United finds loose bolts on several 737 MAX planes

Boeing's latest 737 MAX crisis deepened on Monday after United Airlines said it had found loose bolts 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 of the 737 Max 9 aircraft after a panel blew off an Alaska Airlines-operated plane flight not long after taking off from a Portland, Oregon, airport on Friday, forcing pilots to scramble to land the plane safely.

United, one of the two US carriers that fly this Boeing model, said its inspections found bolts that needed tightening on several panels. That disclosure heightened concerns about the production process of 171 MAX 9 jets that have been grounded.

A source told Reuters that United has now found closer to 10, instead of five, airplanes with loose bolts, and that the figure may increase.

The discovery was first reported by The Air Current.

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 grounding of the 737 MAX family in early 2019.

"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 per cent on Monday.

Boeing's largest single-aisle model in production has a panel known as a door plug to replace an exit that would be installed on planes configured to carry more passengers. Most operators use the lower-density version with the door plug.

People familiar with the process have said the panel is fitted in two stages, first by supplier Spirit AeroSystems and later completed by Boeing. Investigators said they would examine both manufacturing and maintenance records.

Aviation services provider AAR Corp dismissed reports that it had performed worked near the door plug.

The US 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," United said in a statement.

The FAA gave the go-ahead on Monday for airlines to inspect grounded jets using an approved process by Boeing.

Alaska Airlines said it also needs additional approval from US regulators before starting inspections, so it has not had a chance to look for any additional loose bolts yet.

The inspections are expected to take several days, forcing the cancellation of numerous flights but one senior industry source said the timing was increasingly unpredictable and that the FAA, under a recently appointed leader, would be cautious.

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' shares fell 11 per cent.