737 MAX family in March 2019 that lasted 20 months after two deadly crashes killed 346 people.
"This changes a lot because it is now a fleet problem. This is a quality control problem," said US aircraft safety expert John Cox.
Investigators said on Sunday it was too early to determine the cause.
Boeing shares sank 8% on Monday.