[Source: Reuters]

United Airlines (UAL.O) has found loose bolts on multiple 737 MAX 9 aircraft.

It said, referring to the Boeing model grounded after a cabin panel blew off an Alaska Airlines-operated (ALK.N) plane in mid-flight Friday.

United found issues related to the installation on several panels that were being inspected following the accident, it said in a statement. The disclosure raises concerns about the production process of 171 Boeing MAX jets, mostly operated by U.S. carriers Alaska and United Airlines, that have been grounded while safety checks are made.

Boeing shares sank 8% on Monday as the latest setback for the jetmaker has left it trailing rival Airbus after a series of production delays hampered its recovery from a lengthy 737 MAX safety grounding in 2019.

The U.S. Federal Aviation Administration (FAA) and Boeing declined to comment on the loose bolt reports.

“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. These findings will be remedied by our Tech Ops team to safely return the aircraft to service,” United said in a statement.

Alaska said it has not yet found any loose bolts because it has not yet started inspections.

The FAA gave the go-ahead on Monday for airlines to inspect grounded jets using an approved process by Boeing, but both Alaska and United are awaiting another FAA approval before they can begin.

The intensive inspections of the 171 MAX planes may take several days, forcing the cancellation of numerous flights.

The FAA on Saturday ordered the temporary grounding for checks of MAX 9 jets installed with a 60-pound (27 kg) panel, called a plug, that detached from a full Alaska Airlines Flight on Friday and crashed into a Portland suburb without harm.

The plug closes a hole that could be used for an extra emergency exit door on planes configured with relatively high numbers of seats.

Boeing issued the detailed instructions, approved by the FAA, to air carriers on Monday. Alaska Airlines said on Monday it also needs U.S. regulators to “approve operators’ inspection processes to ensure compliance,” and the airline itself needs to develop “detailed inspection instructions” for its technicians to follow.

The FAA said planes would remain grounded “until operators complete enhanced inspections which include both left and right cabin door exit plugs, door components, and fasteners.” Boeing supplier Spirit AeroSystems’ (SPR.N) shares were down 11%.

The Alaska Airlines pilots turned the plane around following depressurization after the door plug tore off over the weekend. The flight returned to Portland with 171 passengers and six crew on board. No major injuries were reported even as oxygen masks deployed and personal items were sucked out of the rectangular opening where the panel had been.

The panel was recovered on Sunday by a Portland school teacher identified only as “Bob” in the Cedar Hills neighborhood who found it in his backyard, U.S. National Transportation Safety Board (NTSB) Chair Jennifer Homendy said.

“Our structures team will want to look at everything on the door – all of the components on the door to see, to look at, witness marks, to look at any paint transfer, what shape the door was in when found,” she said.

The force from the loss of the panel was strong enough to blow open the cockpit door during flight, said Homendy. “They heard a bang,” Homendy said of the pilots, who were interviewed by investigators.

Homendy said the cockpit voice recorder did not capture any data because it had been overwritten. She again called on regulators to mandate retrofitting existing planes with recorders that capture 25 hours of data, up from the two hours required in the U.S. at present.

Boeing and Spirit, which made the panel, have been grappling with ongoing production setbacks over several years. Since the 737 MAX was grounded in March 2019, Boeing shares have fallen by more than 40% while Airbus shares are up 25%.

Alaska Airlines cancelled 141 flights, or 20% of its scheduled departures, on Monday. The carrier had said travel disruptions were expected to last through at least mid-week. United, which has grounded its 79 MAX 9s, cancelled 226 flights on Monday, or 8% of its scheduled departures.

Of the 171 planes covered by the order, 144 are operating in the United States, data from aviation analytics firm Cirium showed. Turkish Airlines, Panama’s Copa Airlines and Aeromexico said they were grounding affected jets.