Boeing's latest 737 Max crisis deepened 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 from a Portland, Oregon, airport on Friday, forcing pilots to scramble to land the plane safely.
How many planes with loose bolts have been found?
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.
The incident occurred not long after take-off and the gaping hole caused the cabin to depressurise. Source: AAP
What is the door plug panel and why is it used?
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 work near the door plug.
The US Federal Aviation Administration 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.
How long will the Boeing 737 Max 9 jets be grounded for?
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."
Alaska Airlines pilots turned the plane around and landed it safety, and no major injuries were reported even as oxygen masks deployed and personal items were sucked out of the plane.
"It was really important to figure out whether it was that specific aircraft from Friday night," said Anthony Brickhouse, an air safety expert at Embry-Riddle Aeronautical University.
"The fact that United has now found some aircraft with loose bolts, that just means that the investigation is going to be expanded."
A diagram of the 737 Max 9 door plug posted by the US National Transportation Safety Board on Sunday shows four bolts - two in the upper corners of the plug and two lower hinge brackets - that secure the plug to the fuselage.
The plug is further fastened in place by "stop fittings" at 12 different locations along the side of the plug and the door frame. Those components hold the door plug in place and prevent it from being pushed out of the airframe.
Why is the cockpit recording missing?
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, NTSB Chair Jennifer Homendy said.
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 US at present.
Alaska Airlines cancelled 141 flights, or 20 per cent of its scheduled departures, on Monday. United, which has grounded its 79 MAX 9s, cancelled 226 flights on Monday, or 8 per cent of its scheduled departures.
Of the 171 planes covered by the order, 144 are operating in the United States, aviation analytics firm Cirium said.
Turkish Airlines, Panama's Copa Airlines and Aeromexico said they had grounded affected jets. Indonesia said it had suspended use of three jets not covered by the order.
